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MG MGB Technical - Allison Electronic Ignition Problems? 77 B
|I have a 77 B that has been converted from the factory ignition to the Allison (aka Crane I think) ignition. It still uses what I believe to be the original distributor with the box on the side.|
The car died on me the other day after about 8 miles. I verified that it was pumping fuel to the carb (downdraft Weber) because it acted as if it was running out of gas. Since it had gas, I suspected the coil. After it cooled down for a while, it started again, but then died about 5 miles later (in the dark in a pouring thunderstorm).
I later drove it to where my 68 GT is parked, and I borrowed the coil from the 68 that has a points ignition. I put the 68 GT coil on the 77 roadster, and the car failed to start (red herring here I think). I swapped the coils back, and it did start. However, I then drove a couple miles and shut it off for a few minutes, and when I returned to the car it failed to start until it had cooled for a half hour or so.
This morning while everything was cool, I put the 68 GT’s coil into the 77 roadster, and it started right up. This leads me to believe that perhaps it is the Allison box that is getting hot rather than the coil, and being warm was the reason the GT coil failed to start it the day before.
That said, the 68 GT coil is a 12 volt with the higher resistance, and the 77 roadster is a 6 volt coil with the lower resistance. I measured 2.9 ohm and 1.3 ohm, but I have one of those cheap Harbor Freight meters, so take the numbers with a grain of salt.
So, is it likely that some component in the ignition system, such as the Allison box, is failing at high temperatures, or is the coil the only candidate for failure? The 6V coil gets hot enough that it is uncomfortable to hold your hand on it, but not so hot that it is impossible. It is, of course, pretty hot under the hood anyway.
Here is some other info that I am trying to make sense of, not knowing if the figures are as they should be:
When not running but with the key on, there is 7.9 volts at the plus side of the coil,
When running, there is 10.9 volts at the plus side of the coil.
There is 12.49 volts at the battery when not running.
There is a Lucas resistor reading 10 ohms, (at least I think it is a resistor). It is fastened to the car with one of the coil mounting bolts. I read 12.1 volts on either side of this resistor (if that is what it is) when not running. I read more when it is not running, but it is the same on either side. Is that normal?
My tentative plan is to grab the points distributor off of the 68 GT and install it in the 77 roadster. Then I have to decide which coil to use. It seems odd to me that there is 10.9 volts to the coil when running if this is a resistor system feeding a 6 volt coil. I thought the 10 ohm resistor I mentioned might be for the coil, but I guess it is not because I pulled the wire off of it while running, and it kept running. It is a small aluminum rectangular cube. Anyone know what it is? I thought my car used resistor wire to feed the coil.
Is it likely that the Allison box is causing a problem that shows up after running a while and works when it cools down?
Is 10.49 volts too much to be feeding a 6 volt coil, thus making overheating the coil the likely problem?
If 10.49 volts is too much for the 6 volt coil, can I put in the 12 volt coil and run it that way still with the Allison ignition?
If I put my points distributor in, which coil should I use?
Sorry to be long winded, but I couldn't seem to condense it. I am heading out shortly to get the GT distributor, so will be offline for a while.
|C R Huff|
How many wires are there to the + side of the coil?
Are there two, white and light green? If so, you have the resistive wire in the ignition feed to the coil.
|Dave O'Neill 2|
There are two wires that are white with a green tracer on the + side of the coil. They are joined into a single terminal where they plug into the coil. In addition there is a red wire on the + side of the coil, which I assume is the power feed to the Allison box.
So, it seems I have the resistive wire. It seems odd to me that it runs as high as 10.49 volts when running. I would have expected less, but I don't really know.
|C R Huff|
|Charlie. The original Allison system was an optical system and was a common points replacement system back in the mid-70s. It also had (from memory--used one on my Corvette) a finned aluminum device having transistors which was part of the system. Yes, over time, these systems will fail and need to be replaced.|
The points distributor from your GT is a direct drop in and will run with either coil with no problems. The point of dropping the input voltage, except on starting, was to extend points life due to US Federal emissions requirements. If you have doubts about the coil on the 77, they are common at most auto parts stores. Purchase one of about the same size marked "12 V for use with a ballast" and simply hook it up as the old one was hooked up.
The voltages you are seeing are not uncommon and I see them on my 79 which has a similar system to yours. Everything is nominal and, when running, the 12.5 V of the battery gives way to 13.5 to 14.5 volts in the system. Batteries need to be recharged at above their rated voltage and all systems run higher than battery voltage unless there is a charging system problem.
|You should be seeing 6 volts at the coil with the key in the run position. While cranking the engine over, you should see 9 volts. Your voltage is too high and is probably overheating the coil causing it to fail. The resistor has most likely failed and is delivering full battery voltage to your ignition system. The unit, bolted to the side of the coil, is most likely a noise suppressor installed to cure radio interference from the ignition system. A common item in the '70s. RAY|
|Okay, thanks Les and RAY. I now now a little more from doing experimentation, but what I know only leaves me more confused. Here is what I have done.|
I drove the 77 B the 8 miles over to my 68 GT and took the 25D points type distributor out of it. The 77 ran fine over there, but by the time I got the dist out of the GT, the 77 roadster would not restart. Being smart, I took a beer with me and drank that in the shade waiting for the 77 to cool down.
When it cooled down enough, I drove the 77 over to a friends house about 7 miles away so we could work on our MGs together. It ran fine on the way there, I shut it off when I got there, tried to restart it right away, and it failed to start. I stuck a screwdriver into a plug wire, had my friend crank it while I held it close to the thermostat housing, and there was no spark.
So, I pulled the 45D distributor out of 77, disconnected the Allison system, installed the 25D points distributor, left the 12 volt coil from the GT in it that I had already driven over there with, set the static timing, and fired it up. Then I set the dynamic timing.
Then I shut it off and tried to restart it. It did not start. We drank another beer while it cooled, and then it did start. I backed out of his driveway to head home, and it died and would not restart.
We pushed it back into the driveway and I swapped back to the 6 volt coil. It started, and I headed out for the 12 or so mile trip home. I barely made it. I had to keep the revs up, or it would stumble and threaten to die. As I pulled into my drive, all I had to do to shut it off was take my foot off the throttle.
So, will an ignition switch act like this? That is, will an ignition switch work for a while when cold, and die after running a while. If that is possible, can I just hot-wire the + side of the coil from a good live wire and go road test that configuration. It seems that I have changed most everything else out and the problem persists.
This would not be so important at the moment except that we have had high water around here lately, and our road was washed out. We patched it up to get out, but it was rough and I managed to bash a 3 inch dia hole through the oil pan of my 99 Volvo V70 daily driver, and I need to use the 77 MG to fill the gap until I can fix the Volvo.
Below is a photo of my road. The square box in the right of the photo is the mailboxes, and the road is right on the other side of that. The photo was not taken at the maximum height of the water.
|C R Huff|
|One other thing I forgot to add, but I doubt is the problem: The fuel in this car is old. About half of it is from last year, and the other half is from the year before. I have gotten it run down to about a quarter tank now, so I could refill or pump it out and give it to my 46 John Deere Model A, which would be happy to get any fuel at all. |
But, I doubt old fuel is the problem. I did start a trip to Zion in Utah with this same car with 3/4 tank of 3-year old fuel a few years ago and had no problems.
|C R Huff|
The "ballast" is a resistive wire (white and pink) which is spliced into the coil supply from the fuse box (white) and runs across the slam panel to emerge as white and light green at the coil. This gives a running voltage of "around" 6 volts. The other wire (white/blue)on the coil originates at the starter solenoid and delivers whatever the battery can deliver. This voltage depends on battery condition and state of charge, resistive losses on the battery to starter lead and connections, and temperature. Battery output drops with lower temps, also the starter draws more from the battery churning a cold engine, mainly because the oil is more viscous.
This set up was adopted to give better cold starting, i.e., a decent spark was obtained from a 6v coil when battery output was low and starter drain high, but only on cranking. The ballast delivers a more coil friendly voltage when the key springs back to "run".
10.9 volts is too high and not coil friendly, but 7.9 would be reasonable running voltage.! Something is wrong there, probably down to a PO messing about with the wiring. You can find the resister wire splices, they are quite prominent "bulges", so you can check continuity, resistance and voltages. Quite often this wire has been by-passed, less usually shortened.
But you need to check this and the cranking voltage at the coil.
Or you could just put a new 12 volt coil feed from the fuse box (white), and use a 12V, 3ohm coil. Modern batteries are much more capable than those available in the 60's and 70's. The original 2 x 6v batteries delivered a highish cold cranking ability while being reasonably compact.
|Charlie. Had somewhat similar issues with my 79 MGB. It turned out to be a bad ignition switch that intermittently would fail. Mostly after the the car was at normal operating temp. Also, it may be time, as well, to change out your distributor's ignition. |
So next time the car fails, test the ignition switch with your meter. You may have both a switch and ignition problem.
|The original electronic and after-market electronic ignition systems can't really be used together i.e. mixed and matched. The later factory system 45DM4 did use a box mounted on the inner wing, the earlier 45DE4 didn't. It's very unusual for the later system to need replacement, they are extremely reliable and the pick-up and electronics were used on a wide range of marques and models. The 45DM4 box may well be there but not doing anything, or it may be a box for a different system i.e. your Allinson. The earlier 45DE4 system used an additional resistor mounted by the coil, but this didn't use a box on the inner wing, everything else was inside the distributor body. But again this could be left over from a previous system. So it sounds like you could have bits of at least two if not three systems hanging around, you really need to work out which are no longer required and disconnect and remove them as they are only causing confusion.|
You are right in your coil measurements, chrome bumper cars used a 12v coil of about 3 ohms primary resistance, rubber bumper used a lower resistance - about 1.5 ohms to begin with, but by the time the 45DM4 ignition came in, which is variable dwell, they could well be 0.8 ohms as apart from maximum revs only short pulses of current were used, so lower resistance coils could be used for a fatter spark without overheating. As mentioned a length of resistance wire concealed within the harness was used in series with the rubber bumper coils, and this also has a resistance of about 1.5 ohms, reducing the current to much the same level as the original 12v coils - in the 1.5 ohms coil anyway.
Having said all that a rubber bumper car configured for a 6v coil ought to start and run with a 12v coil, unless there is something else wrong, the spark will still be there but weaker.
After that things go a bit haywire. When turning on the ignition on a rubber bumper car, you may well see 6v between the coil +ve and earth, but only if the points are closed, or the electronic trigger is 'closed' i.e. passing current. If either of these are 'open' you would see 12v from the coil +ve to earth, but zero volts between the coil terminals. The difficulty with electronic systems is that many only pass a current through the coil when the engine is running or you are trying to start it, they deliberately don't pass a current if the ignition is switched on without starting - for more than a few seconds anyway - to prevent the coil from overheating and getting damaged.
With the engine running the situation changes. Measuring between the coil +ve and earth on a rubber bumper system the coil is being switched between those two values i.e. nominally 12v and 6v, but in practice when the alternator is charging it is more likely to be between 14v and 7v. With a points dwell of 51 degrees, which equates to 57%, the voltage between the coil +ve and earth will *average* 10v. With an electronic system that has variable dwell i.e. very short current pulses reducing the voltage down from 14v you could well see 12v or higher. The voltage needs to be measured between the two spade terminals on the coil, and only if that is higher than 6v is there likely to be something wrong which is bad for the coil.
What is your ambient temperature when you are measuring the temperature of the coil? Coil temperature is directly related to ambient, as the coil can only start dissipating heat when it gets hotter than the surrounding air, until then it just gets hotter and hotter. For example in the UK at an ambient of 8 to 10C i.e. 46 to 50F both my 12v and 6v coils only got up to about 40C or 104 F which is barely warm to the touch. But in summer at 27C i.e. 80F the 12v coil measured 67C or 150F and the V8 measured 62C or 143F and the roadster in particular was very hot. 6v coils do run cooler as half the energy is being dissipated in the ballast resistance and only half in the coil. The roadster coil does get very hot in summer, but I have no running problems. But heat can be a factor in the failure of any electrical or electronic component.
So you've tried both distributors and both coils in the problem car, and whilst one combination did start and run it still had problems. That says to me you have more than one problem.
The tach is one of the major diagnostic tools for ignition system problems that occur while you are running. If it drops like a stone the instant the engine loses power, but the momentum of the car is still spinning the engine, then it is an ignition LT problem or you have lost the feed from the ignition switch. Additionally if the ignition warning light has come on then you have lost the feed from the switch, if not it is a problem through the coil and whatever trigger i.e. points or ignition that you are using. If the tach continues to register then it is HT or fuel, or condenser failure if you are running points.
|Allen, It did seem to me that the voltage I was seeing at the coil was more than I expected, but then I temper that by knowing I have a cheap multimeter. And, now I have a 12 volt coil on the car. That, I suppose now means that I am not providing enough voltage to it since it is still fed by the resistor system. And, you are right, I do need to check voltage at the coil while cranking.|
Gary, I am considering your explanation of the ignition switch being a problem as pretty likely now that I have changed out the coil and distributor. Unfortunately, I forgot to check voltage at the coil at a times when it failed to start.
Paul, I have the 45DE4 distributor. You have explained some things that puzzled me. While not running, I did once measure battery voltage at the coil, and another time I measured the 7.5 volts. I did not realize that this would be normal depending upon where the optical chopper wheel had stopped. Also, I had not thought about the resistor wire dealing with alternator voltage while running.
Given your description of coil temps, I think overheating is probably not my problem. It was 90F (~32C) when I was trying to decide if it was overheated. Given the ambient temps and the under hood temps, the coil was probably no hotter than should be expected.
On my last drive home, I did notice that when the car was stumbling (sounded like 2 cylinders had quit) the tach was rock steady at about 3000. I did not think quickly enough to check it when it was dying completely. It did seem necessary to keep the revs up to prevent the car from quitting.
So, at the moment I have the 25D points distributor and the 12 volt coil, and the car failed to operate properly. The Allison box is disconnected. I do still have that metal box that I think is a Lucas resistor bolted on with the coil mounting bolt. I did unplug it while the car was running, and the car continued to run.
I think my next step will be to hotwire a full 12 volts to the coil, and see what happens. That way, I will be delivering full voltage to the coil, and will be eliminating the existing wiring and ignition switch.
I will report back the results of that experiment.
Thanks for the help,
|C R Huff|
|I ran a hot wire through a switch in the cabin from the battery to the 12-volt coil. It seems to work better. I noticed better throttle response. However, if I get it fully warm, shut it off, and try to restart right away, it fails to restart though it does now sound like it is trying.|
I suspect Paul is right that I have more than one problem. Making this last change is still a big improvement. I drove it around a bit and the last leg home was 16 miles. It ran fine for the entire 16 miles except for dying at a light once, and it took a few minutes before it would restart. When I got home, I shut it down and tried to start it right away, and it would not start.
But, I let it sit for 5 minutes and it started fine. Before, it had to sit for about 45 minutes before restarting. Also, this car has always been a bit stubborn about starting hot. It would start, but act like it needed to clear out as if it was flooded.
I ran the fuel gauge into the red and added nearly 12 gallons of fresh fuel. Tomorrow I have to go to western Kentucky, so that will be a test of about 315 miles round trip. I will see how it does.
|C R Huff|
|"so that will be a test of about 315 miles round trip. I will see how it does."|
That's the kind of test I like :o)
|Charliy, are you still running the original Stromberg carburetor? Your fuel may be boiling due to the extreme heat put off by the catalytic converter. Also, check to see that your fuel tank isn't creating a vacuum after a short drive. This can be accomplished by removing the fuel cap right after the engine dies. If you hear an inrush of air, chances are that there is a blockage in the vapor recovery system. This can mimic your problem. RAY|
Good suggestion on vacuum in the tank. I never thought to check that, but it is probably not my problem since I did verify that there was no spark to the plugs when the car died. The carb has been changed to the 32/36 downdraft Weber. I hope sometime to switch to SUs.
I made the 315 mile round trip to Western Kentucky yesterday, got home about an hour after dark, and had no problems with the car. I did not try to start it right after shutting down, but did have short stops for fuel and supplies, and had no trouble restarting.
The trip was done with the 12-volt coil, the 25D points distributor, and a hot wire run through a switch from the battery to the coil.
So, I have found the work around, though not the initial problem. I suppose it has to be either the ignition switch or poor connections somewhere. There is a small chance that the resistor wiring system was delivering too much power to the 6-volt coil and too little to the 12-volt coil, but I don't think this is too likely.
The car has demonstrated anemic electric power for a long time. I started working on it last fall when I discovered that my erratic radio reception was related to stepping on the brakes. The brake lights are also switched power, and stepping on the brakes resulted mostly static from the radio. Also, the turn signals (switched power) are sluggish to start and not as bright as they should be, while the 4-way flashers (not switched power) work fine.
I did discover a poor connection at the white junction box near the right side fender well, which connects four brown wires. The connector was hot, and moving it to try to check the voltage in and out resulted in killing the engine. I replaced the junction box with two inline blade type fuses. However, I had trouble getting the solder to flow well, and may have a cold joint. But, I believe this is not switched power. I also found that there was no ground between the chassis and the engine/transmission, so I added one and that improved starting, though there had been no trouble with cranking.
When the Allison box was connected, it was getting its power from the + coil connection, meaning it was being powered by the lower voltage of the resistance wire. Does anyone know if this is correct? Does anyone have another idea in addition to either poor connections or a bad ignition switch?
Thanks for your help, a toast to you in the photo,
|C R Huff|
|Charlie... Cheers to you also.|
About a year and half ago, my 79 B had electrical issues. In the end it was ignition switch, as well as goodly number of terminal end connector replacements. The final blow was the battery packed up.. Long story short, replaced about 15 terminal ends, and an ignition switch, and of course the battery, and now have a good working electrical system.
Do you (or anyone else) know anything about the quality of the currently available replacement ignition switches?
Also, RAY, I forgot to mention that the cat on my car has been moved downstream to the end of the exhaust header that was installed to accommodate the Weber conversion.
|C R Huff|
|Ah, grist for the mill. The color of your B is the same as mine, so that bodes well. A common fault, similar to yours, is poor grounds. Most of the under dash grounds can be found connected to a stud, at the base of the windshield, on the passengers side. I had unusual electrical problems for two years before I traced them down to a loose nut that retains these ground wires. After securing the grounds, my problems went away. RAY|
|Charlie... I purchased my ignition switch through The B-Hive located in Clemsen, SC. The fellow there is Gordon Srickland. I forget how much $$$. reasonable.... |
When you remove the switch, there is a very small screw on the bottom of the casing for which you will need a very fine screw driver to loosen. Pretty much it is a straight forward swap.
Again, it would be wise to go over your car's terminal ends and replace them after cleaning the tabs they plug onto. I used a small Dremel with the conical attachment. It made quite a positive difference. Starting with the battery connections and just kept going through the fuse box, etc, etc.
|Also, my 79 has no cat converter and sports a header and Weber. I have had no problems with the Weber unit. As it has been quoted, 95% of carburetor issues are electrical.|
|"Also, the turn signals (switched power) are sluggish to start and not as bright as they should be, while the 4-way flashers (not switched power) work fine."|
They do get their power from different sources but the flasher units are also completely different. Hazard flashers are designed to flash anything from one to four bulbs, where as turn signal flashers are designed to flash two, and do give a different flash rate if one of them fails. On the original 2-pin flashers they flash more slowly as the voltage drops, even when both are working.
The connector block with four browns is in the main power supply from the alternator and battery to the rest of the cars electrics bar the starter. Bad connections there (i.e. getting hot when you have several things switched on) will affect everything with low voltage, i.e. flashers and ignition.
Usually when adding electronic ignition to a ballasted ignition system the electronic module needs to get its power from a full 12v switched ignition source such as the white or white/brown at the fusebox, and not the coil +ve.
|The complete ignition switch/locks are, I think Land Rover units, so I assume quite good. I've had one on my V8 roadster for 5 or 6 years now with no problem.............fingers crossed.........touch wood............where's my rabbit foot?|
That is interesting. I went looking for my ground fastened to a stud under the dash on the passenger (right) side, and could find no such thing. Then I drank more coffee looked again. I found a cluster of black wires all butted together with a heavy duty shrink wrap on their ends. They are fastened to nothing, and they are too short to reach any stud or screw.
There is a stud with a nut on it directly below the passenger side windshield wiper, and there is a small screw below and left of the ground wires. Unless the wiring harness were moved (and that may not be possible) the grounds won't reach either location. The ground cluster wires only extend about 2 inches out of the harness, and it is about 4 inches to the stud. Any idea how long these ground wires should be?
Another interesting observation is that there is some body color paint overspray on some of the blue harness wrap in this area, and I would think the factory would have painted before wiring. So, could someone have been working in there and decided to get rid of all those pesky ground wires?
I am familiar with Gordon at the B-Hive. Thanks for the tip on the small screw. I have done terminal cleaning on this car, but only under the hood.
I do have 2-pin flashers for both the signals and hazards. Since some of the signal lights are dim, but are bright on hazard. Would this mean that the local grounds for the lights are good, and the trouble is upstream of that on either a hot or ground?
Before I replaced the junction box for those brown wires from the starter, they were too hot to hold onto at the junction. I should recheck that to see that my solder joints remain cool.
It sounds like if I reconnect my Allison box, I should use full 12 volt to supply it. It did seem odd to me that they would design it to run through an existing ballast. On the other hand, a friend of mine who got a used spare unit with his B, showed it to me and it has one of those ceramic resistors in the wire that feeds it. Maybe a manual for the Allison is available online.
Do you know if the Rover unit is available as switch only? I would think it would be a lot cheaper not to buy the lock part.
|C R Huff|
|Looking through the wiring diagram in the Haynes (page 201, 1989 copyright) it seems that it is normal to have a bunch of ground wires come together, but not to be grounded at the point where the come together. So, it may be normal that the grounds I found under the dash are not supposed to be grounded at that point.|
|C R Huff|
Not sure about switch only availability, mine was replaced because of lock problems. Also the under dash earths usually terminate with a 1/4' EYE and are bolted with the wiper motor clamp bolt. Not sure about US cars though.
|Thanks Allan. I just double checked and there are no grounds at the wiper motor clamps on my USA 77.|
|C R Huff|
|" it seems that it is normal to have a bunch of ground wires come together, but not to be grounded at the point where the come together"|
Ditto brown wires and red/white (gauge illumination) from time to time. There are at least three of those junction points in the earth system behind the dash of a 77.
FWIW my UK 73 and 75 have the earthing point under the dash top, front right-hand corner. Close to the wiper motor - above it, but not it's mounting bracket. However that earth feeds dash stuff, not any of the external lights.
If the flasher bulbs are visibly brighter with the hazards on compared to the turn signals then there is definitely a bad connection somewhere between the brown and the greens. You need to do voltage checks both sides of the ignition switch (brown and white) and fusebox (white and green) at the very least. The green needs to be drawing current, the heater fan is probably the most convenient for this as it takes quite a bit. Info on doing a full indicator circuit volt-drop check is here http://www.mgb-stuff.org.uk/indicators.htm, potentially (ho ho) around 60 connections where you can lose voltage.
|Well Paul, it looks like you, RAY & Allen were right about the ground under the dash. I failed to find it until I compared my car to your photo, and then there it was. I haven't tried to clean it yet, I only verified that it is not loose.|
Thanks for the link to your website for the signal voltage testing. I also backed up to one step before that page to get the background instructions for doing the test. I get to save the step of disconnecting the coil since I now have that on a separate switch. I think I will run test with the flasher bridged and with the car off, but with a small battery charger hooked up. That way I won't have all the heat and noise while doing the testing.
I don't know if I will get to it this weekend or not. Some other things have to get done.
A note to anyone clicking on Paul's link above: It didn't work when I clicked it and I think it is because the period at the end of the sentence prevents it from working. Copy and paste it without the period and it works fine.
|C R Huff|
|Charley, I only started to have problems with my electrical system after I got the car back from the body shop when I had the sills replaced. The windshield was removed and, in the process, the the ground wires were reconnected to the stud, but the retaining nut wasn't tightened. This led to all sorts of problems until I discovered the loose connection at the stud. RAY|
|"It didn't work when I clicked it and I think it is because the period at the end of the sentence prevents it from working. "|
Indeed. I usually deliberately leave a space after a link and before any punctuation, but not this time.
|Well, the car has continued to die when hot even after the successful trip to Western KY.|
I have not found time to deal with the weak power through the turn signals and other wiring, but as far as running the car, I think I have bypassed all that with the hot wire from the battery to the coil. It faded and died yesterday at a stoplight. While checking it, I decided to pull the fuel cap, and I did hear the air sucking into the tank. I disconnected the vent line at the separation tank in the trunk, and stuck the end of the line outside of the trunk, and laid the trunk lid down on it to hold it in place.
The car did restart (reluctantly after a 10 minute break) and I thought maybe I had found the problem. When I got where I was going a few miles down the road, it died again while I was trying to park. I pulled the fuel line off of the carb with the key off, and the line was still under pressure evidenced by the squirt of fuel that came out. I pumped some fuel into a bottle, and it seemed normal except that it took a couple seconds before the fuel started filling the bottle.
After it restarted, I drove about 15 miles to my next stop. I left a wet rag on the coil for this trip, but I also never let the revs go below 2000 since it seems more prone to dying from idle or low rpms. When restarting while hot after dying or shut down, it starts running at such low rpm that you can't be sure it will pick up and run, but clears in a few seconds and is drivable again.
When I got home, I hooked a hand vacuum pump to the line fitting on the fuel separator in the trunk. I could not draw a vacuum on it. After verifying that no raw fuel came out of the separator, I put a clear line on it and checked that I could both blow and suck air through it. I could, but I could feel resistance to doing so. It was not like blowing through a simple open pipe. Is this normal?
To review, this problem has persisted with the Allison electronic ignition, coil, and distributor. The problem persists with another coil and a points equipped distributor, and it persists with the points system fed directly from the battery.
I did just check voltage at the battery. I had 12.58 standing and 14.20 running. With the voltmeter plus on the batt and the voltmeter neg on the gearshift, I got about 14.17. One other clue is that when I just started it up, by the sound of the fuel pump I could tell it was working rapidly for a while and then slowed as if the carb required significant fuel to fill it even though the car had only been sitting for about 16 hours. It seems that a lot of fuel is evaporating on hot shut down. The pump is a low pressure Facet (I think Facet, at least it looks like one) that I picked up in a California NAPA a few years ago for my GT.
|C R Huff|
|There comes a point when you clutch at straws.|
Is the Webber float chamber and the fuel lines well separated or insulated from the exhaust?
The Weber downdraft has a 1/4 inch fiber isolator/insulator between the manifold and the carb base. There is no heat shield. The fuel lines are not insulated. I would have to inspect their routing to comment.
I talked in my last post about the fuel pump playing catch up when I started it to check the voltage this morning. However, I only ran the engine for a minute or so, and when I went to start it a few hours later, it did the same thing, though maybe not quite as long. So, it might be catching up with pressure rather than filling an empty carb.
I took it for another trip today, in fact going to the same place I went yesterday. It never died, and had no trouble restarting. However, when I got home it was idling at about 600 instead of the roughly 900 I had set earlier, and it was stumbling a bit. Today it was only about 85 F (~ 30 C) while yesterday it was about 94 F (~ 35 C).
Also grasping at straws, has anyone ever heard of a failing catalytic converter causing a problem like this? In the fall of 2012 I drove this car to Southwestern Utah, and crossing the continental divide put me over 11,000 ft, and a lot of time was spent at over 8000. Not having SUs, I couldn't lean the carb and I think I damaged the cat. The exhaust doesn't have the characteristic smell of a controlled exhaust anymore.
|C R Huff|
|Fuel cap thing first. If you have the emissions plumbing as seems the case, you should not get any vacuum in the tank at all, it should always be at atmospheric pressure. This is different to the UK and pre-emissions cars, as they use a sealed tank and a vented filler cap, but the cap has a spring-loaded valve which does allow a small vacuum to develop in the tank while running.|
Your tank should be at atmospheric pressure via the separator and charcoal canister, which is open to atmosphere at its bottom port. If you can feel resistance there is a restriction, which may be the charcoal granules gummed up, or may be a pinched pipe somewhere.
However difficulty in restarting after removing the cap indicates it is not vacuum-generated fuel starvation problem, as having once removed the cap and filled the vacuum, the next time you switch on the pump should almost immediately fill the carbs.
Left even a few hours after a hot switch-off in warm weather is quite likely to evaporate a significant amount of fuel from the carbs. Under those conditions I can hear the fuel boiling in my V8 carbs, vapour is being pumped out of the vents, and switching on after it has cooled does result in significant pump chattering. It doesn't affect starting though.
You say: "I pulled the fuel line off of the carb with the key off, and the line was still under pressure evidenced by the squirt of fuel that came out. I pumped some fuel into a bottle, and it seemed normal except that it took a couple seconds before the fuel started filling the bottle."
There should be a significant spurt - a fuel pump chamber's worth - of fuel if you pull a pipe off a carb just after having switched off the ignition. If it's just a dribble, and it took a couple of seconds with the ignition back on before fuel started coming out, then that looks like fuel starvation to me, from problems back through the lines, pump and tank. After the significant spurt when you switch on a again it should start pulling fuel immediately. If the carbs and lines are full should should only get one or possibly two clicks when turning on the ignition, no more than that. Do that again but measure it, it should deliver a minimum of one Imperial pint per minute, and in practice more than double that, in steady series of pulses with minimal bubbling.
A blockage in a cat or elsewhere in the exhaust would cause running problems, but the beat in the tail pipe should be very muted or muffled, and the effects would be much more obvious at wide throttle openings than smaller.
Your battery voltages are fine. But are you getting the same readings between the coil +ve and the distributor body (ideally the points plate by getting a thin wire through to the inside somehow).
|"There should be a significant spurt - a fuel pump chamber's worth - of fuel if you pull a pipe off a carb just after having switched off the ignition."|
Should read 'up to a fuel pump chamber's worth' as the points could be on the verge of closing to pull the diaphragm back ready to deliver the next chamber's worth, so in that case you may only get a very small spurt. But the point about the fuel being pumped out of the open pipe almost immediately i.e. within a fraction of a second when you turn on the ignition still stands.
|I've had experience with plugged catalytic converters on a Ford product. Actually 3 failures. In each case the car lost power and wouldn't operate above idle. All the problems originated with failure of the notorious Ford Thin Film Ignition Module. Misfires overloaded the cat with un-burned fuel and it melted the matrix.|
I don't think you have a plugged converter but it may not be as active as it should be. My Toyota converted failed this way, not active but not plugged.
Sounds like your Facet is not the continuous type but a diaphragm type. There should be some sort of inlet screen. Maybe it's plugged with rust and you are not getting proper suction.
See if there is a screen and if it needs cleaning. I took my old Hardi SU clone out for installation of new hoses and found the intake screen chock full of rust. Since I didn't want to take the pump apart (parts unavailable) I flushed the rust out with solvent.
Mind you this pump continued to feed even with all this rust. But a plugged inlet might be affecting your pump.
Lots of luck.
I agree with you that the vacuum I found in the tank is not THE problem. It seems that it may be A problem. But, since relieving the tank vacuum, I also disconnected the line from the tank to the fuel separator and stuck the open end of the line to atmosphere out of the trunk, and the car died again a short distance later. My fuel check after the car died probably shot about a tablespoon of fuel out with the key off. There was about a 2 - 3 second lag during which the key was on and no fuel was pumped into my bottle.
I have just verified that the resistance to air flow from the tank to the charcoal canister has nothing to do with the fuel separator. Air passes easily through the separator. The resistance is felt when checking the line from the trunk through the charcoal canister. Air will flow through this system, but it has some resistance to flow.
One other thing to note. This intake manifold has the coolant line running through its base, and it is connected. Given the amount of heat rising from the exhaust manifold, I don't know if this would serve to cool or heat the intake.
I did just check the plumbing at the charcoal canister, which is compromised by the conversion to the Weber downdraft. Looking at the late model diagram (figure 3.41) on page 91 of the 1989 Haynes, here are my deficiencies:
Hose # 8 (vapor lines) is only connected to the canister. The other end, which I think would have been connected to the Stromberg carb, is simply open to atmosphere, and will freely pass air.
Hose # 11 from the running-on valve is not connected to the valve. Nothing is connected to the valve at that location, and that port on the valve is not capped.
There is a hose that likely is Hose # 11, but one end is connected to a vacuum port on the intake manifold and the other end is plugged.
The vacuum advance module is connected to the Weber carb and not to the intake manifold.
Here are some more voltage readings with the car not running:
Coil +ve to thermostat housing 12.08
Coil +ve to points plate with points closed 12.02
Coil +ve to points plate with points open 12.61
Coil +ve to distributor body points open 12.61
Points + lug to points plate points open 12.61
I suspect that you are right that, if my cat has been compromised, it is like your Toyota and not like your Ford. I can't say that the car performs any worse since the suspected failure.
I should do a check of my fuel filtration. I just checked my record, and if I ever replaced the filter under the hood, I did not record doing it.
|C R Huff|
|"This intake manifold has the coolant line running through its base"|
I'm aware of coolant being circulated through auto-choke systems but not through the intake manifold itself. That will be heating the manifold to thermostat temperature or higher, but only when the engine has warmed up, which seems odd. It may have a slight cooling effect as regards catalytic converter heat output, but usually the intake system is being cooled by the vapourisation of the liquid fuel as it comes out of the jets. Not that I think it has anything to do with your problems - unless that system was added immediately before the problems started.
You need to disconnect the tank line from the canister and check those two independently, to see which is causing the restriction. Could be both of course, and it may not be the cause of your problems now but as something that is 'not right' it could get worse.
Hose 8 would normally be connected to the float chamber vent, which should be present on your Weber.
Hose 11 would normally be connected to the inlet manifold, and provides the vacuum when the valve is actuated to suck the fuel out of the jets. That would only be applicable to variable jet carbs like the SU and Stromberg.
Given those two disconnections you might as well remove the canister and valve and all their plumbing, plugging any ports on the inlet manifold. Ordinarily would need to put a small filter on the port on the back of the rocker cover, but given the Weber I doubt that is providing any vacuum to the front tappet chest cover for crankcase ventilation. Without that you should really put a PCV valve between the inlet manifold and the tappet chest cover, and the filter on the rocker cover. You could also put a small filter on the engine compartment end of the tank pipe. You could remove that pipe and the separator and put a vented fuel filler cap on the tank, but if ever someone fits the correct non-vented filler cap you would splutter to a halt after a few miles for yet another reason :o)
Vacuum advance connection to the Weber is fine, I'm assuming this inlet manifold doesn't have the port for that which the original would have had.
Those voltages are fine, but really you need to check the points plate one with the points closed while sucking on the vacuum advance pipe to twist the points plate back and fore.
The 2-3 seconds lag between turning on the ignition and getting fuel is the biggest problem that I can see. There should be two non-return valves in the pump - one in each port, and that stops fuel draining back into the tank i.e. it keeps the lines full. If yours are faulty then when you disconnected the line at the carb, being higher than the tank the fuel would have run back, which could take a couple of seconds and a few clicks to refill. That normally wouldn't happen while running, but it will significantly reduce the effectiveness of the pump, which is why you need to do a timed delivery check of at least one Imperial pint.
|I forgot that you have a downstream fuel filter which you should check or replace. My Mk I doesn't have a filter. |
The other filter I was referring to was a screen on the inlet of the pump. I know the Facet rotary always on pumps require a filter on the suction. SU's don't want a filter because if it clogs you can burn out the coil. But they do have an internal suction screen for larger stuff like rust. I don't know about your pump.
As mentioned you could have a bad valve on the pump.
I'm sure the forum will help get this solved.
All the best
In the attached photo you can just barely see where the intake manifold coolant line enters the intake manifold below the base carb. You can really only see the radiator clamp at the very bottom near center of the photo. I think the idea of the heated manifold was to promote vaporization.
Also, you can see where the tappet chest breather hose connects to the bottom of the air cleaner. I don't know if this provides enough vacuum to the crankcase to do any good. I will try to test that sometime this week. Both the heater and breather hoses have been hooked up since I have had the car.
I checked the fuel tank venting today. It was the line rather than the canister that was causing the restricted flow. I blew it out with compressed air, and got a puff of dry rusty dust. It flows better now and I have reattached the fuel tank vent lines. I don't really want to remove the charcoal canister because then I am likely to loose or misplace it, and at some point I may replace the Weber with SUs that could use the canister. The float vent of the Weber is not constructed to connect to the canister since the vent is just an open rectangular hole inside the air cleaner.
Actually, there is an unused port on the inlet manifold if that would be a better place to hook up the vacuum advance. At the moment, the 25D dist is connected to the Weber vac take off, which I am guessing is ported. My electronic is the 45DE4.
Glad to hear the voltages look okay. I will recheck with my vac pump moving the points plate.
I haven't done the fuel pump volume test yet. I do have an electronic SU that I intend to install.
I definitely need to replace my downstream fuel filter. I don't have one upstream of the Facet, but perhaps I should. As I mentioned above, I plan to put the SU on it. In part, I need to plug up that hole in the trunk. I bet the check valve is bad on the Facet, which would explain why the pump seems to play catch up even when the car has been off for only a short time, and was not run long enough to get hot.
I didn't get much done on the MG the last few days because I allocated my car fixing time to getting the busted oil pan off of my Volvo. What a pig of a job that was. Now I have to do it again at the junk yard, and then install it.
I did take the MG out Friday night to go to a Blue Moon party about 15 miles away. With the cool evening/night temps(high 60sF, ~ 20C), the car ran trouble free. So, whatever it is, it does seem to be temp sensitive.
|C R Huff|
|Couple of things.|
The air filter looks dirty.
The air intake being above the exhaust centre branch means it's getting very hot air, not exactly conducive to volumetric efficiency.
Also I would presume the float chamber(s) also get pretty hot. Both these situations would be helped by cooler ambient temperatures.
I would be clamping off the water feed to your inlet manifold.
|If your Facet is a diaphragm type then you probably don't want an inlet filter. I am pretty certain you should not put an inlet filter on an SU type. If the filter plugs then the diaphragm can't return to neutral and things burn out. They can pass some trash with no problem. The internal screen stops the big stuff but generally does not plug tight due to mesh size.|
A points type fuel pump doesn't have much that would be temperature sensitive, maybe the capacitor or diode. Rare. An electronic pump could be temperature sensitive.
In fact, I cleaned the air filter yesterday, but it did not dry in time for me to oil and reinstall it. I will have it back on today.
I have pondered clamping off the coolant to the manifold, but given the temps involved above the exhaust, I don't know if the coolant would be heating or cooling the manifold. I used to run Cummins engines in large trucks, and most of the turbocharged versions had an aftercooler to cool the intake charge. The aftercooler did its job by running engine coolant through a heat exchanger that was integral with the intake manifold.
And, though it may be apples and oranges, my Model A John Deere has an intake manifold that is encased by the exhaust manifold.
I don't know if my Facet is a diaphragm type or not. It is the small silver cube type, if that tells you anything. I have seen that David DuBois (sp?) cautions against a filter before an SU pump. The pump I plan to swap in is an electronic (no points) SU that I pulled off of a parts car I used to have.
|C R Huff|
As far as i can remember the Facet cubes don't have any moving parts, and, an inlet filter is essential.
A petrol/air mixture doesn't need pre-heating, unless outside temps are freezing, it's plenty volatile enough and the hotter it is the less you get in the combustion chamber, too hot and it becomes a vapour which, I think but am not sure, becomes a problem. Engine tuners and racers jump through hoops to cool the charge. Nothing to lose by cutting off or by-passing, the hot water supply to the inlet manifold and insulating the float chamber(s) from the exhaust.
|"If the filter plugs then the diaphragm can't return to neutral and things burn out."|
'Tother way round. If the diaphragm can't suck fuel into the pump chamber when pulled back by the action of the solenoid, i.e. away from it's 'at rest' position, that is when the solenoid can burn out. It is only when the diaphragm approaches the fully pulled back position that the points open to release it, and push fuel up to the carbs.
The tank should have a filter sock on the pickup, and the SU pump has a filter on the inlet anyway as shown in the attached.
|Charley the Facet cube is constantly on electronic type pump. Generally considered reliable if noisy (this can be fixed with different mounting schemes.|
It does require an inlet filter (check comment in Moss catalog or Facet online). Facet and Moss sell these or you can find something at NAPA or other parts stores.
As Dave DuBois points out even the Facet pumps fail eventually. Possibly sediment has damaged the pump internals. I have no idea how these things work and the Facet site isn't any help. But Dave DuBois says they have a steel shuttle or piston. This can wear out or corrode resulting in little or no flow. Usually they do not fail electronically.
Since the fuel flow seems suspect and the pump may be worn, you might consider replacing the Facet with another. They aren't expensive.
Be sure you have the correct pressure for the Weber. Facet has multiple models with differing flows and pressures. Make sure the outlet is NOT below the inlet. Horizontal is okay but outlet above inlet is best.
Hopefully you and the contributors can solve the problem and you can enjoy your classic ride.
The Facet I have is the low pressure version, but I don't recall the exact value. It does not have much use on it. I bought it in California when I was having trouble with the SU in my 68 GT, and I changed them out in Oklahoma on my way back to Kentucky. But, in so doing, I found the problem was a loose ground at the license plate mount plate.
Therefore, I switched back to the SU after my return, and subsequently put the Facet on the 77 roadster. The pump probably has fewer than 10,000 miles on it. It does not have a filter between the tank and pump, and I would have to look to check the orientation between the inlet and outlet.
There is not much new to report since I have not really found time to work on the problem. I did take it on another over the road trip of about 200 miles to northern Kentucky and back to pick up some equipment with which to fight the Emerald Ash Borers in the woods where I live. I had no problems on that trip, but the temps were down a bit at about 86 F (30 C).
It seems that the problem is directly related to temperatures getting up around 90 F or more, and I'm glad to say we have not had those temps for a while. Initially when trying to sort this problem, I definitely had no spark on the two occasions that I checked it, but there is also evidence that I may have a fuel problem as well. When I am not depending upon the B as a daily driver, maybe I can assess the condition of SU carbs that I have.
|C R Huff|
|Charlie... My 79 B gets fussy over outdoor temps over 90 + degrees when it comes time to idle at a light or slow traffic. Drove the car to work this week when the AM temps were in the 70's. The car ran great. The trip home, the temps were in the 90's. The car didn,t enjoy that|
|Really the only thing you can do now is to wait until it happens on a hot day, preferably when you have the time and are in a convenient position to fiddle.|
You need to check if you have spark or not. How are you checking for spark? Some clip-on 12v timing lights won't function when cranking as their electronics needs more than the 10v that is typically available. Either use the neon in-line type, or power the light from another 12v source.
If you find the spark is missing then you need to do voltmeter tests on either side of the coil while cranking. This is much easier with a points distributor than on an electronic system.
If you are powering a 12v coil direct off a 12v source i.e. the white at the fusebox, then on the coil +ve you should see about 12v with the ignition on, and about 10v when cranking.
On the coil -ve you will probably see 0v or very close to it with the ignition on, as usually the engine stops in one of two positions with the points closed. If the points are open you should see the same as on the +ve.
When cranking you should see an average of about half battery voltage, i.e. about 4-5v, depending on what type of points distributor you are using and the dwell.
You need to do these tests when the engine would normally start, but is prevented from doing so somehow, so you can verify your test methods and know what to expect when it won't start.
I'll leave fuel for another day.
Well, misery loves company.
The first time I checked for spark (the first time the car died) I removed the HT lead from #1 plug, and shoved a piece of primary wire into it and held the other end near the t-stat housing. The second time I did the same thing except I used a Phillips screwdriver into the plug cap.
I am currently powering the +ve coil with a temporary line I have wired directly from the battery to the coil through a switch. I have not checked for the voltage at the coil +ve while cranking. I don't think I ever checked voltage at the -ve coil lug.
Currently I have the higher ohm coil fed with full battery voltage, and the 25d distributor with points.
As you say, it seems that it will take another hot day before I can catch it in the act again.
Thank you all for the help,
|C R Huff|
|Charlie... I can't help but feel a major part of you car issue is the 90+ degree weather. If it runs well at speed but not in traffic in the hot weather, I wonder if it is not fuel vaporization due to extreme heat.|
Another item could be vacuum leak. I discovered the 4 manifold nuts to the cylinder head had loosened up over time and the 4 nuts that hold the Weber in place were not very tight either. Major improvement once tightened.
Also, my car does not have a heat shield. I'm sure that
Doesn't help the problem either.
|" I wonder if it is not fuel vaporization due to extreme heat."|
90F is not that hot, we get more than that in the UK on occasion.
I have never been able to imagine vapourisation, about the only time it could is with HS carbs in the jet pipe where the heat shield is defective.
If any vapour develops in the rest of the lines between pump and carbs then as soon as the fuel level drops and the float valves open, pump pressure will push fuel and vapour through until such time as the floats rise to close the valves.
It can happen on the tank side of the pump, but that is why the pump is fitted at the rear of an MGB and not in the engine compartment as earlier.
Finally these cars have run in desert states with temperatures far higher than 90F for 50 years without there being a general problem.
The first thing to do is establish whether there is a spark or not when the problem happens. Only if there is a good spark at all plugs at the right time is it then worth looking at fuel.
I should check all those nuts. I put a new manifold gasket on it a few years ago, and I don't think I have ever gone back to check it.
I did check for spark twice, and found none either time. Once was with the electronic ignition low ohm coil and once was with the points type hi ohm coil. But, the system wiring has been changed since that time, and I have not been able to check spark during failure since those changes.
I suspect there is some fuel issue just because the car has always been a bit stubborn to start when hot after a short shut down. I have always suspected some Weber issue is responsible for this. Maybe heat soaking causes fuel expansion dribbling into the intake manifold, since a hot start seems to require a few seconds of running to smooth out.
|C R Huff|
|I agree, hot starting difficulties if you have a spark are more likely to be due to fuel expansion overflowing into the inlet manifold. However that wouldn't happen when running - unless the heat shield isn't adequate for the Weber.|
My experience with a weber with a facet pump...and Ive seen it multiple times, is that the lowest rated facet pump is 5 lbs. The stock mg pump is usually in the 2 lbs pound range. Many times Ive seen the facet pump flood out a weber carb as the needle and seat cant hold the higher pressure back. I usually install an in line fuel regulator to solve it.
On the cars I did this on the owners always told me there milage improved. Maybe this explains the dribble that keeps the car from starting right up?
There isn't a heat shield on this car (unless you count the fiber separator between the carb and manifold. But, I don't recall seeing a heat shield on other Weber installations.
It is possible that it is not a Facet, and is only a Facet look-alike. It came from a NAPA store. I know it had a low pressure rating, but, I don't remember the numbers. I think they had a higher pressure one that was rated at something like 5 - 7 psi, and I bought the lower one.
|C R Huff|
|I don't know a lot about DGV carbs but I've never seen a photo of one with a heat shield. Most downdraft carbs don't have one. You could fabricate one I suppose.|
Your symptoms don't seem to be flooding but are heat related.
With points ignition there isn't much that is temperature affected. Capacitor could be but they are not electrolytic and usually work or don't. You could replace it but be sure to get a better one. The coil normally should work or not work. So, I am at a loss.
I still suspect the fuel pump. Normally the Facet pumps don't fail electrically from what I read. But it could happen. It definitely needs an inlet filter and the orientation checked.
|If your cat is still below the carb, even further away it could have an effect when you stop. You asked about cat damage, if it has become choked to any degree I can imagine that it would get hotter than it should. When it won't hot start lift the bonnet and apply wet cloths to the bottom and sides of the carb, and see if that makes any difference.|
Can you look through a Weber and see if any fuel coming out of the jets? In any event if you disconnect the fuel pump before trying a hot start you would eliminate the effects of excess pressure overwhelming the float valve.
And if you disconnected it shortly before switching off it would eliminate flooding from expansion as well.
That's after checking the spark to make sure that it is still there.
Heres the napa one I usually see...Here are the specs.
Part Number: BK 6102001
Product Line: Balkamp
Check Valve : Yes
Electric Or Mechanical : Electric
Fuel Pump Fitting Size[s] : 1/8"-27 NPTF Int.
Fuel Pump Fitting Type[s] : NPTF
Fuel Pump Gallons Per Hour : 30 gph
Fuel Pump Pressure Rating : 3 - 4.5 psi
Gas / Diesel / Both : Gas & Diesel
Installation Kit : No
Manufacturer : Facet
Manufacturer Part Number : 40288
Minimum Dry Lift : 24"
Operating Voltage : 12 Volt DC
Product Application : Industrial, Ag, & Marine
Shut Off Valve : No
Contents : Fuel Pump, Instructions
3 TO 4.5 PSI is to much....It needs a regulator.
Thats been my experience.
|Re Catalytic converters. I've lost 4 over the years. Three of these were on a Ford throttle body fuel injection V-6. In each of the Ford cases the car totally lost power due to converter plugging (the substrate basically fused). The car wouldn't run above idle. It happened suddenly in all three cases.|
The last one was a Toyota V-6. "Check Engine" light diagnostics revealed a non active converter (poisoned or inactive catalyst). In this case the car ran fine.
So, except for heat thrown off by the converter it doesn't sound like any of your problems are related to the catalytic converter.
I ran the Crane/Allison system for years, even replacing on or two of them. They do get hot and stop working. For years on extremely hot days I might be driving along fine and, randomly, the car would just quit like you turned the key off. Wait five minutes and fine again.
The last time the system died I replaced it with a Piranha system which fits completely inside the distributor. You mentioned having a 25D distributor (what I have) so this should work fine.
|Richard Smith 1|
|Okay Guys, here are some responses to your suggestions, and the latest, but not conclusive, updates.|
Gary, I did tighten the manifold and carb base nuts as you suggested, but I just did it and haven't driven it since.
I agree that the hot start issue is likely to be fuel expansion pooling into the manifold. I have checked with the back window unzipped, and I do get black smoke on a hot start. It seems it would be difficult to check for overflow by looking down the carb since having the hood open would make it cooler, and opening the throttle to look into the carb and at the bottom of the intake manifold would activate the accelerator pump in the carb. I suppose I could try opening the hood as soon as I park and see if it makes a difference. Disconnecting the fuel pump may have some promise for diagnosing the problem. The cat is not below the carb anymore. It is below the car just after the pipe levels out and heads toward the rear. I have tried wet towels to the carb when it wouldn't start, but I don't know if that "fixed" it or if it was just the time involved for it to cool.
I am pretty sure my pump is not 5 psi, but it might be that 3.0 to 4.5 you mentioned. It has been too long since I got it for me to remember, but I did know I needed low pressure when I got it. I don't have a pressure gauge that would be any use down in those ranges. I have not gotten around to putting the SU electronic on it yet. I do get poor fuel mileage at around 22 - 23 US. However, the plugs are not black.
I don't think I have ever seen a heat shield on a downdraft Weber either. As I mentioned to Steven, I haven't done any checking on the fuel pump yet. At some point I do need to check the entire exhaust system because it has a rattle, and I think the rattle may be internal.
Generally when mine dies, it fades out rather than acting like it was shut off. Also, I had the same problem with the Allison paired with a 45D distributor, and with the 25D equipped with points.
So, here are a few more observations. While working on it, I noticed that the paint had been cooked off of the steering shaft where it passes by the exhaust header. I should say that the paint is an industrial enamel that I put on it, not the factory paint. Also, the shaft is quite close to the header. I am pretty sure this is a new development, or I would have noticed it, and if it were not new it seems it would have have happened on the trip to Utah and back.
I decided that the burnt paint might mean the timing was too retarded. I had set the timing at about 32 all in with the vac adv disconnected. But, when checking this I did not mark my pulley and do not have a dial back timing light. So, I was just estimating another 12 degrees past the 20 mark on the timing cover.
When checking the operation of the vac adv with a hand vac pump, I did notice the idle speed up a couple hundred or so as I advance the timing with the pump. I seem to recall that this can be quite normal even when the timing is correct, but I have also heard that the Weber likes a lot of advance, at least at low RPM. It was while doing this that I noticed the burnt paint.
Before my next drive, I cranked the advance wheel on the 25D while it was running to add a couple hundred RPM to the idle. This certainly helped with the dead spot that the Weber is known for at tip in of the throttle, but I don't know if it was because of the timing change or because of the idle now up to about 1200. It was while doing these checks that I saw the paint burnt off of the steering shaft.
Either way, the throttle response is better, and I did a lot of around town driving the other day when the temps were about 90 - 93 F, and I had no problems other that a slightly stubborn hot start, which the car has always had.
I apologize for not being more aggressive with diagnosis and repair, but you may remember from my July 20th post, I am using the MG as the daily driver so I don't want to mess with it too much. I got the Volvo up on ramps and got the oil pan off, but I haven't made the trip to the junkyard yet for a replacement.
|C R Huff|
|"Generally when mine dies, it fades out rather than acting like it was shut off. "|
That does sound more like fuel than spark.
"When checking the operation of the vac adv with a hand vac pump, I did notice the idle speed up a couple hundred or so as I advance the timing with the pump."
That is normal - unless the timing is over-advanced to begin with.
Thanks for confirming that idle speed-up is normal when timing is advanced, even when timing is correct. In my case, based on improved throttle response, I think I had the timing retarded. But, the problems it caused were introduced by me while trying to cure the initial problems.
On the dying slowly sounding more like fuel, that may usually be true, but I have seen failing coils mimic running out of fuel.
|C R Huff|
|"but I have seen failing coils mimic running out of fuel."|
Possible, but that would also be visible on the tach showing zero when the engine has lost all power but the momentum of the car is still spinning the engine.
The times I remembered to look, the tach did not drop. So, that may rule out the coil. Also, I am still using the 12V coil from the 68 GT, and I had the failure with both coils and both ignition systems. At the moment, the car is running pretty well, and I will not likely do much to it until I get the new Volvo oil pan and get it installed.
|C R Huff|
|I've had two of the Allison/Crane ignitions on a 68 GT I drive. The first, an Allison, died in pretty much the fashion your describe, CR, and was mounted inside the engine compartment, which seems to be pretty typical.|
However, the guts of the Allison and the Crane (essentially the same) were/are manufactured by "potting" them with a resin compound with only some important transistor cooling fins exposed to the air. During the life of the unit set inside the engine compartment, all the rest of the components (resistors, capacitors, etc) do not get much cooling because of that potting and that situation, typically, for most electronics, tends to shorten their life.
Having dealt with similar installations of other electronic components in environments similar to or worse than an MGB engine compartment, I used that experience to reason that perhaps I might get a longer life if I mounted the new (and almost identical)Crane unit replacement somewhere other than the engine compartment. I chose to mount it under the dash beneath the Abingdon Pillow inside the cockpit where temps are usually cooler.
The upshot has been that in 26 yrs of driving this car, the Allison which was about 7 yrs old when it died in the engine compartment, has been surpassed by the Crane unit which I still use today.
Of course, your luck may vary, but it would not surprise me if your current unit's problems come from having spent its life in the engine compartment. Dried out capacitors/condensers, accelerated aging, etc. It probably won't do you any good to move your current unit inside now, but if you do get a replacement with a "black box" like the Crane's, you might consider placing it outside of the engine compartment. FWIW
Mounting the "black box" under the dash does sound like a good idea. I don't know how old mine is since it was on the car when I got it. Because it originally had the Opus system, it likely has been with the car for a long time.
I don't know if you saw it, since this post is so long, but I have had the car fail with the Allison, AND fail with the points distributor. I expect I have multiple problems acting up, but I haven't tracked them all down. It is doing okay at the moment, but I would not claim that I have fixed it.
|C R Huff|
|What was the out come Charley?|
Unfortunately I never really arrived at a positive conclusion. The last thing I did to it was advance the timing, and I have had no trouble since. However, we have not had the mid-90s F temps to contend with. Also, I had the problem with both ignition systems and with several timing settings. Therefore, I could not come to a conclusion. I do suspect that there were several problems combined that made figuring it out difficult.
|C R Huff|
|Well.....thats a low point in my day....I was hoping for at least a good story! Thanks for letting me know anyway! :-)|
|Charlie: Your car and mine seemed to have had the same problem. After the correction of vacuum leaks and going over the recent tune up, it runs very well now. Also, since the temps and humidity are not in the 90's now has made a huge difference.|
Went to an MG event this past weekend (135 miles away) with the temps in the 70's and 80's and much lower humidity, the temp gauge stayed to the left of center and the car ran great.
Discussed this issue with another MGB owner at the event who had a similar experience. His comment was, too much heat build up under the hood. He solved it by wrapping the exhaust manifold.
I have yet to wrap my exhaust manifold, after I do that, I will be paying close attention to performance.
I did notice though, when the temp gauge climbed past the half mark, the car did not run nor idle well at all.
This thread was discussed between 19/07/2015 and 06/10/2015
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