Welcome to our resource for MG Car Information.
MG MGB Technical - Damn MGB won't run
|1972 MGB, HIF carbs... rebuilt the engine, had the carbs rebuilt, replaced the vac advance... it won't run. Never has.|
1. The car is properly static timed at 10 deg (middle notch on the timing chain cover). It is not 180 degrees out because I took the valve cover off and looked... cylinder one both valves are closed (up) and the piston is up (you can almost see it in the spark plug hole)
2. We have a spark, otherwise the timing light wouldn't flash.
3. The car has gas at the carbs. With the ignition on, gas will pour out of the gas line if I pull it off the carb.
So... um... aside from junk the piece of crap because I'm so mad at it, what should I do?
|Hi, John - I understand your frustration, but with as much as you've done at one time, it's entirely possible (and completely human) that some item has been overlooked. Here's a few things I'd try to verify myself:|
Back up the engine rotation about thirty degrees with a wrench on the alternator pulley, and look at the valves on #1 again. Are they still both closed? Stick your thumb over the #1 plug hole, and rotate the engine back to TDC. Are you definitely on the compression stroke, so that it blows your thumb away from the spark plug hole?
Pop the distributor cap and verify that the rotor is pointing at the #1 plug wire. I know you set timing statically, and you're dynamically seeing a spark at the #1 plug, but static timing only says that the points are opening for "some" lobe, and not necessarily the correct one. Then, the wiring pattern is 1 - 3 - 4 - 2 counterclockwise at the distributor. Many people (myself included) have gotten this wrong in the heat of the battle.
Finally, since you've got to have fuel to go with your compression and spark, I've always sprayed a bit of ether starting fluid into the carb throat to eliminate the fuel issue. Fuel into the carbs may not mean a rich enough mixture to fire, while the ether pretty much guarantees it.
If none of this produces any results, then I'm just as lost as a goose - Best of luck - - Alec
All holes plugged in the intake manifold?
Have you tried a shot of starting fluid down the carbs just to check?
|Have you check each plug for spark? Normally the timing light is used only on lead one. Physically check for spark at all plugs by holding against the block (with insulated pliers of course). |
Does the spark look strong (difficult to tell I know)! Have you doubled check the points gap, or if you have EI are you absolutely sure it is working?
Try rotating the dizzy just a little bit and see if you can get signs of life. Once the car is actually running you'll be resetting the timing anyway. I couldn't get mine started with the book timing initially. Have you set the mixture at the carbs to an average point? On HS4 carbs this is 12 flats down from the bridge, don't know about the HIF.
You have checked fuel is getting to the carbs. After a failed attempt at starting you should be able to remove a plug and smell petrol on it, indicating that fuel is getting into the cylinders.
Have you done a compression test?
If you have fuel in the cylinders and have compression than that leaves the spark. If it is at the right time then it must be weak or not happening at all. If after all the check above and from other people had failed I would replace all parts of the igintion system. So plugs, leads, dizzy cap, rotor arm, condensor and perhaps even the coil. The ideal would be to borrow these components off of another car that runs as then you know they are good.
Have fun and if it's a new engine don't forget to bed the camshaft in when you do at last get it running!
|I D Cameron|
|WRT the timing, with the engine not running I prefer to see the spark at the plug by turning the engine with the fan belt (helps to have all plugs out) whilst touching the spark plug tip of No1 cylinder (radiator end - I know you know that but it rules another thing out!) to the block. Hold the plug in a rag or wear gloves. This way you can see the valves are closed at the moment of spark, check the timing, and by moving the plug away from the head get an idea of the strength of the spark. Finally, I'd make sure the plugs are new and correctly gapped are to spec.|
Fuel, compression, spark, it's one of the three. How old is the fuel?
|It could also be air, a friend of mine rebuilt his carbs over the winter a few years back, then fitted them to the engine, he stuck duct tape over the intake ports. A week or so later the air cleaners seemed to be getting in his way, so, he fitted them to the carbs. Come spring he had forgotten the duct tape...|
|I agree with some above comments that you have to determine if you're actually getting fire at the spark plugs. You say you have spark, but is it at the points or at the plugs. I can recall more than once dropping the spring loaded electrode that carries the coil fire to the top of the rotor and didn't even know it had dropped out when opening the cap. Check the firing order by following the spark plug leads to the distributor (should be 1-3-4-2). A good static setting for the carburetors is "11 flats". That is, turn the adjustment nut on the bottom of the SUs in until it snugs against the springs and back off 11 flats. I think that's 1 and 3/4 turns. You should be close enough at that setting to get it to start and tune later.|
I certainly understand your frustration and offer this piece of advice because I've been there. When it gets to this point it's hard to think clearly. Take some time off from the car and then go back when the frustration level abates a little. Don't sell it off at a bargain price and regret it a few years down the road.
|Sell it to me while you are mad!|
Calm down, check the basics and give yourself time.
|Not only 1 3 4 2 for firing order, but with *anti*-clockwise on the cap.|
Double check the firing position, ignoring the TDC mark on the pulley. Both valve should be closed more or less all the way up on the compression stroke and down on the expansion stroke, not just at the top.
Starting point for the carb jets is turn them up until the tops are flush with the bridge, then down two full turns, either HS nuts or HIF screws. That equates to 12 flats, any higher (or less further down) than that will result in a weak mixture that may not fire even with the choke out.
And is the choke operative? Easy to check on HSs, less so on HIFs.
Check each plug lead is causing the timing light (inductive pickup type) to flash, and check both 1 and 4 are flashing just before TDC. If some do and some don't, or it is irregular, either cap or rotor breaking down is indicated.
After cranking a bit take out the plugs. If they are wet it is flooded. If they are dry and there is no fuel smell there is no fuel getting through, check there is fuel in the float chambers (again easy on HSs, easier said than done on HIFs). There should be a strong fuel smell without being wet.
You *must* be missing something basic, remember first principles and forget esoteric stuff.
|Paul Hunt 2|
|I took another crack at this MGB again yesterday. I have verified the firing order, the spark, rotation, etc. One thing that is a little different is, my #1 cylinder on the distributor is closest vac advance, not towards the front of the car. I am going to assume this means I put the distributor dog in a few grooves out of standard positioning. As long as spark plug #1 fires when the crank pulley is at 10 deg I know I have that close enough to rule out ignition problems.|
So my problem I think is gas, so I took the suggestion to spray starting fluid in the carbs while cranking the engine. This did not work. I would atleast expect a clunky shuddering rough partial running, but I didn't even get that. So am I wrong and need to go back to the ignition??
I am 100% certain that I got the cam and the crank lined up when I rebuilt the engine. The timing chain gears only fit on the cam and the crank one way... they are keyed, you can't screw that up. On each of the gears there is a dot, and these two dots should be right next to each other, correct?
|Hi, John - and thanks for getting back with your results. Your description of the timing gears certainly sounds right, but the distributor wiring being so far out of place bothers me. In a standard setup, the distributor rotor will be pointing pretty close to the #1 sparkplug when the piston is in full compression.|
That's what I keep coming back to. As you know, there are two instances of TDC in a four-stroke engine. One is between the exhaust and intake strokes, and the other one is between the compression and power strokes. This is when the sparkplug should fire, and I still wonder if you're firing at the wrong time.
As Paul noted above, both valves on a single piston will be closed, and both rockers will have a little "wiggle" in them for almost all of the compression and power strokes. So, if you're not able to feel the air being pushed out of the #1 sparkplug hole, you can "rock" the engine 30 or 40 degrees on either side of TDC and check that both rockers are still loose to confirm you're at the correct firing point. Then, take it back to TDC (you can verify TDC with a broom straw in the sparkplug hole - if it breaks off, no damage is done ) and once more look at the distributor rotor and how it relates to the high tension leads.
I think you've got a great attitude about this problem. And, I guarantee the solution will be very obvious, once you find it (they usually are). We had one fellow on the midget board who troubleshot his coil wiring for two hours, just to find that he had not put the distributor rotor back on the shaft before he installed the cap. We've ALL done such things, just many don't have the honesty to admit it <grin>.
Best of luck, and please let us know what you find - - Alec
|John, likewise I think if the ignition was operating correctly, you should get some attempt to start --- at least a couple of cylinders igniting once in a while. My hunch is it's the ignition. It's been many years ago, but I recall not installing the distributor dog correctly after complete teardown, hot tank and total rebuild. However I can't remember how I discovered it and what to suggest. Some others here have more recent knowledge. It seems that when you install the gear, it has to be in a certain position if your engine is lined up at TDC. Glanced in my Haynes manual and haven't found it yet. I have an '80 and the position of the wires from the cap relative to the distributor are as follows. The vacuum advance is almost perfectly vertically aligned and to the left or rear of the engine when looking in from the passenger side. If you consider an X shape when looking at it from the same position, #1 is on the lower left of the of the X, #2 is at the upper left position, #3 is at the lower right and #4 is the upper right position. I think your vintage (1972) aligns the same. Just found it John, it's on page 50 of my Haynes manual and Figure 1.26. The slot in the distributor drive should look like this / (more or less). If it's like this / you're ninety degrees out. The entry position when inserting it will be 90 degrees to clockwise from the final position because of the gear.|
|My above reply may confuse you about the position of the distributor drive slot at the last. Apparently, something about the software on this site doesn't like reverse slashes. The correct position is like a forward slash on the keyboard and the incorrect one is like a reverse slash. Sorry about an illustration that didn't work so well.|
|There is a picture and comments on the dzzy drive dog on Chris Betson's website at: http://www.octarine-services.co.uk/faq.htm|
It is as Rick says, but a picture is worth a thousand words and all that!!
|I D Cameron|
|Me thinks your dizzy is 180º out of position.|
Bear with me as I explain...
On an MGB, the cam & crank are first installed with
the dots on the cam and crank chain sprockets
lined up directly opposite to each other, and both
IN & EX valves on the #1 cylinder are closed.
This is how it's done on most OHV engines.
...BUT...on an MGB engine...
...if you stopped here, and then installed the
distributor drive spindle...then your distributor
is timed 180º out of position!
Here's what you're supposed to do:
After you've correctly installed the crank and cam
(dots directly opposite to each other) - THEN turn
the crank ONE MORE 360º ROTATION - so that
the IN and EX valves on #4 cylinder are "rocking"
(not fully closed, but not fully open, either).
Since the cam spins at 1/2 times the crank,
this equals = 180º of cam rotation.
THIS is when you install the distributor drive
spindle - as per the official shop manual.
If you happen to have the timing cover off of
the engine while doing this, you will see that
the dots on BOTH the crank and cam spockets
are now positioned along a straight line at
around 2 o'clock - NOT opposite to each other.
THAT is the correct cam position for installing
the distributor drive spindle.
|An easy way to check if the dizzy is 180 degrees out is to swap two of the dizzy cap leads. I think it is 1 and 3. Can someone confirm this? The car should then run if that was the problem. |
|I D Cameron|
|Interesting, Daniel - as I understand your explainantion, this means that when the dots on the timing gears are nearest each other at TDC, then the #1 cylinder is actually between the exhaust and intake strokes, in the "overlap" area of the cam, and you've got to turn the crank 360 degrees to get to the firing point. I didn't know that, but it sounds perfectly reasonable.|
The good news is that the worst problem that John can have is a misaligned distributor drive, which has been the guess of most of us here. That should make him a little less angry about his car.
Iain, I believe that you would have to swap TWO pairs of wires to correct a 180 degrees out distributor. Since the firing order is 1 - 3 - 4 - 2, it looks to me as if 1 and 4 would have to be reversed, as well as the 3 and 2 wires. Swapping ONLY the 1 and 4 would correct a distributor that was 180 out, AND that had been wired in a clockwise direction on the cap, too.
Above decision reached with a crayon and a piece of cardboard, to take it as you will <grin> - - Alec
|John. Might I recommend that you visit my website, www.custompistols.com/ and go to the MG section, then the articles section. There is a tech article on ignition system timing which might be of use to you. There is also an article on trouble shooting the ignition system and one on performing a compression check. None of these may correct your problem, but it will give us a common basis upon which to do our troubleshooting. As Paul H observed, "You *must* be missing something basic..".|
My belief, from what you have posted, is that this is the case. My theory is that you have done the static timing incorrectly and that is the root cause of the non-firing problem.
Static timing is, at is best, an inexact process. I statically timed an engine last week prior to starting it for the first time in three years. Set it to 10 deg BTDC. When fired up, the timing light showed 0 deg BTDC at 900 rpm. The engine started, but I had to set the actual timing using a strobe light to get it correct.
One of the problems with static timing is that all it indicates is when the points open. The engine must be on the compression stroke if the system is to work properly. (All of this described in the tech article.)Then, it is possible to "time" the engine either correctly or about 60 degrees off. It depends on how you rotate the dizzy when doing the static timing procedure.
All of this is covered in the tech article. Please read it, do the checks, and let us know what happens. I think you statically timed the dizzy 60 deg off which would explain your problem. But, cannot be certain until I know you followed the procedure I have outlined.
Good Luck. Been there myself a time or two.
|John, just for the fun of it, switch your plug wires 180 degrees. It only takes a few seconds. Why not try it- you've tried almost everything else! |
By the way, I'm still batting 1000 - I have NEVER installed a distributor that wasn't 180 out after a rebuild! No I'm not proud of it. It just happens.
|By the way John, If you point your timing light at the marks while someone is turning over the engine, you will se whether the timing is set properly or not, with the exception of it being 180 degrees out. If its 180 out, it will appear "correct".|
|What direction is the rotor pointing when #1 cylinder is at TDC of the compression stroke? The spark plug wire should go from the the distributor cap terminal nearest the rotor to # 1 spark plug. It doesn't matter how the dizzy is installed if you match the cylinders to the rotor. |
|All very good advice. I have only worried about dizazy orientation when installed "way" off as the only thing being bothered is teh ability to see the points during tune-ups and the vaccum cannister bumping something and preventing proper timing. Then I pull the gear and reposition.|
My suggestion is to sit back, relax, and do a tune-up.
Yup, a tune-up. Per the manual.
"Why?" you may ask.
The tune-up proceedure in the manual is:
1) comprehensive, that is it covers all the systems.
2) methodical, that is it asks you to do the same things in the same order every time.
and 3) written down, you can mark with apencil when you've done a step so you miss nothing.
This is a tried and true technique for me to eliminate "silly" problems when I can't get a car to run right, regardless of marque.
Two other observations:
First - I always set the engine at TDC before installing the plug wires for the first time so I can start with the most appropriate wire in the most appropriate hole in the cap and go from there. The static timing can fool you! Make certain you are looking at the light as it goes "on" rather tahn as it goes "off." What do I mean? It is very easy to set the static timing at the spot where the points are closing! This gives you an extremely retarded ignition (gee how would I know this?)
Second - The "11 flats" would, of course, be 11/6 rotations or one flat shy of two full turns. and absolutely meaningless on an HIF!
Your manual should give the proper "first time" carb set-up but I think it is "adjust jet lean (up) until the top of the jet is flush with the carb bridge then adjust rich (down) two turns."
G'luck, John... I think the static timing and wire order are your bugbears.
|I don't think this one has been covered yet.|
Some MGBs have the timing pointer near the top, at say 10 or 11 o'clock, others have the timing pointer at the bottom. The notches on the pulley have to be different depending on which you have.
Perhaps you have a pulley that goes with a bottom pointer but have a timing cover with a top pointer.
|There could also be a delaminating crank pulley. Which is why the advice to go back to first principles is best, one step of which is to determine TDC on the compression stroke by looking at the piston and valves, and only then look at the pointer. As long as you get the lead the rotor is now pointing to on No. 1, and the others on 3 4 2 counting anti-clockwise, the plug leads are correct. Even if that means the rotor is not in its recommended position it is only a difference in appearance, not function.|
|Paul Hunt 2|
|I wonder if the condensor may be faulty? If it is not functioning properly, the distributor will deliver a mild spark, maybe enough to trigger the timing light, but not enough to fire the air/fuel mixture. To check, take off the dizzy cap and, with the ignition on, open the points with a screw driver. There should be a strong spark at the point contacts. If just a weak spark, change the condensor.|
|Or connect the coil lead to a plug laying on the block, and flick the points open while withdrawing the HT lead from the coil. You should be able to get a spark to jump at least 1/4", and possibly 1/2" or more. A failed condenser will result in a spark that will barely jump a plug gap, as well as lots of sparking at the points.|
|Paul Hunt 2|
|You say you are getting spark but not where...Pull all the plugs and ground one against the block to check for sparking when cranking..(keep this plug away from the open spark plug holes to avoid a surprise) or maybe if you have a particularry non-mechanically inclined associate, have him, (or her) hold a finger on the spark plug while cranking.|
IF no spark at the plug, try a new ROTOR in the distributor. Yes it sounds too simple, but you can burn an invisible hole through the top of the rotor and send the spark to ground down the dizzy shaft and never see it.
Ask me how I know this and how long(in days) it took me to figure it out after I got the car towed back home.
It was a local auto mechanic who suggested this to me, said it's a common problem on some Chevys, so much so that GM had sent out a service bulletin on it.
It costs < $3.00 to find out.
|A clip-on timing light will diagnose rotor and cap problems - if it flashes on the coil lead but one of the plug leads it is usually the rotor that is faulty, if it flashes on some plug leads but not on others then it is usually the cap. Unless 'the other one' is known to be nearly new it is best to replace both.|
|Paul Hunt 2|
|I found the spark to be very weak. It was enough to trigger the timing light, but it was pale orange and wouldn't jump from the plug to the block. So I changed out that brand new condensor I had put in with the one out of my Magnette and it started right up, reved up to 2000 rpm, and died. Turned key again, same thing. I figured I had the idle turned up way too high. I turned it down, and took a phone call... came back and its dead again. I think its burning up condensors. The Magnette is in pieces so I can't swap it back and see if it works there. |
So, what kills condensors?
|Hi, John - The concensus for some time on the board has been that the main thing that causes condensors to fail is a batch of bad condensors. Consider this: the coil has two windings internal to it, a Low Tension (12 Volt) winding and a High Tension (20,000 Volt) one, but there are only three terminals on the coil (the SW that goes to the ignition SWitch, the CB that goes to the Contact Breaker, and the spark output that goes to the distributor rotor). Where - you might ask - does the lower side of the HT winding terminate?|
Is it attached to the coil case? NO! It actually is tied internally to the CB terminal of the coil. This means that, as the points open, and the coil fires, the high voltage spark is actually across the condensor for a small part of its duration. A weak condensor can easily fail under this scrutiny.
Now, that was a deliberately simplified view of what happens. The actual effect is much more complicated, involving "ringing" of the coil's oscillations, the reverse sparking that occurs through the ionized zone between the distributors microscopically opened points, and additional grounding effect back through the vehicles ignition wiring. It's all about back EMF.
One thing tha can strain your condensor (and all of the High Tension components) is spark plug gaps too large. Twenty Five tousandths is all Joe Lucas planned for on standard installations. Purely for trivia's sake, some coils from the 20's and 30's actually had a spark gap built into their outer case, so that if a spark plug wire fell off, the spark would have a safe place to discharge. Maybe they kept their engine compartments clean and dry, back then?
Sounds like you're very close. Please try a different brand of condensor. - - Alec
PS: Since the bottom of the HT winding goes to the CB terminal of an old, negative ground coil, can anyone explain why reversing the LT connections when switching from Positive Ground to Negative Ground is a good idea? I personally think a new, modern, negative ground coil is a better alternative.
|You might also look for a fractured wire or short that you could have disturbed in changing the condensor. The advance plate moving as you revved to 2000rpm may have caused the problem to reappear.|
|Steve has a good point - there should be a very flexible 'tinsel' wire between the moving points plate and the body of the distributor, check this is sound. Another possibility is the conductors fractured inside the insulation of the wire between the points and coil where it passes through the distributor body, this is being constantly flexed back and fore as well. However either of these are likely to stop sparking altogether rather than just making it weak.|
Unlikely you would 'burn up' a *good* condenser with any fault on the car, more likely to be a bad batch.
Switching the connections to the coil when changing polarity reverses the current flow - or rather *restores* the direction of current through the coil to what it was before and hence the direction of HT current flow. The 'correct' way is said to be slightly better. However 'wasted spark' systems seem to fire current one way through two of the plugs and the other way through the other two, and I have seen references to keeping the plugs with the cylinders, again something to do with the plugs firing less effectively if the current through them is reversed after some time spent firing 'the other way'. So with changing polarity perhaps it is more a case of not *changing* the direction of current through 'old' plugs, rather than one way being better even with new plugs.
There are only 12v and 6v coils in the Leyland Parts Catalogue, so I suspect the original SW/CB coils are identical to the 12v +ve/-ve coils except for the external markings.
|Paul Hunt 2|
|Re: HT polarity - http://hometown.aol.com/bgahc/01_jwha_coilpolaritysparkplugs.html:|
"Spark plugs are sensitive to polarity and they will not function as efficiently it the secondary coil voltage is of the wrong polarity.
"The reason for this sensitivity is as follows. The centre electrode has a higher temperature, normally, than the ground electrode so that more electrons will be released from the hotter metal surface of the centre electrode than from the ground electrode.
"If the centre electrode polarity is more negative than the ground electrode (which is connected to chassis) then the earth electrode will attract electrons from the centre electrode; but if the coil secondary voltage polarity is reversed, the voltage needed to cause electrons to pass from the earth to the centre electrode (i.e. to make the gap conductive) is increased."
However this seems to be contradicted by - http://en.wikipedia.org/wiki/Ignition_coil:
"In a 'waste spark' system two spark splugs share the same coil. In this system both spark plugs attached to the coil are fired simultaneously. While one cylinder is toward the end of its compression stroke, its 'piston pair' is nearing the end of its exhaust stroke, and both plugs are fired. Although the spark plugs are manufactured identically, if re-installed after extended use, they should go back into the cylinder they came from. This is because the electrons align themselves favorably with one direction of current flow. While one plug receives positive potential (voltage) at the wire side, the piston pair receives negative potential."
|Paul Hunt 2|
This thread was discussed between 24/03/2006 and 03/04/2006
MG MGB Technical index
This thread is from the archive. The Live MG MGB Technical BBS is active now.