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MG MGB Technical - Dstributor becomes fugitive

I heard this rumor about installing an electronc ignition in a B and that it was as easy as pie. I picked up a crane fireball igntion, and put it in. I started the car up and it roared to life, and sounded great; infact it purred as aposedto burbled. But the it clanged and shut off. I checked under the hood and the distributo cap had separated from the rest of the ditributor(clips to hold it closed still firmly in place), and the whole works had rotated about 90 degrees. I'm convinced what stopped it from turning further was the fact that is snagged on the oil pressure line.

I pulled the cap off and my optical trigger was still in good shape, but when I grabbed my rotor, I found that I coul now rotate it about 150 degrees clockwise, and back again.

Someone told me about little weights on the distributor, and that one may have fallen off... I'm not too sure, I haven't had a chance to take it apart yet...

any suggestions?... .the distrubutor is for a '67B motor

Thanks,
Dave
Dave

I am really confused? The cap came off, but the springs were still in place? Did the distributor cap come apart? Also the wieghts inside the dist are for vacuum advance, these only allow about 20 degrees movement. 150 degrees of movement sounds like something internal broke. I would take it apart and see what it looks like.
J Arthurs

The vacuum advance is controlled by manifold vacuum, the weights and springs control the centrifugal advance. Have you looked at the tab in the rotor that holds it in place on the dizzg cam? J's suggestion to pull the dizzy and examine it is correct. I use a Crane fireball ignition in both my B's and have found after installing the shutter the rotor will hit some aftermarket distributor caps around the high tension terminals. I use a Lucas cap have no problems with them. I saw a TR3 do about what you describe,
only it knocked the dizzy out of the clamp. A screw had dropped down in the advance weights and jammed it. Interesting, the car had a new pertronix in it, installed by a Triumph mechanic.
Clifton Gordon

hmmm.... I looked at the rotor, and the tab looked to be fine to me. One thing I noticed was the whole shaft deal that the rotor tab goes onto turns, it's just that the rotor gives me more leverage. I'll give it a second look though.

I'm not too versed on the whole distributor thing, but the first thing that ran through my mind was that the rotor had come up, hit one of the leads on the cap and that had caused it to A) twist in rotation with the rotor, and B) when it refused on the oil pressure line caused the cap to move the only way it could: up.

Cliifton, If you say you have this system on both your B's, then I have a question for you(in addition to this problem). The instructions mentioned that I was supposed to find a snap ring and a washer and O-ring, right below the rotor. All I found was a flathead screw. Did you have these bits?, and what was the procedure you followed when installing yours?

thanks a lot for your input guys. I'm usually pretty good with stuff, but I'm a bit in shock which is causing me to be a bit flabergasted. I'll take it all apart tomorrow and let you all know how it goes.

Thanks again,

Dave
Dave

Dave. Please disregard most of the above. It is, quite frankly, BS. The weights do not control the vacuum advance, nor is your vacuum advance connected to manifold vacuum on your model. You should have your vacuum advance connected to the rear carb. Now, to your problem. You need to remove the rotor and see if the distributor drive shaft has more than about 20 degrees of movement in it. The drive shaft has two parts, the shaft itself and the cam, mounted on a shaft, which is secured to the drive shaft by a screw on the top of the shaft. When you remove the rotor you can see the screw. The vacuum advance is not a player here. It moves the base plate in relation to the distributor cam when vacuum is applied. This could not cause your problem with rotor movement. The next thing to look at is the rotor itself and the inside of the distributor cap. If the rotor was not seated properly, it could be damaged and can, in turn, damage the distributor cap. Lastly, tug the cam shaft (distributor cam shaft, on which the rotor fits) up and down. If it moves up and down, it is loose on the distributor driveshaft. It may have moved upwards sufficiently to allow the centrifugal advance weights to fly free, which would cause a banging sound. If there is movement in the distributor cam shaft, you will need to remove the distributor and disassemble it to see exactly what has happened. The workshop manual and my website (http://www.custompistols.com/), click on MG articles, should be of help. Les
Les Bengtson

J. You are a new guy. Do not let my comment get you down. You made a mistake. We all make mistakes and our fellow contributors catch them. I posted a strong response to get Dave back on track. The Lucas 25D has been a research project of mine for several years. Please do not take offense at my characterization of your post as "BS". It was, but you show a certain potential which I would not want to discourage. You seem to be somewhat inexperienced (perhaps young or somewhat new to the game?) but seem to be observant and to not be afraid of sharing your experiences. Keep posting. We need you.

Ronald. "Got Yah". You did not read the post fully and notice the significance of the "67". Since I consider you, fully, my equal in this game, "Tag, Your It". Les
Les Bengtson

Dave: Didn't find anything but the screw. Les also Mentioned the screw. My comment about the
rotor was made because the shutter disc for the Crane system causes the rotor to be positioned
a little higher and with some distributor caps the rotor will hit the high tension contacts inside the
cap.For my distributors Lucas caps work ok. With some other caps you can feel and hear the rotor hitting the terminals in the cap.

I never said the vacuum advance was connected to the manifold, I said," Manifold vacuum controls the vacuum advance". I'm aware of the "sensitive" ported/manifold vacuum issue, I have one of each. I still
believe that the vacuum souce is the same in either case, but the throttle plate position on HS carbs
with the ported hookup does control when vacuum is applied to the advance.

How does Ronald fit in this thread? What did I miss?

Regards, Clifton



Clifton Gordon

Thanks a lot guys. I'm oging to take it all apart after I get back from work today. And now I have some comfort about what i'm supposed to find. As for the advance issue, yeah, I know the weights are for centrifugal advance, that only makes sense, and the vaccume advance on my car isn't hooked up. I have newer SU carbs (with the float bowls underneath) and there simply isn't a place for the vaccume attachment, so I've been running purely on centrifugal advance.

once again, Thanks, and I'll let ya know
Dave

Sorry Clifton. However, you are also one of may favorites and it is nice to get the occasional "got 'cha". Actually, the source of all vacuum is the downward movement of the pistons. Where you tap into this vacuum will have some decided effects on how much vacuum will be available and under what conditions. I was feeling very "froggy" last evening after my daughter received the news that she had maxed out the nation-wide standardized test to get eight hours of college credit for her high school European History course and I had just found out that, for the first time in twenty years, I have been drawn for elk this year. Much better tasting than beef.

Dave. It is slightly possible that the distributor has moved slightly upwards, but, generally, this will allow the distributor drive shaft to turn completely around. In other words, the connection between the distributor shaft and the drive dog comes adrift. I have seen this happen one time and it produced a distinct "crack" and the engine stopped running. I think I would start by pulling the distributor out and stripping the upper half to see what you find. Also a good time to check and see if the cross pin which holds to bottom piece to the distributor shaft is still there. As to the HIF carbs, the UK versions had a vacuum take off on the rear carb. These carbs are available, new, from Burlen in the UK configuration, but you have to order them from England. There are some threads in the archives on how to add a ported vacuum attachement to the rear HIF carb. Let us know what you find. Les
Les Bengtson

Les; No problem, congratulations to your daughter on her achievement, I'm sure you are
very proud of her. Congratulation on the drawing for elk. For my education is that a lottory
giving to the right to hunt and shoot an elk?

Just to keep some car content in this respone, I agree that vacuum does originate on the
downward movement of the piston.

Regards, Clifton
Clifton Gordon

Clifton. Many thanks. Yes, Arizona uses a lottery system and this is the first time I have been drawn in twenty years. I will be taking the rifle that a friend used to hunt in Iran back when the Shaw was still in power. After my friend's heart attack, he could no longer shoot it. Since this will probably be my last serious hunt, I will take it along for the trip. Les
Les Bengtson

well..... i took all the bits out of my distributor and what I found wasn't too pretty. Like I said earlier, a guy I know had mentioned the weights coming loose could have caused the cam to have a bit too much play. Well.. I found my centrifugal advance weights in a pile of scrap metal and bits. Included in this pile of bits was one of the posts that the weights hang on, and one of the posts the little springs hook to.

Lets just say it's a bit of a mess. But the good news is with the cam out of the way, the saft doesn't move without the motor going!

My question now is, will the distributor off my old engine (1975, 18V) swap right into my current 1967, 18G, engine?(it's a 75 MGB, but I spun a baring in the other engine so I took the one out of my parts car, a '67B) and to avoid some tricky work aligning the spiralled gears and stuff, can I swap the top part and leave the crank part of it in place?

thanks,
Dave...
Dave

Dave. It will physically interchange with no problems and you do not have to remove the distributor drive. However, I doubt the mechanical advance curve is correct for your car. The first step in researching this is to check out Paul Hunt's website, the address should be on one of his posts, and compare the two curves. If the 75 distributor is a little retarded, no problem. If, however, it is advanced then you could get engine damaging problems. I believe you will find this to be the case. Also, the 75 models, if I am remembering correctly, had the vacuum advance made inoperable with the base plate being pinned in place so it could not move. This would give you a straight mechanical advance only distributor, which I do not like on a road car myself. There are a couple of options left to you. Simplist is to contact Brit-Tek or other supplier and order a Lucas 45D "Euro Spec" dizzy suitable for use with your car. The second route is to pick up a used Lucas 25D, specification number 40897, the correct one for your car, and have it rebuilt. Bob Schaulin at gbmg@aol.com can supply the used distributor and John Twist at University Motors Ltd can rebuild it. This is the way to go if you are concerned about originality and can wait a while. If, however, you would rather be driving, call Brit-Tek. Les
Les Bengtson

Can John Twist supply the correct springs, Les, and if so where does *he* get them from!
Paul Hunt

Les,
No offense taken. I meant to say 'mechanical advance' instead of 'vacuum advance', early in the morning when I wrote it. Appart from that my post was right on track. As I said, the rotor should turn approx 20 degrees for advancement. If your getting 150 degrees, I assumed something internal had given way and it had. I always would recommend removal for the 25D's if there is a problem there easy to remove and inspect. Not really young I've had my current B for over a dozen years.


Dave,
If you click this link http://www.mgbmga.com/tech/mgb21.htm
you will see the differences in the two distributors.
J Arthurs

What sort of "engine damaging problems" am I looking at with the newer distributor, Les?.. and seeing as how i have a 67 motor in a 75 B, I'm not too concerned about originality. And seeing as how I haven't had the vaccume advance hooked up for 5 years, that isn't much of an issue to me either.

I'm not too sure I understand what difference the advance would make with the optical trigger in there. the disc that spins and tells the trigger when to fire is in a fixed position turning with the rotor, and the optical trigger is in a fixed position. Now if these items can't move in relation to eachother, then isn't the advance just left over from the breaker point system I took out?
Dave

Dave,
The points in the distributor only open and close contact which allows the coil to release it's charge creating a spark. They have no bearing on the timing in any way. The distributor has a mechanical advance as well as a vacuum advance. If you manually twist the rotor with the dist in car you will see if will move about 20 or so degrees and then spring back. As the RPM's increase centrifugal force advances the rotor and thus the timing. If it's set at 10 degrees static timing and has a 20 degree mechanical advance that gives a total of 30 degrees of advance. If you look at the link above you will see the later engines had more advance than the earlier one. You just want to make sure you don't have too much advance or the engine will ping under load and this can lead to piston damage. I don't know exactly the right combination maybe Les can give you a more definitive answer, but you may want to start with maybe 5 or so degrees of static timing and see how it runs. Basically the lower the static timing the worse the car will run at idle. The more static timing the better idle running, but the more total advance you will get. I think the wieghts inside the distributor limit the timing advance and I think they are actually labeled 18 degree, 24 dergees ect. Maybe you could get an earlier scrap distributor and switch weights? Just make sure the spings you use are tight or the distributor will advance too soon, and cause poor running. Don't know how correct this all is, but I hope it helps to clarify a little for you.
J Arthurs

Paul. Last time I spoke to John he informed me that they had a large collection of springs and a distributor test machine along with someone who specializes in tweaking the springs. Thus, his rebuild distributors are done by the "informed trial and error process" (my words, not John's). I am still trying to locate anyone who has the correct springs as a stock item. I am almost ready to pay to have some custom springs made, but am waiting to get up to Idaho to pick up the distributor test machine I bought from Bob M before doing so. That was supposed to be sometime next month, but I have to drive to Florida to help go through my mother's house instead. But, I will get up to Idaho and will move forward with the spring project. Les
Les Bengtson

J basically has it right. The points control when the spark happens--the closing of the points allows the coil to charge and the opening of the points induces the charge in the secondary windings that causes the spark. Basic timing is set by rotating the distributor until the points are opening at a particular point in relation to the rotors sending the spark through the distributor cap terminal to the number one spark plug. An an "electronic ignition"--an optical or magnetic spark trigger do is open and close the circuit, charging the coil when the circuit is made and discharging the coil when the circuit is broken--the exact same thing that points do. Now, the fuel/air mixture burns at a constant rate whether the engine is doing 600 rpms or 6,000 rpms. Thus, an engine that is timed perfectly at idle would give insufficent burn time at higher rpms and power would be lost. Hence, the mechanical or centrifugal advance system which allows the timing to "advance" or spark the system earlier to allow more efficient burning of the charge. Retarded timing, firing too late in the cycle, leads to loss of power and excessive heat build up. The mixture is still burning as the exhaust valve opens. Over advance, firing too early, leads to loss of power because the piston has to fight its way upwards when the mixture is truing to force it downwards. This leads to both over heating and piston damage. Hence, the engine/car manufacturer determines, through testing, the most correct "advance curve" and Lucas would build a distributor to that specification, although, normally, a little conservatively. Better to be a degree or two retarded through out the curve than being a degree or two advanced as no engine damage would result from this design. Now, what does this really mean? It means that, under rotational force, the weights tend to fly outward and move the distributor cam in relation to the points or triggering device. The weights are restrained by springs which affect how rapidly they are allowed to influence the points cam and therefore, governs the "rate of advance" or the "advance curve". To limit the total amount of advance, there is a cam arm or stop which is marked 10 deg or 18 deg or some other number. This is the total amount of centrifugal advance the cam stop will allow, registered in DISTRIBUTOR degrees. But, we measure our timing in CRANKSHAFT degrees, not distributor degrees. The crankshaft makes two complete revolutions for the distributor to make one complete revolution. Therefore, one degree of distributor advance equals two degrees of crankshaft advance. Thus, the ten degree cam stop equals a total mechanical advance of twenty crankshaft degrees. The later distributors, which commonly had 18 degree cam stops will provide 36 degrees of crankshaft advance, or significantly more than was desirable for the early engines. Add in the fact that high compression engines need less total advance that low compression engines and that most of the later US spec engines were low compression, and you see why you need a distributor designed for your early, high compression, engine rather than using a later distributor designed for low compression engines. Les
Les Bengtson

Ok, I get what you're all saying now. But just to get this one out in the air... If the vacuum advance gives me ten degrees of advance, plus the 20 from the mechanical, then with only the mechanical hooked up I'm only getting 20 degrees total as aposed to 30. So if I only use the cetrifugal advance with the newer distributor, will I still exceed that 30 degree total, or will I be safe without the vacuum advance hooked up?

I now understand the problem. Combustion BTDC is a big problem, and I don't want to go there, but if I can get away with the distributor I already have, then that would be much more convenient than searching out a new one. And I really don't want to spend the cash to rebuild a distributor that i'm going to eventually sell when I rebuild my 75 engine.
Dave

Dave,
Just put it in the car. A good rule of thumb for timing is to advance the distributor untill it reaches it's highest RPM and then back it off a little. Go for a drive and at around 2000RPM or so in for forth, floor it. If the engine pings just back it off a little untill you find a nice medium between good idle and low pinging. It might work great, you never know with older engines in MGB's. I have a '73 and had the same problem as you. I put in a distributor from an early B and it has been running great ever since.
J Arthurs

J. The reason it "works great" is that you are running an early dizzy in a later model car. Check out the advance curves. The low compression engine of the 73 had more advance, hence, you should have no problem, just some loss of performance. Dave has exactly the opposite problem.

Dave. If it were my car, I would fit a proper distributor. I tend to keep them and would rather not have to rebuild the engine sooner than I normally would. As J notes, it might be possible to get to some point where the advance curve will be suitable for use with your car by setting the static timing earlier than normal, thus you might be able to set the total advance for 32 degrees at the crankshaft at 3500 rpm and the curve might work for you. But, to be really sure that it would work properly would require several hours of on road testing or a trip to the dynamometer. The dynamometer would be the easiest and best way to ensure you find out exactly what is happening. At that point, however, you may find that the dizzy will not work properly in your car at some rpm range and you have spent $100 or so to find this out. Better to call Bob Ford at Brit-Tek and get one of his distributors, the 45D "Euro Spec" that will work with your engine, then build the 75 engine for better performance and use the new dizzy in it. Best of both worlds type thing. Les
Les Bengtson

"(points) have no bearing on the timing in any way" - *When* the points open is *precisely* what controls timing.

None of (the few) distributors I have show maximum advance on the weights, all are limited by a stop on the base of the cam piece. And these are marked in distributor degrees, which are half crank degrees, of course, typically marked '10' for 20 degrees.

We all talk about 'curves' but in fact most such curves are a straight line with a 'knee' in it. A true curve probably would be better and I think is what modern ECUs give. Therefore in a 'perfect' mechanical distributor the 'curve' is probably *very* conservative over most of its range, only coming close to the ideal in one or two places if at all, one of the reasons why modern systems give so much more performance *and* economy than ours.

Flooring the throttle only checks for pinking/pinging against centrifugal advance since there will be little or no vacuum advance with a floored throttle, and there is more chance of getting pinking at low rpms than high all other things being equal. Backing off the throttle will advance the timing as vacuum advance increases hence more likelyhood of pinking, but a lighter throttle means a smaller cylinder charge so less likelyhood of pinking. The upshot is you can quite easily get part throttle pinking but not full throttle so you need to check a range of throttle openings, engine speeds and loadings (e.g. inclines) and keep backing off until it doesn't pink in any of them, not just full throttle at one rpm.

Les - I asked about John Twists springs on another thread before I got to this.
Paul Hunt

Paul. John does not have access to factory springs for his rebuilds. He has access to, he told me, a large number of springs and, by trial and error, combines and tweeks them to produce the proper advance curve. Hence, there is still not source of "drop in" springs which may be installed without the use of a distributor test machine to verify that the distributor will function correctly. Les
Les Bengtson

Thanks to Les for his excellent post describing centrigugal and vacuum advance.

At the risk of being a bit too esoteric here I would offer a small clarification lest there be some confusion on too much advance.

Note that timing specs almost always refer to BTDC. Why would that be if we really don't want to fire before top dead center?

Well, the reason is that, when the spark plug fires it starts at a small point at the spark plug. What we want to happen is a smooth controlled burn, i.e. an expanding 'flame front' that progresses at a smoth rate over the top of the piton.

At some point as the flame front travels across the combustion chamber there will be a point at which a peak pressure occurs. We want THAT EVENT TO OCCUR SLIGHTLY AFTER TDC.

Since the movement across the combustion chamber takes time and takes the same amount of time regardless of engine speed, it is necessary to make the spark happen BEFORE TDC so that the maximum pressure is achieved AFTER TDC. As the engine speed increases, the spark must be done earlier and earlier in order to make the max pressure occur when we want it to.

Hope this helps more than confuses. Took me a while to get this straight in my head...
Richard Smith

This thread was discussed between 16/07/2002 and 19/07/2002

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