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MG MGB Technical - Adjustable Cam Gear

We have a '74 MGB race car, running in Vintage to '72 specs. We have raced this car for two seasons, encountering problems in the latter part of the second season with holding the timing. It would, inexplicably, advance the timing anywhere from 4 to 10 degrees. As you might imagine, we checked all the usual suspects ...... nothing amiss. In the course of our attempts to discover the solution to our problem we tried two different distributors, both mechanical advance, one with points/condensor and one with pertronix. Initially, when the timing was re-set, it would remain in the proper position throughout the weekend. As time went by, it would not, and finally could not be set to provide any power. We have completely disassembled the engine, and are ready to re-build, with new components. The engine was already fitted with a DeLong variable cam gear (yes, this may have been the problem - not being properly tightened - it wasn't obviouly loose, but it might have moved, under stress - double row). Does anyone have any advice to offer before we go ahead and put it together?
Has anyone encountered this phenomenom before? Any and all help would be appreciated.
Fisher Jones

I have a Piper venier-adjustable camshaft sprocket on my engine and so far it's been quite adequate in holding it's setting accuratly. The variable camshaft sprocket may not be the culprit. It could be that the camshaft drive chain is wearing prematurely due to insufficient lubrication. If the timing seemed to be wobbling when checked with a strobe then it could be that your camshaft chain tensioner is malfunctioning. You might consider using the tensioner offered by Advanced Performance Technology. I use it in my engine, too. If you want to reuse the OE tensioner be sure to check its bore for ovality (.040" Maximum)
Steve S.

Thanks for your response. The timing wasn't wobbling, it actually changed, to varying degrees of advance. I'm putting a new chain and tensioner and crank shaft sprocket on the re-build, along with a new APT cam.
Fisher Jones

What about the pulley? Is it older or new? Is the timing changing or the mark changing? If the "proper" setting was poor power, then you were likely retarding the spark. Several folks have had trouble with older pulleys delaminating and indicating faulty timing. APT has a nice stainless pulley/harmonic balancer with degree marks over a wide range. (You don't have to take the engine apart either, if this is the problem.)
-let us know what you find. Tom
Thomas Lennon

That was a good point Tom, and we looked at that in the beginning. The pulley is an older one, but there isn't any delamination on it. We marked both pieces of it, and it isn't slipping. When we would initially set it, and we were running 28 degrees advance, we could get around 7,500 to 8,000 rpm. But, when the timing moved we could only get around 4,000 rpm. It always advanced, anywhere from 4 to 10 degrees. We're really stumped, except for the adjustable gear. We are open to any suggestions, and of course, we'll report our eventual findings. We have another race at the end of June, so we hope to have all the bugs ironed out by then.
Fisher Jones

Long shot. You may have a sheered distributor drive gear pin allowing the gear to slowly turn on the shaft. Good luck at your June race.
Leland Bradley

Fisher. There are three things which could affect your timing--the crankshaft pulley moving (which you have checked and found good, unless the woodruff key slot has widened or the key itself has been lost) the timing chain or the adjustable cam gear. Since I assume you adjusted the cam to the timing you wanted, why not check the cam timing again? If it has changed, you know your adjustable timing gear is the probable culprit. I have never seen a timing chain "jump a tooth" on an MGB, but, if you find the cam timing is off, pulling the timing chain cover should tell you if the chain has stretched or the tensioner self destructed. Les
Les Bengtson

Hi Fisher
Check out the distributor base plate, once removed you should find that it can be moved freely against the advance retard plate. They can, and do jam together. This can also happen when a screw that is too long has been used to attach the points/pick up etc to the base plate, if this has been done by some ham fisted mechanic then i have even seen the two plates bent.
I hope this helps.
BTW Regards to Liz
John

Thanks everyone for your comments, but perhaps I wasn't clear enpough in my original message. The engine is already disassembled, we decided on a complete re-build. Before we took it apart we looked at everything that we could think of that might have been the cause of the problem. The timing chain and tensioner were in good condition, we tried a couple of different distributors (both of which we went through with a fine-toothed comb). We are completely re-building the engine, with new parts in most cases, to include a new APT cam. The only thing that we could think of that might have been suspect was the variable gear. Nothing else made sense to us, as Les said, there are really only three components that could cause this phenomenon. The new timing chain and tensioner shouldn't be suspect, and the distributor is in tip top shape. We haven't purchased a new timing gear, believing this one to be showing no signs of wear. All we could think was that it had been slipping just sufficiently to allow this advance in the timing, and wondered if anyone else had encountered similar symptoms. If anyone can think of something else, we'd be glad to hear it, any suggestions are well worth checking out. Sometimes you begin to think that you can't see the wood for the trees, that you are overlooking something so simple that you just didn't think about it.
Fisher Jones

John, I've jsut seen your posting, is this really you? Send me an e-mail, please.
Fisher Jones

This thread was discussed between 24/05/2002 and 26/05/2002

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