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MG MGB Technical - distributor jumped gear teeth?

we rebuilt our 79 mgb; was running great; got about 100miles on it. then started idling very erratically; discovered timing was way off. to get it running ok again had to (apparently) advance timing to ~25 deg BTDC (crank timing mark is off of timing cover scale).

did my distributor jump a few teeth on the camshaft ??

thanks
frank
frank

Frank. The basic engagement of the distributor drive gear and the camshaft cannot be disturbed because the distributor housing holds the two in alignment. Unless these two parts are worn far beyond anything I have ever seen, they will not cause a problem.

I have seen distributors which were not properly tightened down move out of alignment, in some cases popping out of engagement with the block.

It would be possible, but I have never seen it myself, for the timing chain to jump a tooth, or two, if the timing chain tensioner was not properly released. But, the purpose of a timing chain tensioner is to provide tension on an old, worn chain. With the new ones, the tension would not allow such a thing to happen for many thousands of miles.

More information about what you are running, including what the carb is, would be of value to us in helping you to define what may be the problem.

Les
Les Bengtson


Possible sticking advance mechanism in your distributor. Was stuck advanced when you timed it; after driving a while the stuck part let go, retarding your timing.

It's not possible for distributor to 'jump a few teeth' - it simply cannot happen. Ever. Do not open your engine.

It is, however, perfectly possible that the distributor wasn't properly tightened down after you timed it. Then, as you drive, the timing drifts.

My advice: reset the timing correctly and see if the event recurs. If it does, look into something sticking in the distributor. If not, chalk it up to human error.
Sam Good

You have the 45D distributor, where the cam is retained by a plastic washer that pops over a ball on the end of the spindle.
If the rotor arm seizes to the shaft, sometimes pulling it off pulls the whole outer shaft upwards, allowing one or both of the bobweights or springs to come adrift.
This can cause erratic timing at idle.

Check that you can twist the rotor arm clockwise about 15 degrees, and that it springs back by itself.

You can dismantle the distributor and remove the points plate whilst the distributor is in the engine.

Also the late cars have a ridiculous amount of vacuum advance at idle, if the vacuum capsule has failed, or the pipe has come off, the timing at idle will be miles off.

Test the capsule by sucking the carb end of the pipe, you should be able to get your tongue to stick on the end of the pipe, and see the plate inside the distributor moving.
Martin Layton

thanks for the inputs.
Everything on the car is stock; we've got the zenith stromberg carb, and a ton of california emissions equipment.

the collar bolt on the distributor hold down was stripped; we replaced that last nite.

but..to get it to run, the crankshaft timing mark is off the timing chain cover scale.

i will check the vacuum suggestions.

thanks
frank
frank

Frank. What was the condition of the harmonic balancer when you did your rebuild? I begin to think it possible that your problem is a bad harmonic balancer--the outer ring of metal, having the timing marks on it, may have slipped in its relationship to the inner ring of metal which attaches to the crankshaft.

Were it a distributor drive gear problem, that would affect the position of the distributor, not the amount of advance required for the engine to start and idle. Your timing light is measuring the supposed actual time of spark plug firing. An internal problem with the distributor would give the same effect--the actual firing of the spark plug would be within the markings on the harmonic balancer, but the distributor would have been rotated to an unusual position.

You need to verify the markings on the harmonic balancer. Martin is correct that the vacuum advance provided a great deal of advance at idle. But, it was set with the vacuum advance line plugged and the engine idled on the mechanical advance only. Your engine should be able to do so with no problems.

Check for a bad harmonic balancer and let us know what you find.

Les
Les Bengtson

the 'harmonic balancer' theory seems to make logic out of the illogical symptom's i'm seeing.

i took a close look; its not obvious where the balancer is sectioned.

i'll put a strip across the whole thing, as well as mark a new timing line on the timing chain cover, in line with the timing mark on the balancer, identifying the current timing mark. i'll monitor both markings.

thanks
frank
Frank

Whilst a slipping harmonic balancer would cause problems when you came to retime it, it certainly wouldn't put the *actual* timing off to cause the erratic running you first noticed.

A 79 would originally have had manifold vacuum and this *must* be disconnected and the manifold port plugged before checking timing, or you *will* see 25 degrees BTDC or more.

Paul Hunt 2

This thread was discussed between 21/07/2007 and 24/07/2007

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