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MG MGB Technical - Timing Change

I was driving my 67 B-GT last summer and the car suddenly stopped running. I have spark and fuel. After towing it home I found out that the timing had jumped and was off by about 45 degrees. I can get the car to run (poorly) if I turn the distributor. Any idea what happened in there? Did my timing chain jump a gear? Thanks Guys!
Todd Nickleski

Todd. I have never known a B series engine to "jump a gear", even when the timing chain and tensioner were sufficiently worn that there was audible timing chain drag on the inside of the cover. I had this problem, recently, with my 68GT and found it possible to change the timing chain sprockets, chain, and tensioner with the engine still in the car. You have to remove the radiator, unbolt the motor mounts from the frame, and lift the engine front somewhat. The engine has to be firmly supported when detached from the chassis and raised to protect hands and fingers. It would allow you to inspect the alignment of the timing gear sprockets and the condition of the chain and tensioner.

I have seen distributors, which were not fully clamped in place, turn in use and throw off the timing. If that happened, the engine may quit running, depending on the amount of shift.

I have heard of the harmonic balancer delaminating and causing the timing marks on the outer circumference to be out of their proper relationship. That, however, would only affect the perceived timing as measured with the pointers and harmonic balance. The actual timing is controlled by the relationship of the crankshaft/cam/distributor and would not be affected by the movement of the outer portion of the harmonic balancer in relation to the inner portion of the harmonic balancer and the engine would have continued to run.

Thus, "the car suddenly stopped running" may well indicate a problem with the timing chain and tensioner and would be a good place to begin your researches. If the relationship of the sprockets is correct and the chain and tensioner are in good condition, the problem will become very interesting.

Les
Les Bengtson

Todd,

if the timing chain would have jumped, it aught to be 90 deg. at the crank and there would be serious missfiring, not mentioned by you and very uncommon on the b-series engine.

I would take out the dizzy and check it or have it checked, as well as the internal drive that links the dizzy shaft to the cam. Have a look at the workshop manual and you will find detailed information how this dizzy drive shaft is to be removed.

Report your investigation here again, please.

Ralph
Ralph

Hi Todd - FWIW - My son's metro ( A series engine) had the timing chain jump off the sprocket - but it then jammed behind it - not back on at a different position - so I doubt if this would be the cause of the problem - unless the teeth on the sprockets are very badly worn - only way to find out is to remove the timing chain cover and have a look.cheers cam
Cam Cunningham

You don't have to remove the timing chain cover to check the chain alignment. With the rocker cover removed turn the engine to No1 TDC. Then rock the crank about 10 degrees either side of the TDC mark at which point 7&8 valves should be rocking. What happens when you put a strobe on the engine and where are the timing marks then ?
Iain MacKIntosh

Just a further point. Check the distributor in line with my comments on the Distributor Install position thread.
Iain MacKintosh



My 80 MGB had similar systems. The car barely ran, and it was getting two timing flashes; one at about 45 degrees BTDC, and a very faint flash at about 13 BTDC. I sent the distributor off for rebuild, and sure enough, the vacuum canister had failed completely. A general rebuild with new vacuum canister fixed the problem. If you have, or can borrow, another distributor, you can trouble shoot this possible solution. Good luck.

Chuck
Chuck Hassler

I've never known of an MGB timing chain jump a tooth, and it would have to jump several to be 45 degrees out.

The centrifugal mechanism may have failed as well as the vacuum advance mechanism, and either can cause the timing to go way out. Remove the vacuum pipe from the carb or manifold and suck on it hard, and you should see the points plate move clockwise. You should be able to hold the vacuum by sealing the end of the pipe with your tongue, it only releasing when you move your tongue off, when the points plate should flip back smartly. For centrifugal advance you should be able to turn the rotor *anti*-clockwise against spring pressure, and it should return all the way when released slowly. There should ideally be no slop at all in the fully clockwise position, although with used distributors some is almost inevitable.

The distributor may also have been loose in the clamp, but this should have been immediately obvious. Make sure it is fully pushed home into the drive dog, with the clamp-plate bolted up to the block there should be no gap between the underside of the flange of the distributor body and the face of the clamp plate.
Paul Hunt 2

This thread was discussed between 24/01/2007 and 25/01/2007

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