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Q & A: Cam Timing

Hello John,

I have a stock 1971 MGB that appears to have jumped a tooth on the timing chain. The engine will start and run but has a pretty bad shake in it (no knocks) and is down on power.

I was pulling away from a light when there was a "ba-bing" noise (sounded more like a spring breaking?) and the roughness shake was immediately apparent. I dove the car home and checked the timing. The timing light showed that it had retarded itself about 20 degrees (no marks on that side to be exact) so I put it back to 14 degrees by the light. The distributor wasn't loose and the dwell on the points was correct before and after whatever happened to move the timing.

My multiple questions are:

1. Have you ever heard of a double row chain jumping? (crude check of the slop in the timing chain shows about 12 degrees of slop without the motor running--assumption that the tensioner isn't pressurized when turning the engine with a ratchet,,,)

2. Is the crank pulley a solid steel piece or a harmonic balancer that may have slipped?

3. Is it easier to raise the engine up to get the crank pulley off or is it smarter to drop the steering rack?

4. Other people have speculated that something in the flywheel/clutch may have broken free, creating the imbalance to put the shake in the engine. (Shake is there at idle with no forward movement of the car)

Thanks in advance for any insight you can offer.

Best regards,

Jim

Jim,

In my nearly forty years of servicing MGs, I've yet to see a timing chain slip, so I doubt that is the problem. There is a method for checking cam timing:

Set the #1 inlet valve to 0.060" instead of the... read more