MG Car Repair: MGB pinking at high revs, abnormal combustion, combustion chambers

Question
My MGB engine was completely rebuild some years ago and has not covered a great mileage since.  When the engine gets hot it, exhibits 'pinking' at 2500 to 3500 rpm.  If power is maintained when this pinking occurs, the engine temperature increases.

At least the sound is identical to pinking although this cannot be reproduced at lower rpm in the usual manner.  Additionally, the engine is very difficult to re-start when hot.

I would appreciate your comments.

Regards,

Richard

Answer
Hi Richard,
Engine pinging is an indication of Detonation (abnormal combustion) and that is very destructive to the engine. (If that is what it is)

You need to know, and the easiest way to tell is to check the ignition timing at 2500 RPM and reset it to at least 5 degrees retarded from that setting and take the car out and try it again to see if it affected the noise. If it eliminated it but the timing was correct and you are now 5 degrees retarded from correct timing, then you need to look to other causes of Detonation.

1. Low octane fuel
2. Wrong heat range number of the spark plugs. (too hot)
3. Excessive carbon in the combustion chambers.
4. incorrect ignition timing
5. Carburetor mixture too lean.
6. Engine overheating. (even though Detonation can cause overheating so can overheating cause Detonation)
7. Part of a misalignment head gasket sticking into the combustion chamber

It is unusual that you get it at 2500 - 3500 RPM and not at low RPM with a quick snap of the throttle. But that can happen. If you can duplicate the noise without a load when hot You may have a mechanical problem. Keep in mind that an MGB's top torque is at about 3500 RPM. Your symptoms do not indicate a rod noise or anything lower. If you can duplicate the noise at 3500 RPM with no load you should do a cylinder kill test by shorting out each spark plug while it is making the noise to see if it is more on any one cylinder.

Usually when I get something like that I try setting things wrong to see what affects it and what don't, like retarding the timing and setting the carburetors to a rich setting. If no change affects it and you can get it under "No Load" conditions, then it is most likely a mechanical noise and different tests must be done to isolate the problem.
let me know,
Howard