Morris Minor Owners Club

I know that if the ignition advance is insufficient, there is a chance of overheating and a sluggish engine. But now I also read that a bottom end rumble can be a symptom?

Good. No replies means it is nonsense!

Retarded ignition will manifest as overheating, poor performance, hard to start and thirsty on fuel, but I've not heard anything about 'bottom end rumble'. Where did you read of it being a symptom?

Are you hearing piston slap, pre ignition or knocking? Could be worn out bearings, worn cylinder wall and rings or just badly timed ignition.

Engine is absolutely as new. I have the rumble thought from somewhere down in this topic
viewtopic.php?p=660212&hilit=up+hill#p660212

Oh yes, I see....not heard that before.
Are you experiencing any symptoms in your car, or is the question merely academic?

Yes, merely academic,

I am not sure I really follow the question. When is this rumble apparent? At idle? When the engine is under load going up a hill? When the revs are raised at a fast steady speed?

It is possible to have the ignition timing set correctly at idle but for the timing to be incorrect when the revs are increased or the engine under load due to non-functioning centrifugal advance and/or vacuum advance. It is also possible to have an incorrect advance curve for a particular engine by fitting (for example) a Lucas 25D4 distributor that was set up for a different type of engine.

To check the advance curve is beyond my ability and is something a distributor specialist could do on a special machine but to check the advance works at all is not difficult.

To do some basic checks: warm up the engine to normal operating temperature, ideally by going for a short drive. Remove the vacuum advance pipe from the distributor and block the end of the pipe with sticky tape. Attach your strobe. I am assuming your strobe can measure the rpm. Reduce the rpm using the idle setting on the carburettor to about 650/700 rpm. The idle may be a bit rough this low, especially with the vacuum disconnected. Now check the timing with the strobe. It should be roughly 4 degrees BTDC. Now increase the revs and check with the strobe that the advance has increased. Now connect the vacuum pipe again and increase the revs and check that the advance has increased more. You have now done a basic check that the centrifugal advance and the vacuum advance are working (but not that they are working to exact specifications). You can raise the revs considerably and check the maximum advance. This is the most advance the engine can get and for a 1098cc A-series it is around 30 degrees. To measure this properly you need a strobe that can be set with the degrees of advance and when set with the degrees of advance on the strobe, the timing mark is read at the TDC pointer.

Or, take the car to a good rolling road tuner experienced with classic cars and they will do all of this for you as well as optimise the fuelling. It's money well spent in my opinion.

Stephen