H that keeps running after it is shut off

Assume that you're using the term "diesel" to mean it keeps running when you switch off the igntion.

Dieseling is generally caused when an engine is running hot enough to turn carbon deposits in the combustion chamber into a source of igintion. That is, even when the spark plugs are shut down, there's enough heat in the crud in the chamber to ignite the gas.

That's become a common problem with lean burning emission control engines. So a lot of modern engines have solenoids which shut off gas flow when the igntion is turned off to prevent this problem. The first, easy, step to try would be to close the throttle before you switch off the ignition. Next trick would be to turn the main jet to rich, or counter clockwise, a turn or two -- it's that screw pointing to the rear and down at a 45 degree angle from the carb -- a richer setting won't generate the extra heat that causes the dieseling. If there's still no fix, try lowering the float setting so there's not as much gas available in the bowl of the carb. Worst case would be to pull the head and the pistons and clean off all those carbon deposits which are causing the problem.

You might be right, but it's Sunday and so I was being charitable by assuming his handyman knew what he was doing. Least nothing was mentioned about his changing the ignition timing.

If the plugs are the wrong heat range, or just not torqued down right so they've become "glow plugs"; he could just reinsall the old plugs and see if that cures it. Another idea would be to retard the ignition timing a bit, as I think every 5 degrees of advance can raise chamber temps by 75 to 100 degrees. Net cost of the above is zero -- the kind of fixes we like!

Maybe the best advice is to read the color of the exhaust smoke and/or pull a plug and compare it to one of the hot/right/lean series of pictures of plugs here on the web. I don't know if plug gap or dwell is going to be a make or break item on a low compression non-turbo engine, but your main point seems to be that the cause or the cure can be on either the fuel or the fire side of the engine; and that's the truth.

Interesting. Still have two strobe timing lights and noticed that my distributor was "scattering" spark at high RPM. Shimmed up the shaft inside, and that made it more uniform - but never messed with the advance weights or springs; probably make a good winter project.

Have also noticed that a few turns of the main jet will make a dramatic change in the EGT levels, so I fully agree it's easy to overheat and burn valves, and so on.

Thanks again!

Pete