my harmonic balancer has shifted and I cannot find the proper timing setting. I get the vehicle running well and idling well but the motor will diesel when I shut it off. Do I need to advance of retard my timing to fix this? Its on a 1985 f250 7.5L 460 with duraspark II and a Edelbrock 600cfm Model # 1406 with the mixture screws out 2 1/2 turns and the vacuum advance hose unpluged from the Distributor and pluged with a Golf Tee. Thank You
Yup, I agree....dieseling is due to an idle that is too fast. Advance the timing as far as you can before the engine starts pinging. Then, set the idle where it should be and your dieseling should go away.
You should replace the harmonic balancer. If it has slipped, it could fly apart and cause some serious damage to your engine, never mind the outter ring flying off and coming through the hood or other body parts.
I keep hearing folks say that 'too high idle' will cause dieseling. While I agree that a high idle will allow more fuel/air to get sucked in as the engine is shut down, there still needs to be an ignition source.
Since all engines are hot when they are run for a bit, and since not all engines diesel when they are shut off, it leads me to think that there is another contributing factor.
Granted, a high idle speed not only allows more air/fuel to get pulled in at shutdown, it also creates a bit higher compression of that mix during shutdown as well. It's possible that the increased compression may allow it to get set off easier with the hot engine. That said, the engine alone should not be hot enough to do it.
While I would look to correct the high idle, I'd also be looking at ignition timing (incorrect timing can cause excess or abnormal heat), spark plugs (they could still be glowing indicating incorrect gap, defect or heat range), and in my view the biggest issue in older engines - carbon build up.
Carbon is basically charcoal and when it get's hot it glows and stays that way for a bit. That could be the ignition source.
Reduce idle if you can, check timing and plugs and clean out the carbon.