RAG

My friend has a '68 Cutlass, 350 engine, 4 barrel carb correct for '68. I rebuilt the carb and changed spark plugs. The car is not driven on a regular basis and is stored in a garage. It diesels when the ignition is turned off, even if you leave it in gear and turn off the ignition. The engine had been built by Lynn at Mondello's in California a few thousand miles ago. Any suggestions or experiences with severe dieseling?

rocketraider

What octane fuel is he using?

First thought was idle speed too high but shutting it off in gear should override that.

Check base timing and play with that.

Then again it might just be who built this engine...

oldcutlass

It can be hot spots in the combustion chamber, too high of idle speed, or voltage feeding back through the coil+ circuit.

JohnnyBs68S

Not sure if the 4bbs have this, but my 2bbl carb has an idle stop solenoid that releases the throttle plates and lets them close further when the ignition is turned OFF to help prevent this exact problem. If your carb has one, make sure it is adjusted properly (IIRC, disconnect the solenoid and adjust the stem to obtain a 400-ish RPM idle).

Interestingly, my 69 Toro didn't have this solenoid, and it suffered from dieseling too. After I sold it to a friend, he broke a half-shaft and when he started it to steer it into his garage (NEED that power steering on a Toro), putting it in gear did nothing to load the engine to get it to stop running on. We literally choked it to death by putting a plastic garbage bag over the carb. The only solution that worked for me when I owned it was a half-tank of 100 octane each fill-up.

Kensey

I'm going to have to check this on mine! I have a 1972 Delta 88 with a 350/2 bbl. Same issue. I also want to check the points and timing.

Koda

How does it start when warm? If it cranks hard, could be too much timing.

66SportCoupe

X2 or idle speed may be too high.

Sugar Bear

If it was originally a 4BBL it's high compression and will need premium fuel.

Good luck!!!

bccan

For the most part, dieseling comes from one source, a supply of fuel and air that allow it to try to run despite no “intentional” ignition. If you check the carb, there is a very high probability that the primary throttle plates are not closing properly at idle. They are slightly open, allowing air to be drawn, which in turn draws fuel from the idle and/or transition slot in the carb body. It is this mixture which can then encounter enough heat to cause some mistimed, incomplete source of ignition.

Whatever condition has brought about the throttle being cracked at idle is what likely needs to be found. It could be in the carb, an ignition issue or a vacuum leak. Even a nasty carbon condition needs a fuel mixture to ignite. Find the reason(s) for throttle opening, fix it to allow the throttle to be closed and drawing only idle air and idle fuel through the proper channels and the dieseling will very likely be fixed.

cjsdad

I once owned a 74 Ford Torino that would diesel on for minutes. I could turn the key off, lock the doors and get a slurpy at 7Eleven, come back to the car and it would still be running. Turns out Fords also used synthetic cam gears same as GM. My Dad showed me how to check for timing chain wear by turning the crank forward and then backwards. Changed the timing chain and gears and solved a multitude of issues with that car. That shouldn't be an issue on a new engine, but consider the reputation of the builder.

Greg Rogers

Too hot heat range of plug will also do it. I see people mentioning timing, but isn't timing out of the picture after ignition is turned off? I suppose that if timing were enough to make combustion chamber so hot to cause dieseling it could be... I've seen plugs taken out of cars that had ran so hot you could see the discoloration on center electrode and tell that it probably was red hot when driving. (glow plug?)

RAG

Thanks for all the responses, have fowarded them to the owner.

pettrix

Iron heads on 10.0:1 + compression ratio, can diesel on 91 and even 93 octane if idle is high or timing is above 30 degrees. Lots of factors at play, even heat and carbon buildup inside cylinders.

Dieseling is bad but I would be more worried about detonation occurring when at high RPM. It will destroy the pistons and pound the upper rod bearing into oblivion.

An E85 tune (105 octane) will eliminate any dieseling or detonation when driving. Or get a set of aluminum heads

Either way. Detonation will end up destroying that engine if not fixed.