Engine diesels on shut off?

I have a 383 ‘69 rr, after market cam and enlarged intake valves. It is dieseling at shut off, I have double checked the timing and it’s correct. I’ve run this motor since 2012 and resetting the timing has always cured any dieseling. I don’t know what else can cause this, any ideas are appreciated. Thank you, Paul

Usually the idle is too high, causing dieseling.

I would agree.

Also any chance you got a bad tank of gas.

What is your compression ratio and what fuel do you run?

As carbon builds up over the years octane requirement can go up.

A couple of other suggestions if reducing the idle does not help. !\1. if it's an automatic, shut it off in gear. 2. if it's a stick, while in gear, turn the key off and slowly let the clutch out at the same time. Takes a little practice to get the timing of both actions to coincide for a smooth turn off

Agree! I had the same problem but my engine needed the higher idle. My fix was back down the idle on the carb (to get it back on the idle circuit) and then I increased the initial timing. Problem solved.

It could also be too lean. Maybe a vacuum leak.

What do you mean, the timing is “right” hopefully you are not timing to 1969 specs. This fuel needs a little more time to complete its burn.

My c/r is 9.0:1 and I run 91 octane premium.

The timing is set at 10 degrees BTDC, idle is 700.

Oh, it’s a stick also. I will work on the suggestions given and respond back.
Thank you to all who responded, I really appreciate it!
Paul

I always wondered why my car would diesel but every race car I ever saw would shut off clean.

Give that thing some initial timing. It will probably respond well to 15-20 Btdc. Remember what ever you put into the bottom also goes to the top. You don’t want to be 40 total.

I wonder too.

The combustion chamber temperature is too high and that can cause carbon in the cylinder to glow and continue to ignite fuel. This used to be quite a problem in the late 70's and early 80's when manufacturers where trying all kinds of things to get their engines to meet emmisions. Crappy fuel that ignites easily, too high of an idle speed, a carb problem, too much or too little ignition timing can all contribute to the condition as well as an escalated engine coolant temperature.

Exactly why manufacturers used idle solenoids. For the younger guys (and gals), you would set the idle speed to spec with the solenoid. Then when you shut the engine off, the solenoid allowed the throttle blades to close.



Tim

[]Usually the idle is too high, causing dieseling. That's what cure my 440 I set it at 900 rpm