Recently started newly rebuilt 427. It was tuned by builder on dyno and broke-in. Motor runs on after I turn ignition off. I've got maybe 30 miles on new motor. I know this is either from incorrect timing or excess gas in motor usually from carb. problem. Motor runs great and was "professionally" tuned, so I am thinking a carb. problem. Should I go ahead and rebuild carb. or should I mess with the timing a little to see if that helps? Any opinions are always appreciated.

 

I would check to see if your idle is set to high. If its holding the throttle plate open it can cause dieseling.

 

On a new engine i would say that your spark plugs are the wrong heat range and the tips are so hot that they are glowing causing the run on.

 

You can either retard the timing and loose performance or install a throttle stop solenoid. I chose the solenoid and solved the problem. Basically the solenoid is energized by ignition in run mode. It holds the throttle at idle speed until ignition is shut off then retracts and closes the throttle plates. This in turn shuts off air and fuel. Problem solved.

 

Running on is NOT caused by ignition timing. Once you turn off the ignition, there is no spark! It makes no difference where the timing is at if there is no spark.

Too high of idle speed as mentioned will cause this, the solenoid is good advice. Too hot a set of plugs as mentioned, possible. The biggest reason is the octane is too low. With higher octane they won't run on pre-ignition otherwise known as dieseling.

 

Probably sticking throttle. Gotta have air.

 

Hot plugs?? BS.
If you are running an auto look at the throttle stop. If not, look at the throttle stop. Reduce your hot idle to 750 rpm.

 

Run-on after ignition shut-off (aka, "dieseling") is most commonly caused by excessive throttle plate opening at idle. Normally, excessive throttle opening correlates to high idle, but not necessarily: If the timing at idle is inadequately advance (either retarded or inadequate initial timing, or failure to run vacuum advance hooked up to manifold vacuum), the engine will need to be "forced" to idle by cranking in more throttle plate opening. This excessively opened throttle will allow run-on after ignition shut-off.

This can be fixed in one of several ways:

First, if you have a little bit of a cam, make sure your initial timing is in the 18-degree range and that you are running vacuum advance hooked up to manifold vacuum to allow full "suck-in" of the vacuum advance at idle: At idle with vacuum hooked up, you should be running actual timing of about 30 degrees. This will allow you to close the throttle plate opening as much as possible, decreasing the amount of air into the engine at shutdown, and allowing a smoother, lower-rpm idle speed. The advanced timing will also lower exhaust gas temperatures, and can eliminate any hot spots and glowing plugs contributing to the run-on.

If your distributor does not have vacuum advance, get your initial timing up into the 24 - 26 degree range with a very short centrifugal curve, limiting total to 36 (or wherever you found the "sweet spot" during dyno resting).

If, after getting the right amount of timing into it, and after decreasing your throttle plate opening and idle speed, the engine still runs on, you will need to install an idle solenoid that closes the throttle plates at shutdown.

Lars

 

I agree with both Lars and gkull, I've seen the plug change to a cooler plug quil dieseling, further more I'd be careful who you call a moron. My two cents.

 

I had this issue when I installed my new engine - I put it down to a fairly rich idle mixture (needed due to lower vacumn from hotter cam) sooting-up some of the new and sharper edges of the combustion chambers etc and creating carbon hot spots (and resulting running-on) a few hundred miles and some carb tuning later the issue went away - it still happens occasionally after prolonged low speed/inner city driving and with non-premium gas - engine just needs a good 'thrash' to clear!

 

Thank you all for the replies. Now it's time to get to work!!!! It's been since Oct. that I've been working on this car. It's easy to get anxious and a little discouraged with such a long project. My last project was restoring '52 Chevy pick-up that took nearly two years and I didn't really want to get into another "big" one. But when a matching #'s 427 blows up 75 miles into the trip home- whadaya goin' to do!!! I'm gettin' there though, slowly but surely!!! Thanks again for all the help.

 

Working on the vettes' dieseling problem and think I have found the excessive gas. Holley carb. # 8106B. I've been told it's a 750 CFM carb. When idling there is fuel "leaking" from one side of one of the 4 barrel ports. By leaking I mean to say one of those "squirters" are delivering fuel to the motor when idling. Sorry I don't know the correct terminology. I am assuming some type of cut off needle valve or whatever initiates fuel flow to those rear "ports" is "leaking." So can someone point me in the right direction to learn about this problem. Thanks

 

What? Fuel is coming out of a squirter nozzle at the top of the carb when idling?
Or do you mean the venturis in the middle of the carb?
If this is the case your throttle plates are too far open, or your fuel level is to high in the bowl.

My engine had a completely different diesel cause: had a leak in the intake and sucked oil into the combustion chamber. This caused glowing carbon... My previous engine had a higher oil consumption, but did not diesel. So have a look for this. If your oil consumption is very low you can exclude this root cause. But if not if could also be possible.