Hey guys!
Dieseling is caused by too much timing as I know. I have a 427ci BB and the timing at idle is set to 17°. Is this too much? Should I retard the timing at idle?

Günther

 

Correct me if I'm wrong, but if you shut off the ignition and the engine still tries to run, (what we all call "dieseling") What does timing have to do with it? There is no spark.....
just detonation.

M2C Steve

 

"Dieseling" in a spark ignition engine is a term used to describe a run-on condition which occurs after the ignition is turned off. It can be caused by a number of things, but your initial timing setting is not one of them. "Dieseling" occurs when the ignition is off, and therefore no spark is present.
For "Dieseling" to occur, you need the presence of air and fuel, as well as a source of ignition. In carbureted engines, an air/fuel mixture can continue to enter the engine during the coast down of the engine after the ignition is turned off. The engine continues to pull air thru the carburetor, and the carburetor contines to mix fuel with it. This can be aggravated by a high idle setting; more air/fuel is delivered. The source of ignition can be a "hot spot" in the combustion chamber, which remains hot enough after the ignition is turned off to initiate combustion. This can be a carbon deposit, sharp edge, or even the spark plug electrode itself.
Modern fuel injected engines are not plagued with "Dieseling", as the fuel AND ignition are turned off when the engine is stopped.
Some early emissions controlled carbureted engines exhibited "Dieseling", even when new. To control this condition, these engines used a solenoid operated idle throttle position control which permitted the throttle to be held in a more open position to provide sufficient idle speed, but closed completely when the ignition was turned off to prevent "Dieseling".
You need to focus on the causes: Too much air/fuel after ignition shut-off, the "hot spot" (source of ignition", or both.

 

Dieseling is caused when the cylinder creates its own combution heat...without the need for a spark. It's a combination of timing, running hotter than engine design intended (for emissions reduction) and higher compression. A normal internal combustion engine doesn't have enough compression to ignite the fuel charge only with compression heating effect. But, throw the other conditions into the mix, and it can (and does) happen with the ICE. GM used a lot of 'tricks' to prevent this, such as:shutting the air supply off completely, turning on the A/C compressor when the engine is cut off so that it will load it enough to stop, etc.

If you have a hi-compression engine with the emissions stuff still intact, you will have to get 'creative', too. Back in the day, most of us just put the car in gear when we turned the engine off (released clutch a bit after engine "off"). That would 'kill' the engine with no problem.

 

what is your engine RPM at idle? Did this start after you rejetted the carburetor?

 

Two very good explanations.

It does seem strange that you have initial timing of 17 degrees although that is unrelated to the dieseling.

 

I've had multiple 427 engines, and never any dieseling problems. Most of my experience with people having dieseling problems is when they run ported vacuum to the distributor and the throttle plates have to be opened more to compensate for the poor thermal efficiency caused by the "retarded" timing.

 

Hi!
Thanks for your all help! The idle rpm is about 700 and yes, I use ported vacuum. Do you think the ported vacuum is the problem?

Günther

 

Ported vacuum might be the cause but there is no valid reason to use, possible emission reasons aside.

You might want to re-do you timing map including the vacuum advance curve, sound like there are multiple things to take advantage of.

 

Trickle some water down your carb while the engine is running to clean the carbon deposits off the pistons.

 

In most cases, yes. Swap the vacuum connections, readjust the idle back down to 700 (as it will probably want to idle at about 900 with the improved efficiency of the additional timing advance), and the new (reduced) throttle position at idle should choke off enough air and fuel to eliminate the dieseling.

 

Oh no, sorry guys! How stupid am I? I didn´t remember that I don´t use the vacuum advance. The port on the carb is plugged.

So from the beginning. what can I else do to avoid dieseling?

Günther

 

I would still follow 427's advice. At idle, plugged vacuum advance is similiar to running ported vacuum advance.

 

OK so timing vacuum canister on the distributor should be hooked to manifold vacuum?
if this is right I am going to try it for the first time in owning a car just to see what it does.
Right now I use no vacuum and just leave the canister open with nothing hooked to it.

 

But it won't fix Guenther's problem.

 

Why not?

 

Duh- you're right. I was think backwards again. The more advance at idle, the more efficient. My bad.

 

Having NO vacuum connected to the distributor advance can is exactly the same as having it connected to a 'ported' vacuum connection...at idle, at least. Ported vacuum has the orifice just a bit above the throttle plates when the car is idling. So there is no vacuum at idle. Crack the throttle a bit, and now you have 'manifold' vacuum on the 'ported' outlet.

Having no vacuum advance (in either instance) will cause the engine to run hotter and leaner at idle, and that will set up some of the conditions required for an ICE to "diesel".

I suggest that you hook up manifold vacuum to the dist. advance can, adjust the idle speed again, and see how it works then.

 

Where would a ported vacuum be on a demon carb?

 

Closing the throttle plates really does the trick.