Dieseling is caused by hot-spots in the combustion chambers acting like glow-plugs, hence the name. They essentially provide an ignition source after the key has been shut off. Since air\fuel can still enter into the combustion chamber through the carburetor even after the engine is cut, there's your combustion.
Dieseling can be caused by several sources: curb idle too high, vacuum leak, late ignition timing, or carbon buildup. It can also be caused by excessive combustion chamber temperatures, which can be caused by the EGR system not functioning or lean air\fuel mixture. Usually pinging and dieseling go hand-in-hand as they share many common causes.
Lastly, many later carburetors have an idle stop solenoid which is often called an "anti-diesel" solenoid. This is a solenoid that holds your curb idle when the key is on, but as soon as you shut the key off, it retracts and lets the throttle plate(s) shut to almost closed, so that they rest on the conventional curb idle screw. On these carbs, the idle is actually set by moving the solenoid forwards and backwards, usually by a bracket and a long screw.
Moral of the story: there are many possible causes, but I've told you everything you need to know. Tune the carburetor and rebuild if necessary. Spray for vacuum leaks with a can of carb cleaner. Check your timing and verify your EGR system is functioning properly. Doing all of that will cure dieseling 98% of the time.
thanks fellas. that just about covers it
is the idle stop solenoid something I can yank off a junkyard truck or is this a common problem ie they tend to go?
Yes you can get them through a junkyard (or the parts store) but I'd make sure it's actually able to engage before assuming it isn't working. It might actually be working, but just so far out of line with the throttle tab that it can't do anything.
It is located near the throttle arm. Look over the throttle linkage and it is usually pretty obvoius (it's a solenoid). If you're running the stock 1 barrel then it will be on the front assuming it hasn't been removed.
Run a can of Seafoam engine cleaner/decarbonizer through the carburetor. Just follow instructions on the can. In some cases the results are almost astounding.
Also, don't expect the idle solenoid to push the throttle open, many do not have the power to push the throttle open, but do have the power to keep it open once you help it out.
Stopping dieseling is one of the main jobs of a idle stop solenoid, so if it isn't working, that would be the first place to start. I read somewhere that as emissions got tighter, they set the engines up leaner, which means the throttle blade had to be open further to maintain idle speed, which was the formula for disaster as far as dieseling is concerned, so a lot of times they did add a solenoid that would drop back and shut the butterflies completely off when you turned the key off to prevent dieseling.
...so a lot of times they did add a solenoid that would drop back and shut the butterflies completely off when you turned the key off to prevent dieseling.
A neighbor of my father's started working at his grandfather's Ford dealership as a
mechanic after he graduated from high school (late 70s/early 80s). He spent something
like 20 years there before he went and worked at some place that had a fleet of Ford
trucks. The minuscule details about these trucks this guy knows is just amazing, and
he to this day has a 1982 F250 he bought brand new.
Anyway, when I asked him what the Throttle Kicker was for, he explained basically
what you (Franklin2) just said; I don't remember the certain if he explained about the
evolution (engine running leaner as time went on, etc.) but I definitely remember how it
was intended to prevent a dieseling effect.
The Throttle Kicker, at least on Motorcraft/Holley 4180C carbs, is not the the same as an anti-dieseling solenoid. It's purpose is to prevent the throttle from completely closing for a timed period after you lift your foot off the pedal when slowing down. It's to help reduce unburned hydrocarbon emissions. It is a vacuum operated unit controlled by an electronic module.
