Hi, I have a 73 f250 / 390 engine w/ 2 bbl carb and for about two months now whenever i'm finished making my rounds for the day and i turn the ignition off i find that my engine will almost always try to turn over just a couple more times and then send hot fuel blowin right out of the carb, i've been told this is because of my ignition timing and i've been trying to fix that but since i have no emission sticker in my engine compartment or any kind of info on setting my idle speed I don't know how fast i should set my idle. I've currently got it set at approximately 650rpm and i've adjusted the distributer best i could(after testing tdc-10 degrees, 10 degrees of advance worked best and was uniquely marked so i set it too that) but i still have the
quitting problem allthough it isn't quite as serious it is still annoying as hell. Any help would be appreciated, thanx.

 

When you turn off the ignition you lose spark at the plugs (normally). Since your truck continues to run it could be a buildup of carbon on the valves or in the combustion chamber that is igniting the gas. There was a discussion here earlier about how to get rid of the carbon. There are comercial cleaners you can try, go to higher octane gas and take the truck out on the freeway for a good run I have also heard of spraying water into the carb to get rid of the carbon but I have never used this method so it could be an "old-wive's" tale I am not sure.
It also could be as simple as the idle mixture is too rich and allowing the engine to diesel on shutdown. Have you tried to adjust the idle mixture? Also set your idle lower if you can, are you using the tach in your truck or a precision Tach? Sometimes the tachs installed in the trucks aren't really accurate.
Hope these ideas help a little.

 

Justin:

Sparky is right. You could have carbon build-up on your heads and/or pistons. You will go nuts trying to get rid of it. My '81 F100 with an I-6 does it, even more than yours. Long ago I got into the habit of using the clutch to stop it. When I kill the motor I have the trans in 1st and my right foot on the brake. When I turn off the key I just release the clutch to the engagement point and the engine dies smoothly. I reduced the idle to about 525 RPM and this helped too.

I also had to do this on my '68 Mustang with a 289 until I rebuilt the engine and removed all the carbon. It has a C4 auto so I would turn it off with the gear selector in D. Took care of the problem.

A few years ago I read a column on the internet by a petrochemical engineer. He stated that carbon build up is caused by the additives in the gasoline. The gas itself burns cleanly but the additives don't. Each gas company (refiner) uses a different mix of additives to achieve their purposed. To keep the carbon from building up in his engine he just changes gas companies every 5,000 miles. It works for him but I haven't tried it yet.

If your carb is set too rich you will build-up the carbon faster. If you run too lean you can burn-up your engine and it will tend to overheat on hot days. You want to run slightly toward the lean side if you can. This will postpone and reduce the amount of carbon build-up you accumulate over the years.

There was a thread on this topic a few months ago with a lot of good advice. Go back to the archives and try to find it.

Good Luck

Bruce

 

Thanks for some Great Info about this!! I have a 72F250 automatic C-6 390 with the same problem months ago. Seemed that I had the throtle plates open to far in combination with the idle mixture.
As I closed down the idle speed, then re adjusted the idle mixture to get the best Lean Idle mixture things worked out.
I also had to replace the Brake Booster check valve!! Back fire blew it out!!

 

Justin, until you get the problem corrected, if it's an automatic leave in gear, if it's not dump the clutch as you turn off the ignition. Put the engine under a load and it will kill it quicker.



John