What is causing my 300 to diesel and is it hard on the motor? The motor was recently rebuilt (complete)and has under 2,000 miles on it. I had to retard the timing to about 2-3 degrees BTDC to eliminate spark knock. This motor is a 1978 (truck is '86) with duraspark ignition and no emissions equipment. When turning off the key, though, the engine runs-on. Its pretty annoying and goes a long way toward making a new motor seem not so new. Any suggestions besides higher octane gas?

 

Are you absolutely they completely rebuilt the engine? Did you observe this? Was this engine from a salvage dealer? or an engine rebuilding company? Did you get any type of guarantee?

I would not consider higher octane. That engine was made to operate on regular. Did the rebuilder grind the heads down? If they did it would raise the compression and make it knock.

The "running on" after you turn the key off is usally a sign of timing off. Get a book for your engine and find out what the exact timing is supposed to be and set it at that. Be sure you follow exactly the directions for setting the timing and advance. You may be instructed to plug vaccuum lines or disconnet a circut. Just do as instructed.

 

[font size="1" color="#FF0000"]LAST EDITED ON 31-Aug-00 AT 09:06 PM (EST)[/font][p]The motor came out of my '79 F100 (not the original motor-year unknown) and I was at the shop periodically through out the rebuild process. He is a local builder who has a (NAPA) machine shop next to his brother's (a friend of mine) NAPA parts store. They both come with a spotless reputation and the motor with a 30 day warrantee. Motor was completely rebuilt including the head but I'm not sure if he ground the head. As far as the timing goes, this motor was a late '70-s-early '80's with a '78 ig module and accessories. There is no timing spec as the motor and truck is kind of a Mr. Potato Head-everything lists timing for a certain calibration-this one never had a sticker on the motor (but had full emissions when I got it) and I'm mixing and matching engine years, ignition modules, and nixing (capping) all emissions vacuum ports. I have 2 other old 300-6 Fords (78 and 79) with same non-emssions setup- they all run at 6 to 10 degrees BTDC. When timing this motor, I plugged the vacuum advance, set the RPMs to 500 (since no sticker I averaged the other 2 motor stickers) and experimented with the timing 'til spark knock disapeared. Sorry for the long post but this has gotten alittle irritating...could the ignition module be bad?? This was an EFI to carb conversion and I had to learn as I went...

 

Most engine run on is caused by to fast of an idle speed. Check fast idle cam to make sure the choke mechanism is all the way open and on the lowest step of the cam. If ok is there a stepper motor of some kind one your set up? Like you would find for an A/C on idle speed. If so,is it interfering with the base idle speed? A lot of carbs will have something that holds the throttle shaft just off the base idle screw and then let it fall to base idle when the key is turned off. This is a low enough setting so that the engine won"t diesel. If none of this applys just try to find a way to lower idle speed with out compromising idle quality.

 

Hmmmmm...it does seem to idle a little high even after warm up. Goosing the pedal will often bring it down. I'll check idle adjustment to see if that helps. Thank you both for the help.
John

 

How are the ignition parts? Did you go with new
wires, rotor and dist cap? Air cleaner? Plugs? I assume you did but they make a difference with this engine. Mine runs at 900 rpm - a bit high - but does not diesel unless I get lazy and don't keep the tune up.

 

>and make it knock.
>
>
>The "running on" after you turn
>the key off is usally
>a sign of timing off.
> Get a book for
>your engine and find out
>what the exact timing is
>supposed to be and set
>it at that. Be
>sure you follow exactly the
>directions for setting the timing
>and advance. You may
>be instructed to plug vaccuum
>lines or disconnet a circut.
> Just do as instructed.
>
Timing has nothing to do with "dieseling" or "running on". When the ignition is "off" there is no spark to time.

 

The inline six in my wifes van had this problem, after much thought we found it easiest to leave the trans in drive and then turn it off, end of our problem DF