Within the last 3 weeks the engine has started to "Diesel" when I shut it off. It will run for a few seconds. Sometimes it's just one small sputter, others it will sputter/knock for about 3-5 seconds.

I know it's a simple problem-occurrence, but I can't get it to stop.

The carb was rebuilt about 2 months ago. Put in new choke pull-down as well. Everything ran fine until now.

---I have adjusted the float up and down = still diesels.

---I have checked the timing = it's advanced 1 degree. Distributor is locked up tighter than anything. Lube and light prying, and it still won't budge for me to get it to go back 1 degree.

-- I run regular unleaded. Original motor form 1977 with 72,000 miles. Could it be a gas issue ? need leaded substitute ??

What am I missing or need to do?

Thanks for your help

 

Id bet $20 your idle speed is too high.

 

I'd bet your timing is off. Time it with a vacuum guage, it's too old for a timing light to be accurate. Start from there.

 

What??? I beg to differ.

 

I'll try the idle screw. I don't have access to a vacuum gauge.

Thanks

 

really? You think that if the timing chain stretches even a little bit the factory timing setting is still good? A timing light is useless after that.But you know, don't listen to me. I couldn't possibly know what I'm talking about. Why don't you give Tim Meyer a call at TMI? He'll be happy to confirm what I just told you. He specializes in the 351M/400 (although what I said is true of any engine with a timing chain). Here is his website. TMeyer, Inc. Precision Automotive Machining

I'll keep my 20+ years of experience as a mechanic to myself. You guys have fun listening to internet researchers and ignoring the real mechanics that take time out of their day to answer questions.

Good luck and have fun.

 

Its got 72,000 miles on it, I highly doubt its stretched THAT much. Granted, it may be a degree or two off, but a degree or two shouldnt cause his dieseling problem. Been there, done that; my idle speed was set too high.


@olblue77, I highly suggest getting a vacuum gauge. They're only $20 at autozone. Use it get your carb set perfectly, then install it somewhere in the cab to use as an econometer.

 

From a new person to this forum - kinda - been lurking since early 2000's and officially joined in 2006 - a welcome to you as well and thanks for responding.

I will also add my reply to your comment: "Lighten Up Francis".

simple questions were asked and simple responses were given.

I will go about looking into the vacuum. If a mechanic talked to me that way in a shop, I'd take my business elsewhere.

 

Overly-advanced timing can lead to dieseling, or run-on, because this typically leads to heightened combustion chamber temperatures, which is a common cause of dieseling. However, of all the things that can cause dieseling, timing is usually the least common.

Dieseling is caused by 2 things:

1) Path for air (and hence fuel) to enter the combustion chamber after the engine is shut off. This can be caused by too high of a curb idle. Some engines require an idle stop solenoid that maintains the idle speed while the key is set to RUN, but retracts so that the throttle plates can close (almost) completely once the engine is shut off.

2) Heightened combustion chamber temperatures. This can be caused by too steep of a timing curve, or too lean of a combustion mix. Losing the EGR valve (by removing it, or it never opening due to a stuck valve or bad PVS) on an EGR-equipped engine is by far the most common cause of this. If the engine pings at cruise (detonation), then it's susceptible to dieseling, and both symptoms are often caused by the same underlying issue.

A vacuum gauge is a great tool that I highly recommend picking up (they are under $30), although I don't think it would be particularly useful for this specific problem. I personally don't recommend timing with a vacuum gauge because you risk setting the timing too far advanced with this method. Ignition timing is a measure of crankshaft position relative to TDC and you need to measure it with the tool that's made for the job. I don't agree that a timing light is useless on an engine with 72,000 miles on it, although I understand the logic behind the claim.

 

Caution: Be VERY patient here as the casting for the dist is weak in a few places and will break when prying. I have broken two. First, when needing to change the timing at Smog, I broke a ear mounting the cap hold down spring. I was in the street AND in a hurry.... The second, installing electronic ign on my '56 292, broke the shaft casting at the hold down clamp ring @ the block. Don't ask... Both were "rust"/"corrosion" frozen in the block.

Still running the the broken ear on my '78. The second on my '56 almost cost me pulling the engine.... Got lucky, has an outside oil pump, and was able to press it out with a special bracket and 5ton jack.

Soak it everyday and put it thru heat/cold cycles and gently tap it one direction then the other a small amount at a time. Mainly don't be in a rush. You'll need to correct this someday; we do need the ability to time the engine ....

Another reason for dieseling is heat. Do you have a parabolic spacer under the carb? or Just a thin gasket? Is the EGR working correctly? Clogged? Is the idle mixture adjustment too rich?

.

 

thanks guys. I will look into the idle and EGR.

 

For what it's worth, here's the truck in the gallery section I posted back in '06. Still the same, except in now has polished alum. 17" rims on mud tires.

1977 Ford F250 4x4 - Ol Blue 77