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Old 03-12-2008, 12:56 PM
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Dieseling ?sometimes?

I have a 350 sbc in my '55 that will keep running sometimes after I turn the key off.

However, it doesn't happen all the time but maybe 10% of the time.

I have replaced the plugs and the old ones looked pretty good. I also try and run a can of Seafoam at least every second or third tank since I generally buy 87 octane gas.

Any suggestions will be appreciated.

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Old 03-12-2008, 01:08 PM
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What is the compression and initial timing of your engine? If the timing is too advanced or the compression is too high for 87 octane this may occur. Stop running so much seafoam and buy higher octane gas and I bet it will quit.
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Old 03-12-2008, 01:14 PM
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Quote:
Originally Posted by Double_v23
What is the compression and initial timing of your engine? If the timing is too advanced or the compression is too high for 87 octane this may occur. Stop running so much seafoam and buy higher octane gas and I bet it will quit.
haven't checked the compression or timing mainly because it doesn't do it everytime and I understand what you are saying about the higher octane gas which is very well what I may have to do

got a small trip to take her on this weekend so I think I'll put a high octane in her both ways as see what happens

can Seafoam be bad for a car as long as it goes in a full tank but not every fill up ???

thanks,
wr
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Old 03-12-2008, 01:26 PM
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Is your idle too high? Throttle plates open a little too far? Could be close to transitioning from idle to throttle opening, look at your transfer slots.
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Old 03-12-2008, 01:28 PM
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If the idle speed is too high, it will over-run.

Take a look at some 70s Quadrajet carbs, they have an electric idle solenoid. When the key shuts off the power, it closes the carb near completely...just to cure that over-run situation.
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Old 03-12-2008, 01:38 PM
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Great info but I know nothing about a 4bbl carb so bear with me here. I have a 4150 Holley DP (manual choke) and will probably need to find a diagram to look at so I'll understand more of what y'all are talking about.

REMEMBER - - it doesn't do this but about 1 out of 10 shut offs - - OK?
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Old 03-12-2008, 02:14 PM
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The timing and high idle can be related, if you back off the timing the idle will go down in RPM. If you get your hands on a timing light with a tach you could kill two birds with one stone.

As far as seafoam goes, I think if God had wanted the General to use additives he would have put them in the crude.

A well tuned engine shouldn't need anything but fresh gas and a few filters to run cleanly.

This next part is just a theory...the Seafoam could be lowering the octane of your fuel enough to cause the dieseling.
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Old 03-16-2008, 02:52 PM
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Quote:
Originally Posted by Double_v23
The timing and high idle can be related, if you back off the timing the idle will go down in RPM. If you get your hands on a timing light with a tach you could kill two birds with one stone.

As far as seafoam goes, I think if God had wanted the General to use additives he would have put them in the crude.

A well tuned engine shouldn't need anything but fresh gas and a few filters to run cleanly.

This next part is just a theory...the Seafoam could be lowering the octane of your fuel enough to cause the dieseling.
FOLLOW UP - - BUMP
Hey, Double, I took your advice and put 93% octane in her for a High School Reunion trip this weekend. She never dieseled while we were cruising around and having fun yesterday so I filled her up with some more 93% this morning and headed home. Topped her off when we got home and discovered I had gotten 18.8 mpg on the trip home. She never dieseled one time with the 93 so hopefully - - - prob solved- I HOPE

thanks,
wr
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Old 03-16-2008, 02:59 PM
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My truck does it more often on a certain stations gas....use a different stations gas and its fine.....then can get some gas at the problematic station sometime later and its fine....Just a bad batch of gas I guess.


Shane
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