dieseling 351C

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Jan 25, 2006 | 12:43 PM

califasbob

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From: Benbrook, Texas

dieseling 351C

My understanding of why gas motors diesel or run on when shut off is that deposits in the combustion chamber stay hot enough to ignite the fuel vapor even after spark stops. I wonder if there might be other reasons?

My Cleveland is very mild with a Melling towing cam, Edelbrock intake and Holly 600 carb with iron 2V heads from a 351M (with the bigger combustion chambers) and Badger flat-top pistons. I don't exactly know the compression ratio, but I estimate it at 9:1 maximum. I started this motor on premium, after experimenting with 87 (it pinged) and 89 (it didn't) I started running mid-grade gas (89). Even though it doesn't ping, it does diesel at shut-off. Could the fuel be the cause? It's timed at 10 degrees advanced with the vacuum advance disconnected and plugged with total timing in the 30's somewhere (stock DuraSpark ignition advance).

I appreciate all of your ideas and experience!

Regards, Bob

Jan 25, 2006 | 12:50 PM

ford390gashog

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From: Brentwood,CA

your idle is too high , your base timing is to far advanced (verifiy the balancer has not slipped) and running too rich.

Jan 25, 2006 | 01:34 PM

califasbob

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From: Benbrook, Texas

Thanks, idle is about 8-900 in park and 600-ish in drive. I could probably come back down 100 rpm or so. The balancer is rebuilt from Damper Dudes in Redding, CA so it should be a good timing indicator. Spark plugs are nice-n-tan on all 8 cylinders so the mixture seems okay. I'll fiddle with the idle first, then.

You don't feel that the octane is an issue, then?

Thanks!

Bob

Jan 25, 2006 | 03:33 PM

Torque1st

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Get the advance unit calibrated. Then adjust your carb.

Jan 25, 2006 | 03:41 PM

califasbob

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From: Benbrook, Texas

Thanks for the reply, I'll fiddle this weekend starting with the vacuum advance. Eric, what would you shoot for in terms of total timing? 10 initial, 35 total is about where it is with the stock curve right now.

Bob