[300hp]

allright guys, ive got a 383 in my 68 gmc, decently built up, vortec heads, 10:1, msd 6al with probillit dist. ive got the timing about 34 total, about 16-18 initial. when it warms up it will diesel/run-on almost every single time i shut it off for 3-5 seconds. i run 91 octane, and its got a pretty big cam(not sure on specs) it doesnt ping. why is it doing this and how can i fix it? ive seen in the instructions about a diode, but its got a 1 wire alternator on it. is this my problem?

 

[hjdca]

It is a combination of timing, idle rpm and mixture... plus, when warmed up the chamber gets more efficient and the motor wants to run on... If is amazing how the A/F ratio gets richer at idle from 160 deg to 200 deg. radiator degrees... This increased efficiency makes the idle get higher.. Tune the car for good running when it is hot, and put up with the rough running when cold...
Reduce the idle by 150 rpm and richen up the fuel mixture screws by 1/4 all the way around.. that should help..

 

[69ZL1]

Do you have your vacuum advance hooked to manifold vacuum? Most dieseling issues I've seen have been caused by excessive throttle opening due to lack of vacuum advance at idle.

 

[Mike]

Set up properly ,this or the OEM GM version should solve your problem.

 

[berner]

If all else fails, just shut it off in gear.

 

[jeff swisher]

I must go with Mike on this one..I have 2 of those Step up solenoids left from some old Qjets...set idle off of them, You know how to do that?

I had A 350 with 245psi cranking pressure absolutely would not die,,,even in gear at 500rpm she would still run on....When voltage drops out from the solenoid the throttle plates shut completely killing all air and she died instantly.....My uncle who raced asphalt stock cars turned me on to them suckers.
I think they were normally activated when you turned on the AC unit

 

[SWHEATON]

allright guys, ive got a 383 in my 68 gmc, decently built up, vortec heads, 10:1, msd 6al with probillit dist. ive got the timing about 34 total, about 16-18 initial. when it warms up it will diesel/run-on almost every single time i shut it off for 3-5 seconds. i run 91 octane, and its got a pretty big cam(not sure on specs) it doesnt ping. why is it doing this and how can i fix it? ive seen in the instructions about a diode, but its got a 1 wire alternator on it. is this my problem?

==========================

10.1 comp on only 91 oct fuel is likely the issue here even with a fairly lrg cam effectively lower cyl pressure a bit.

Remember,gm stated that even the lower perf bb'cs like 396/325hp-350hp with just over 10.0 compression must be run on a minimum of 98 octane leaded fuel and not 7 octane points lower /non leaded/oxygenated/ethenoled fule thats a totally different fuel blend these 40 yr olde motors were designed to run on.

Get a quality octane booster like Torco's online and mix for approx 97-98 octane and i bet the run on stops.

If you find some real lead booster like kemco lead suprreme 130 or max lead 2000 thats the real deal that works gret but unfoirtuntaley as of 2010 mfg of those real lead octane booster products ceased.

I have hit this same exact issue many times over the yrs when guys are running fairly high comp with todays crappy 91-93 pump fuel.

Some get away with it fine with very lrg cams /tons of gear/stall converter all effectively reducing load on motor etc .

But your motor doesnt like it 91 fuel though theres no detonation it can still run on due to low octane on 91 with 10.0 comp.

also,if its running hot ,has too hot of heat range plugs,doesnt have have a vac adv adding some timing at idle/excessive carbon build up in combustion chambers due to oil use or too rich mix are other things that can also make it run on too.

But in your case my money is on it wanting more octane.

scott

 

[Guest]

Either the anti-dieseling solenoid, or more octane will fix it. Pick your favorite method and you'll be good to go.

 

[SWHEATON]

FYI,KEMCO TYPE REAL LEAD BOOSTER IS NOW AVAIL AGAIN FROM WILD BILLS CORVETTES,SEE LINK POATSED BELOW FOR DETAILS.

I JUST ORDERS & RECIEVED 2 CASES FROM WILD BILL SO ALL IS OK WITH NEW SUPPLIER OF SAME TYPE REAL LEAD BOOSTER PRODUCT.

SCOTT
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http://www.wildbillscorvette.com/OctaneSupreme01.htm
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FYW,when a motor runs on post shut down due to needing more octaner giving it the additonal octane in needs like this real lead booster provide can increase power some too which is another benefit .

Excessive lean idle idle mixture ,incorect-too hot heat range spark plugs ,retarded timing (esp retarded base timing!) and or not running a vac adv making for less timing at idle and lower engine speeds both of which can make a motor run hotter can all collectively add to dieseling/running on issues post shutdown too.

scott