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.