vwtdi

66 L79, electronic ignition. Last summer it started backfiring every now and then and progressively got worse before I put it away for the winter. Got it out last month and had it looked at by my local Corvette guy. Long story short the fuel filter was clogged up with what looked like snot as it was described to me. I thought I was doing the car a favor once in a while by giving it a tank of Sunoco 260 with ethanol treatment added, never again. Along with the filter, the carb was rebuilt, new spark plugs and wires, timing checked, oil changed. It runs great when you get it up to speed, but it stumbles from a stop if you’re sitting at a light for a while, and it wants to keep running when it’s shut off. I have about a half a tank of last year's gas in it which I think is ethanol free reg octane (which it always ran fine with). I got a bottle of Techron at Advance and dumped that in and am trying to get that run through as well as drain the old gas out. Hopefully a fresh tank of mid grade ethanol free and another bottle of Techron will get rid of the dieseling, but I was wondering if the timing may be off. I’ve had it for 6 years and have run a lot of reg ethanol free gas and it’s always shut right down.

Nowhere Man

dads L79 runs like really bad on 89, but 93 runs fine. but sounds like timing is off. unless your widget is breaking up or your cap/ rotor/ wires/ plugs are bad

DansYellow66

Dieseling can be as simple as the idle is set too high but that would not cause the stumbling on acceleration. For that I would check the accelerator linkage adjustment and make sure it’s adjusted to squirt fuel immediately.

leif.anderson93

What's your warm idle speed? Should be 700-750 RPM. Too high can cause dieseling when shutting the car off.
Check your accelerator pump shot is adjusted correctly. Should shoot two nice streams of fuel as soon as you start to apply acceleration. Not adjusted properly will cause an off-line stumble.

JF in MI

Dieseling (and off stop stumble) could also be caused by too lean idle adjustment. Easy to start there.

Sky65

Check base timing with vac disconnected and insure it is correct. Next make sure the vac advance is hooked to manifold vacuum. If hooked to ported vacuum idle speed is too low and tuners tend to raise the idle speed with the idle screw. This opens the throttle plate to far and you get into the transition circuit in the carb and it will run-on and well as lack performance. May also tend to run hot at idle.

Tom

vwtdi

Thanks for the replies everyone, all helpful and appreciated. I’ll report back when progress is made.

Factoid

Given it ran well before and this issue only happens at lower rpm, I suspect the vacuum advance is not returning to base causing the advance to be too high at idle. Inspect the plate in the distributor carefully and ensure it is free of corrosion and moves freely returning fully and freely to zero when released. If you check your base timing at idle and it is too advanced, don’t make any adjustments until you ensure the vacuum advance is working properly. I’ve seen people chase their tails for weeks when they adjust base timing only to then find an intermittent problem when the vacuum advance plate sometimes returns to zero advance and the engine stalls.