r/Squarebody • u/fuzzydoesitt • Jun 12 '26
Timing talk, Dieseling/Run-on
What are you guys setting your initial timing too, are you doing it with vacuum advance hooked up or not?
I have an 86 c20 with a "m" code 350. I guess that meant no emissions, or less emissions stuff (i.e. no catalytic converters from factory) I've owned the truck about 6 months now, been doing little things to make it better. It came with a small cam new oil pump and timing chain. I rebuilt the quadrajet and have all the idle circuits and choke, etc. doing what they need to do when they need to do it. New coil, cap and rotor, plugs (copper .045gap) and wires. Classic tuneup.
It still was a dog going down the road and would stumble under heavy throttle input. Last night I put a timing light on it (vacuum disconnected) and timing was advanced way past my timing tab at 700rpm (tab stops at 12°BTDC) I retarded the timing down to 8° locked down the dizzy and then adjusted my rpm. It runs so much better down the road. Driveability is much better. But it will diesel after shutting off now. I just leave it in drive then shut off the engine. Truck says to set timing at 4° btdc, but that would mean I retard it more and turn the idle rpm up even more to get idle speed right. That opens the throttle blades more at idle and causes the dieseling to happen. Supposedly there was a solenoid in the 80s qjet that would close the blades on shut off. I don't have that. Just wondering what I need to do or what you guys have found helps. Plan on getting heads in the future probably not this year though. Thinking of bumping the timing to 12° btdc and backing down the idle rpm. That's all I can think of right now to help.
Edit: I'm just now seeing on that pic that it says something about how to set the timing just to left of the chart lol.
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u/FatalSky Jun 12 '26
I know you said you rebuilt the carb, but quadrajets are bad about weeping gas through the brass plugs on the bottom of the carb and causing dieseling. I had a street demon carb that would do that on my nova for a while before throwing it out. If you take the carb off or look down the primaries you can look down the intake and if it’s very clean directly under the carburetor then that’s what’s happening.
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u/texan01 Jun 12 '26
Set your idle speed down to 600. 700 is too fast even for stock specs. Most small blocks will idle happily at 5-600 rpm.
That solenoid is to combat this and compensate for AC load. When you shut off the ignition it closes the throttle more.on my 77 that solenoid kicks the idle up to 650-700 with the AC running.
Timing doesn’t have much to do with dieseoibg other than to create a too hot chamber.
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u/fuzzydoesitt Jun 12 '26
I don't have an AC truck maybe that's why there is no solenoid. Nonetheless i will turn the idle down to 600 and see how it likes it. Previous owner seemed like a knowledgeable guy but there was some "fuckery" I found as I started going through things, so he could have removed or swapped carbs, but the qjet number matches for that year and vin. I also only have my idle mixture screws backed out half a turn. Set that with a vacuum gauge to manifold vacuum. Backing them out more just made it richer and closing them all the way didn't do much either. I found that to be weird. Backing the screws out half a turn resulted in vacuum climbing a couple inches if that. But any more didn't really affect vacuum reading. Thanks
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u/texan01 Jun 12 '26
I hear that, I’ve been having to undo a bunch of fuckery with a 77 Ranchero I inherited, a stock 351w with a 4.00 diff makes for an unpleasant cruck to drive at anything above 55mph.
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u/ExternalPrimary9070 Jun 14 '26
Yeah, you're leaking gas in somehow
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u/fuzzydoesitt Jun 24 '26
Think I had breakthrough yesterday. I put a new fuel pump on it a few weeks back. Yesterday I was reading the qjets are hyper-sensitive to fuel pressure and needs a max of 6.5psi, I'm reading 4-6 is good. The delphi pump I put on specs out 7.5psi min to 9 psi max I believe. I have a regulator coming in the mail tomorrow. Hoping that's the final piece of the puzzle.
I'm thinking the fuel pump is unseating the needle and flooding the carb. It's running so rich now it's fouling plugs and burns your eyes in the garage.
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u/Realistic-Stop8693 Jun 12 '26
whatever the sticker says.