Koda

Small question, no need for the manifold vs ported wars, but, with vacuum advance set to manifold, idle should see initial and vacuum timing. If one were to move the vacuum hose to ported, one would lose that timing at idle.

If someone were having dieseling, would that effectively back off the timing at an engine shutting down condition and help?

(I am working on other things like installing an idle stop solenoid and adjusting idle rpm to work on it, and checking total timing, but this is simply a hypothetical question for now)

oldcutlass

Yes, going from manifold to ported vacuum will both lower your timing and idle speed. Check your dwell to make sure your set to 30*, set your timing to spec or 2* higher, and set your idle speed to 650rpm. Timing is not generally the issue with dieseling, most often it's specifically the idle speed being too high.

cfair

Not sure I can directly address your question, but I have my 455’s set to 9-11° initial timing at 750-800 rpm with no vacuum to the advance can and the line plugged to prevent vacuum leak.

Basically I set the intial timing with no vacuum, then reconnect the vacuum can. When you reconnect the vacuum advance can, this always spikes the idle RPM so I back off the idle screw to get back to 750-800 rpm in Park with the vacuum advance can connected.

This generally gets me to timing that doesn’t diesel. I should mention that I’m using non-stock vacuum cans from a Buick which has less advance than Olds used owing to our crap gasohol these days.

The diesel could be from carbon deposits in the cylinders igniting. Run a few shots of carb cleaner through the carb and try a few cans of your favorite cleaning stuff like Marvel Mystery Oil or Lucius whatever or Berryman’s and see if that helps. My view is that with our big engines and big gas tanks, 2 bottles is probably required to make any differnce.

Hope this helps. Just one guy’s opinion.

Chris

don71

I'm sensing a need to remove carbon deposits from the combustion chamber.

I'd want to rule that out early on, as a consideration.

matt69olds

The best way to clean carbon deposited is a slow tickle of water down the carb.

Get the engine hot, a long drive at highway speed is ideal. Immediately, remove the air cleaner, rev the engine, and pour just a trickle of cold water down the carb. The idea isn’t to flood the engine, but enough water for it to bog down just a little. When the cold water hits the hot carbon, it will flake off and get blown out the tail pipe.

Many years ago I worked at a Cadillac dealership. A Fleetwood with the sbc came in with what I would have immediately thought as bottom end noise. The tech that was working on it listened for a few minutes, decided he didn’t think it was crank related. He did the water down the carb (throttle body in this case) followed by GM top engine cleaner. The noise was gone. There were 2 huge black rings of soot, one below each tailpipe. Clearly, this was a car that had never been run hard!! I would have never guessed a engine could be so carboned up for the piston to actually contact the cylinder head, but it was obvious in this case that is exactly what was happening.

Hammerdrop

I have the same dieseling problem with my wife's stock 71 2bbl 350. I've using 93 octane to solve it, but maybe my newly acquired bore scope might be useful here. I'm not sure how much carbon deposits would be normal. Engine has 160k and is original.

Koda

I think I will check for deposits like described. The thing has a more aggressive than stock cam in it, which I did not put in there, nor do I know its specs, so idle is artificially high by a bit. I will get timing and idle rpm and dwell and make another thread. Misbehaviors are a hard hot crank, including not cranking sometimes, dieseling, and I believe it pings a little when romped on. I can get mechanical, vacuum, and centrifugal timing, as well as dwell and idle rpm later this week. Related to engine crud, the carb linkage will stick and be broken loose on first use, it's like there's deposit scum in the primaries at the throttle butterflies. I will pull carb and figure it out.