Im running a early 90s carbed 350 with bolt ons.
holley 600cfm 4 barrel carb, weiand intake, shorty headers, accell hei distributor with a accell super coil.

Im having a problem with dieseling, the car stumbles after i turn the ignition off.
Currently im running AC delco r45ts spark plugs, and im wondering if moving down to perhaps r43ts plugs would improve the situation?

compression was good on the motor so im looking for any suggestions to get rid of this problem, any help appreciated

thanks.

 

Is your idle speed in spec?

 

You have hot spots being sharp edges, carbon deposits, or metal burrs in the combustion chamber(s) that get red hot and continue to ignite the fuel even after the ignition is turned off. You might try running Seaform thru the carb while keeping the rpms up in neutral so it doesn't die on you to clean out deposits.
That won't solve the root cause, or keep it from doing it again.
Is the engine misfiring, have you visually inspected the spark plugs?
Open the hood in a totally dark area and check for arcing around or along the spark plug cables, remember these are not wires, they are made of compressed carbon particles, heat stress, and physical flexing/pulling can and will change it's resistance.

 

idle speed is about 1000 in park 800 in gear.
Runnin 14 degrees of initial timing.

ive changed the plugs, changed wires.
ive run several cans of seafoam over a period of 3 months to no avail. So i doubt carbon is an issue. Plugs are pretty clean, slightly lean but clean.. compression checks out.

so im running out of options..

any ideas?


EDIT: even swapped motors and am still dieseling.. still using r45s

 

I'd lower your idle speed to specs and see if the issue goes away.

 

Agree, start with idle speed first, its the easiest. Try get it down to 800 in park, see if that helps.

Then verify timing, then what rgarcia said.

 

No carbon deposits doesn't rule out hot spots in the chamber.
Running lean, improper timing, and idle as was mentioned can cause this problem, but if you don't think that's possible then get a camera snake and look through the spark plug hole to see if there is anything that can cause a hot spot.

 

guys he has had the same issue with his old 305 and now a completely new 350, that indicates to me an issue with reused components and not the piston/heads causing hotspots.

What is reused? Carb, same plug type, and distributer maybe?

Additionally his headers are FREAKING HOT, even compared to my turbo headers - is the tune way out (jets need changed)? and the motor is running lean and really hot? or rich and dumping too much fuel which is burning in the headers? My money is on a fueling issue

 

ill try the idle speed.

only things i re used was the intake, plug type, dizzy, and carb after the motor swap.

 

John, just buy a new car. Haha.
Did you finally change out your plugs? And I believe your dizzy isn't for that carb cause it looks like an aftermarket oem replacement for a q jet.
Alex, do you still have my manifold gaskets?

 

u believe wrong..
its a non computer controlled Hei with a 4 pin module. And msg alex about ur gaskets im trying to stay on topic here thanks..

 

First set your timing around 6*BTDC (if has a vacuum advance, remove the vacuum line to the vacuum advance and plug it) then adjust your idle speed for 700-750rpm in drive.

If you adjust your idle speed first, your timing adjustments will effect the idle speed. You'll just end up having to re-adjust the idle speed.

Finally, hook up a vacuum gauge (manifold vacuum, not ported) and adjust your idle mixture mixture. Adjust to obtain highest vacuum reading

 

ill be trying this. Now should i set my initial timing back to my current setting of 14* after the adjustment? i seem to be running fine no hard starts or pinging at 14* or do u suggest staying at 6*?

 

No, you would not want to re-set the timing back to 14*BTDC after going through what I described earlier. You'd just end up back where you started.

If you insist on running the ignition timing and idle speed outside of "typical" settings, I would suggest you not do anything other than keeping the transmission in gear when you shut off the engine. Keeping a load on the engine by keeping the transmission in drive (or any other gear) will help prevent the run-on you're trying to eliminate.

 

Loading in gear wont work with an auto.

6 degrees sounds way too low for a non cc setup. Eg mine is 15 initial, just happened to be what the motor seems happiest with. Starts and stops perfectly.

 

are you certain the spec is 6*? i always thought thats what the tbi/tpi guys ran. Wouldnt having a non computer controlled Hei set up change that spec? last time i tried setting the timing that low the car ran horribly backfiring through the carb.

 

thats what i thought too, at 14* initial i have zero starting issues

 

6 degrees or whatever it is, is the base timing you set with a cc system so that the computer has the correct base reference. The computer then adds whatever timing it sees fit on top of that.

Also, I am using manifold vac for the vac advance. Puts my idle timing around 23.

 

yeah thats what i originally thought.

im also running full manifold vaccum advance instead of a ported source.

 

Update: I turned down my idle speed and im still dieseling right now im at 900 in park and about 700 in gear.

im running 14* of initial timing, also have the vac advance connected to full manifold vac.

Ive noticed that if the car gets warmer (about half way on the temperature gauge) it will not diesel. What could cause it to not diesel at a higher engine temp?

Should i bump up the initial timing to maybe 17-18*?

Anyone got any other ideas?

 

Just my experience with this: When I got my car, it would take ages to start, run on like crazy, had no *****, bogged off the line, carb leaked fuel and used insane amounts of it, and needed the idle set really high to even keep running. Holley 4175 swapped in place of the original TPI by some PO.

All of these problems were eventually solved with a full carb rebuild and tune, timing tune, and intake and heads removed and de-coked (as an incidental during cam swap). Particularly the back face of the valves, mountain ranges of built up oil, coke and crud.

The ign timing was extremely retarded iirc, somewhere under 10 init. The carb was in a really sorry state. The primary butterflies were propped way open just to get it to idle at all, and the transfer slots were fully exposed. The secondary idle screw was also propped way too far open. Had jets way too large, and the power valve was leaking. There were gaskets leaking both fuel and vacuum, and stripped throttle-to-main body screws.

I never determined what exactly stopped the dieseling for me, but I think it was just with the carb and timing tune-up stuff, before the heads/intake stuff which came later. Just noticed I'd cured it one day. After re-kitting the carb, this allowed it to idle 800 park 650 gear with the butterflies (both prim and sec) in sensible positions.

Ie, the running on disappeared during a holistic approach to general tune-up.

 

I still say you need to bring you idle down to 750ish in park range, like most folks have been saying on here thats where most dieseling comes from, in my experiences.

 

If the headers are getting hotter than normal, that indicates a lean condition. This could be caused by a vacuum leak or carb out of adjustment. Just at a glance, if you took a carb previously optimized for a stock 305 and dropped it on a warmed up 350, it's going to run lean.