Engine cough and grumble
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Xcudame
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Dieseling. It's usually caused by an unburnt gas and air mixture hitting a hot spot of carbon in the cylinders.
Xcudame
Well-Known Member
When was the carburator rebuilt last? Sometimes too fast of an idle, too rich of a mixture can make it even worse.
Xcudame
Well-Known Member
Check out this post. It's got some great information from the good folks here.
TIMING & RUN-ON
TIMING & RUN-ON
Xcudame
Well-Known Member
Sometimes you can hold the throttle open (higher RPMs - 3000 or so) and spray some carburator cleaner down the throttle bores and clean up a lot of gunk that's accumulated over the years. Hince Gunk Carburator Cleaner. It really should be a relatively simple fix. Good luck!
Chryco Psycho
Well-Known Member
3 causes , timing , idle speed & fuel mixture , also hot spark plugs can add to the problem , mostly just accurate tuning .
Adam
Well-Known Member
If I remember correctly, the main cause of this is an idle linkage screw adjusted too high. This is how it happens: someone wants to get a better tune and turns the idle mixture screws in to lean it up, then they turn in the idle linkage screw to speed up the idle. The engine then pulls fuel from the idle circuit and the jets; enough to almost keep the engine running. I know it sounds counter intuitive but on a warmed up engine, back off the idle linkage screw and increase the idle mixture adjustment screws (rich) enough that the engine will idle at an acceptably low rpm. If you have an automatic trans adjust it so it will idle in gear. This is something you can do in a few minutes and costs nothing.
This assumes your carb is not leaking internally. What kind of carb are you working on? Sometimes a Thermoquad will crack at the bottom of the wells; sometimes a Holley‘s gaskets will shrink if it sits too long, and other causes.
This assumes your carb is not leaking internally. What kind of carb are you working on? Sometimes a Thermoquad will crack at the bottom of the wells; sometimes a Holley‘s gaskets will shrink if it sits too long, and other causes.
heminut
Well-Known Member
Run-on (dieseling) isn't affected by timing. Once the ignition is shut off there's no current to the coil so there is no way the plugs could be firing. It's usually caused by the idle speed set too high and/or carbon hotspots in the combustion chamber.
An old mechanics trick to clean carbon out of the combustion chambers is to pour about a half a quart of transmission fluid in the carb with the engine running. You will have to keep the idle high and slowly pour the trans fluid in. The last bit should be poured fast enough to stall the engine. Let it sit for about 1/2 an hour and then start it up again and using the same process pour about 16 ounces of water into the carb but don't kill the engine with the last bit, keep it running. After doing that, take it out and drive it for about 15-20 minutes to clear everything out.
WARNING! Do this outside, NOT in the shop!
I have actually pulled the heads from a Chevy smallblock after doing this and it was amazing how clean the valves, chambers and piston tops were!
An old mechanics trick to clean carbon out of the combustion chambers is to pour about a half a quart of transmission fluid in the carb with the engine running. You will have to keep the idle high and slowly pour the trans fluid in. The last bit should be poured fast enough to stall the engine. Let it sit for about 1/2 an hour and then start it up again and using the same process pour about 16 ounces of water into the carb but don't kill the engine with the last bit, keep it running. After doing that, take it out and drive it for about 15-20 minutes to clear everything out.
WARNING! Do this outside, NOT in the shop!
I have actually pulled the heads from a Chevy smallblock after doing this and it was amazing how clean the valves, chambers and piston tops were!
Chryco Psycho
Well-Known Member
Not trying to start a fight but timing will cause run on , retarded timing adds a lot more heat to the exhaust valves & combustion chamber , shut the engine off & more heat detonates the fuel being drawn into the engine .
moparleo
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Idle screw not letting throttle plates close all the way. You can just put your hand over the carb and completely block airflow. If it stops dieseling, the throttle is not closing completely. Do a complete visual, underhood inspection to see if there is anything obvious. If you are not really comfortable under the hood, get someone to help you out that is.
The note on ATF through the carb works. Just have to trickle it in BUT what wasn't explained in detail is that it will SMOKE HUGE CLOUDS OF WHITE SMOKE like there is a major fire somewhere ! Needs to be done away from the house/ garage as a neighbor may call the Fire Dept.
Spraying water down the carb will also knock loose carbon out of the combustion chamber.
Pull the spark plugs first to see what they look like. That can tell you a lot about the tune/other possible problems.
The note on ATF through the carb works. Just have to trickle it in BUT what wasn't explained in detail is that it will SMOKE HUGE CLOUDS OF WHITE SMOKE like there is a major fire somewhere ! Needs to be done away from the house/ garage as a neighbor may call the Fire Dept.
Spraying water down the carb will also knock loose carbon out of the combustion chamber.
Pull the spark plugs first to see what they look like. That can tell you a lot about the tune/other possible problems.
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