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383 engine.

blue69runner

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So been driving my car for a while now. It has started a little run on when shutting off the car. like a bur when turning off the car. Have retarded the timing. But still have that. Do you guy's think I have too big of jet's in the carb. This has me stumped. 650 Holley four barrel single pumper on the car. So any thought's would be appreciated or advice. Blue.
 
Depending on the year and model of the car, (you did not specify), some applications had anti diesel solenoid devices, to prevent run on. Idle speed, timing settings (static setting), coolant temperature and mixture will affect the tendency to diesel upon shut down. As mentioned, need addition information to offer possible solutions.
BOB RENTON
 
I'm guessing it's a Blue 69 Roadrunner for some reason.
Most likely you have fairly high compression, carbon buildup, low octane fuel and a mild cam.
You have spontaneous combustion from the hot carbon buildup.
Retarding the timing will have no effect as it's happening after you turn off the ignition.
In fact retarding it will iikely make it worse in time as you are incompletely burning the fuel.
It's likely running too rich causing the buildup.
Spark plug type might help.
Have the heads ever been off the engine?
 
I'm guessing it's a Blue 69 Roadrunner for some reason. Should not assume.....perhaps the pix is the car...maybe not
Most likely you have fairly high compression, carbon buildup, low octane fuel and a mil9d cam. Would be nice to know specifics....usually high idle RPMs cause dieseling on shut down
You have spontaneous combustion from the hot carbon buildup. This phenomenon is called PRE IGNITION
Retarding the timing will have no effect as it's happening after you turn off the ignition. Other than have caused hot residual metal temps leading to preignition
In fact retarding it will iikely make it worse in time as you are incompletely burning the fuel.
It's likely running too rich causing the buildup.
Spark plug type might help. Depending on what is origionlly installed. Too hot vs cold heat range
Have the heads ever been off the engine?
Not sure if its.germaine other than possibly the head gasket overhanging the combustion chamber
IMO....high idle RPMs snd lean fuel mixture caused by a possible vacuum leak. What is the carb model....have any emission control system parts been disconnected or eliminated? Just thinking out loud.....
BOB RENTON
 
germaine other than possibly the head gasket overhanging the combustion chamber
Every head gasket I've dealt with was always larger than the bore diameter.
The reason I asked about the heads was two-fold:
1. If they've never been off I've seen where the cooling passages skim over with rust and block the passages so that area of the head would be quite hot.
2. Also if they've never been off and the engine burns/burned oil they can get a nice build up of carbon causing the run-on.
I do agree with high idle RPM as a contributor.
 
If you want it to shut off and not run on the RPM'S need to be low like 650 to 750 max. Problems in the tune or engine build. Big engine and big cam will want a higher idle speed. 1-Fix and issues and tune it. 2-If going through step one does not cure it install an adjustable idle stop solenoid. 3- EFI.
 
It's most likely carbon build up. I had the same problem with my '69 Road Runner (383) when I bought it.

Simple fix.
Get a glass of water and a length of vacuum hose. Plug the vacuum hose into one of the carb vacuum ports. With the ENGINE HOT, run the RPM up to about 2000 and stick the other end of the vacuum hose into the glass of water and let get sucked down. That will clean the carbon out of the cylinder and end your problem. I did mine about 5 years ago. It's just about ready for another cleaning. Works like a charm.
 
Ok here it is. Motor was rebuilt and heads done. Thumper cam like the purple cam. Running 650 Holley single pumper. Running 80 Jet's and metering plate. Running RPM at around a thousand due to A/C. Almost 10 to 1 compression. It did not do this Until lately it started acting this way. Yes, it is in a 69 Plymouth Road runner with an A833 transmission. 8 3/4 open diff with 3.55 gears. Stock exhaust manifolds. Dual exhaust 2 inch. Plugs seem to be reading right except the back one's show a little rich. Have been thinking of replacing the front jets with 76's in steed of the 80's. So maybe this will give you guys the info you asked for. The car dose run well on the road no stumble. Just replaced the fuel pump because it had a bad diaphragm after 1500 miles to a Holley manual or mechanical. But it was doing this with the defective pump also. Have not seen any vapor lock in the carb.
 
It's most likely carbon build up. I had the same problem with my '69 Road Runner (383) when I bought it.

Simple fix.
Get a glass of water and a length of vacuum hose. Plug the vacuum hose into one of the carb vacuum ports. With the ENGINE HOT, run the RPM up to about 2000 and stick the other end of the vacuum hose into the glass of water and let get sucked down. That will clean the carbon out of the cylinder and end your problem. I did mine about 5 years ago. It's just about ready for another cleaning. Works like a charm.

IMO....when in industry, in charge of the maintenance department including ALL outdoor material handling machinery, specifically 25 ton and 35 ton crawler cranes with magnets, powered by GM/ALLISON 8V-92 TURBO diesel engines running #1 fuel (50% #2 + 50% kerosene), used 2 gallons of butylcellosolve per tank of fuel once/month. Butylcellosolve is an excellent carbon solvent when added to the fuel. Upon engine overhaul there was no trace of carbon build up on the pistons, exhaust valves or cylinder heads. Could be introduced as described by #6 above, without the possibility of water accumulation inside the engine, causing rusting.....
Just talking out loud.......
BOB RENTON
 
If there is a working power valve in the primaries 80 jets are way too big. Stock would be 70 or 72. I used to run 80's in the secondaries with no power valve.
 
My point was you want about 10 numbers less with a power valve.
 
It is not a jetting problem. It is an rpm problem...mostly, but hot running engine wouldn't help. 1000 rpm idle. Those silly cams, whiplash, hogwash or whatever they call them with too much exh duration adds to the overlap. More cam overlap means a higher idle speed. That cam will need 30-40* of timing at idle for best idle quality/highest vacuum. This would allow the engine to idle at a lower rpm, but cannot say if it would cure the running-on. It would help.
 
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