Loud back fire at shut down

[Anonymous-0]

I'm guessing my timing is off. Never did it before, but now when I turn off the ignition I get a loud backfire thru the tail pipe. Runs and sounds good to that point. I did check the archives thats why I guessed timing.
Thanks for any advice you can give.
Jd

 

[Joe Ferguson]

When ignition switch is turned off there is no longer spark so timing does not affect in this situation. This symptom is generaly caused by carbon accumulation in combustion chambers when engine is running the carbon become red hot and when you turn ignition off the engine is still turning for few turns gas continue to enter and the red hot carbon ignites the gas/air mixture The ignition is then random and cause backfires or engine turns backward for 1~2 turns with knocks "dieseling".

A significant test is to start engine when cold turn it off immediately and see if it backfires if not untill the engine is at normal temperature
it is carbon accumulation.

Carbon accumulation is due to excessive oil burning in a tired engine, incomplete gas combustion due to defective/misajusted points, plugs, coil, improperly adjusted carburetor or...old or poor gas quality.




My 2¢

Joe Ferguson

 

[ebbsspeed]

I"ve got a little 2-cylinder Onan in a Case/Ingersoll 448 that does this, it"ll scare the heck out of you. It happens when I shut it off immmediately after running it at full or nearly-full throttle when mowing, etc. If I let it idle for just a minute or two before shutting it off it doesn"t do it. Every now and then I'll do it intentionally to scare my wife, the neighbors dog, or some other poor innocent bystander. Just turn the key off at part-throttle, and let the motor pump the exhaust system full of fuel/air vapor. Then just before the motor stops turning I flip the key back on and BOOOM!! You can actually pop open a muffler seam when this happens, since the backfire is actually a fuel/air mix that's in the exhaust system that expoldes and makes most of the noise.....

Now don't go telling me it's bad for the motor, blah blah blah, I've heard all that before and don't agree. But I do know that it can be hard on the exhaust system......

 

[Jerry/MT]

Make sure that your mixture is not too lean and that your throttle plate is near closed at shutdown. The lean mixture causes hotter metal temperatures in the combustion chambers and the open throttle allows a fuel charge . As the other post said, carbon in the cylinder head can also contribute to the problem.
On my old "72 Ford, they had a two position idle solenoid that closed the throttle plate fully at shutdown to preclude "dieselling"" or "run on".

 

[Jerry/MT]

Make sure that your mixture is not too lean and that your throttle plate is near closed at shutdown. The lean mixture causes hotter metal temperatures in the combustion chambers and the open throttle allows a fuel charge . As the other post said, carbon in the cylinder head can also contribute to the problem.
On my old "72 Ford, they had a two position idle solenoid that closed the throttle plate fully at shutdown to preclude "dieselling"" or "run on".

 

[Anonymous-0]

Thanks for the replys guys. Looks like it's back to the tool box this week end. Beats daytime tv.
thanks again

Jd

 

[Anonymous-0]

The backfire problem at shutdown is usually caused by glowing carbon in the combustion chamber. Try letting the engine idle for a minute before shutdown. You can clean out a lot of carbon by slowly dribbling a little water through the intake when the engine is hot. The water turns to steam and blasts the carbon away. keep the engine running for a few minutes after putting water in to make sure all water is boiled away.

 

[Paul Hearsey]

Jd, further to the advice on lean fuel-air mixture, also check that you don't have a vacuum leak between carburetor and cylinder head. As a teenager I had a carb on an old Pontiac with a warped mounting flange, and the gasket wasn't able to seal the joint properly. I played around with the needle valves and idle speed to get the engine to run better but created the ideal conditions for dieseling: very lean mixture and a throttle valve set too open at idle postion. When I eventually discovered the problem, I CAREFULLY filed the mounting flange flat, re-installed the carb with a new gasket and readjusted the settings. Result: an engine that purred and no more dieseling! Hope this might help. Paul