backfire

Maclab58

Maclab58

Well-known member
My 31 sedan has started to backfire whenever i let off the gas to shift gears. it also will bacfire very loudly when i turn it off after a 15 or so minute drive. I have the GAV turned all the way off. This cured my 28 sport coupe of the same problem. is it the float? it also tends to die when coming to a stop but always restarts easily. what gives?
 
Back firing when you let off the gas is usually caused by a leak in the exhaust system . Usually the leak will be between the exhaust pipe and manifold or between the manifold and block . Backfireing under load will usually be caused by a a vacume leak that causes a lean condition . Stalling apon brakeing is caused by improper float setting . Lowering the float a small amount , to reduce the level of fuel in the bowl of the carburetor has always solved the stalling problem for me .
 
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There may be two different problems here.

Backfiring is normally due to unburned fuel accumulating in the exhaust system until conditions are right for it to touch off. Paradoxically, it is commonly caused by running too LEAN--the spark plug fails to ignite the too-lean mix, which is then swept out into the exhaust. Could be either a vacuum leak letting too much air into the mix, or carb set too lean. In that brief time between gears, the engine is still trying to fire on every compression stroke BUT, with your foot off the gas pedal, the idle jet can't provide enough fuel to support the high-rpm combustion needs. EIther way, opening the GAV should help temporarily.

The backfiring at shut off may be due to a leak at either end of exhaust manifold allowing fresh air for combustion to be drawn in as system starts to cool down. Retarded timing or anything else that overheats exhaust system will aggravate the situation.

If the problem is electrical, it will show up as backfiring under load.

Steve
 
Backfiring after shut down usually have a couple of contributing factors .
Raw gas has got to be collecting in the muffler . May be that the GAV has been run too open . It could be weak ignition . If the spark plugs are black with soot , this is a strong indication of spark plugs that are too cold and not burning all of the fuel . I had problems with Motor Craft TT10 and Champion W16 Y . I now use Champion W18 with good results . In order for the raw gas to ignite in the muffler after shutdown, air must be present , as in an exhaust leak somewhere between the engine and the muffler .
 
Cleaned the carb Monday with some improvement. I pulled the intake and exhaust manifolds off today....... Not flat...... Dropped off at the machine shop to have it machined flat. Will be ready tomorrow afternoon. Hope that's it. Thanks for the advice guys, us newbies would be in trouble without all of the experience found here on the barn!
 
1930artdeco said:
Is backfiring when going down hill with a hi rpm caused by a manifold to muffler leak?

Mike
Click to expand...

Probably not; see my earlier post in this thread. High rpms trying to get by on fuel supplied by idle jet causes too lean mix to support combustion, so unburned fuel accumulates in exhaust system. Opening the GAV 1 or even 2 full turns when going down a long hill will help a lot; don't forget to shut it back down when you get to the bottom.

Steve
 
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