Re: Leaky carb and backfiring

Posted by Joe on March 04, 1998 at 07:57:17

In Reply to: Leaky carb and backfiring
posted by Eric on March 03, 1998 at 23:31:47

Ken's evaluation is on the money.
1. The leaking carburetor is definitely not correct and indicates a problem with either the float itself (possibly got a hole in it or stuck on it's pivot) or it's position (too high before shut off of the needle valve) or of the needle valve itself (grit under the seat or just plain worn)
2. The fact that the car runs reasonably well just indicates that in operation you are using up the gas at a rate equal to or faster than it is coming into the float chamber.I might imagine that in operation, you have the dashboard carburetor adjustment nearly screwed down tight.Normally with a cold engine, the dash adjustment is a turn or two open and a warm engine about 1/4 to 1/2 a turn.
3. The timing of the engine is probably not a factor here if the engine runs well as you say it does.Normally, on mine, best idle operation is with the spark fully retarded: if you advance the spark the engine starts to "roll" or surge in idle although it runs good at speed.
4. The "boom" or bang heard after shutting the car off is as Ken says the explosion of unburned gasoline that has been pumped though the engine after the ignition was shut off and is now pent up in the exhaust pipe or muffler waiting for enough oxygen to complete it's final destiny.
The spin down normally does not create that much unexploded gas vapor in the exhaust train, but if you have a tendency to run rich in your mix (I suspect you do) then any exhaust pipe leakage or the intended ultimate leak at the outlet of your exhaust pipe may cause this chunk of rich vapor to reach its "perfect" explosive mix, leaving you liable to backfire.
If this condition has been ongoing for sometime, (or even if it hasn't) likely a buildup of carbon is present in the cylinders or exhaust system.In operation this carbon is heated more or less to incandescence and some burns off.But some doesn't also.This is the source of your "spark" for backfiring and cleaning of the component parts may help in this regard.
Another contributer can be as the engine rolls to a stop, one of the cylinder exhaust valves is of course open and the engine "backspins" a half or turn or so on its compression, pulling a small amount of unburned charge from the exhaust back into the cylinder or a small quantity of air in through an exhaust leak.The unburned charge is ignited against hot carbon and "whango" the flame wavefront travels out the exhaust pipe to explode in your muffler.A tight system can help prevent this.
Perhaps it's not quite as simple as any of this, but the root cause is too much gasoline.
I would do two things.Check your float to be sure it is not pinholed or waterlogged (gaslogged?) and replace your needle-seat valve with a new one, preferably with a neoprene pointed needle. (The original was not)
If you have a Zenith carb you will enjoy reading Paul Moller's book on the Model A Carburetor.It sells for about $6 and is available from the major dealers.
I hope you can get it "purring" and don't tolerate a leaky carburetor any more than you would tolerate a leaky gas can.They're both accidents waiting to happen.
Let us know how you make out.
Best regards,
Joe

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