Question about 1995 Chevrolet C/K 3500

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What causes our 1986 chevy motorhome to back fire through the carburator? This occurs when you first take off, it stops after awhile when warmed up. Carburator has been rebuilt, new plugs, wires and cap. Only thing we didn't replace was the coil.

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Backfire in an automobile engine typically results from various malfunctions related to the air fuel ratio. Backfiring can occur in carburetor engines that are running lean where the air-fuel mixture has insufficient fuel and whenever the timing is too advanced. As the engine runs leaner or if there is less time for the fuel to burn in the combustion chamber, there is a tendency for incomplete combustion. The condition that causes this is a misfire. The result of a misfire or incomplete combustion is that unburned fuel or flammable hydrocarbons are delivered to the exhaust manifold where it may ignite unpredictably.


When starting an engine, timing that is too advanced will fire the spark plug before the intake valve is closed. The flame front will travel back in to the intake manifold, igniting all of that air and fuel as well. The resulting explosion then travels out of the carburetor and air cleaner. A common air filter will allow the gases to escape, but will block the flame front. On many small marine engines, no air filter is used, but a screen is placed over the intake of the carburetor as a flame arrest to prevent these flames from escaping the intake, and potentially igniting fuel, or fuel vapors in the enclosed sump or bilge of the boat and causing a fire or explosion. Improperly adjusted carburetors that create a lean condition during acceleration can cause the air fuel mixture to burn so slowly, that combustion is still taking place during the exhaust stroke, and even when the intake valve opens. The flame front can then travel up the intake and cause a backfire. In this situation it is conceivable that there is a backfire occurring in the intake manifold and exhaust manifold simultaneously.

Good luck and hope this helps, check the carburetor settings to make sure it's not running too lean and the timing . Keep me posted, be glad to help you get your RV running 100% again.

Posted on Jul 14, 2009

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