backfiring, stalling and no power under load.

[pat]

hello, I've just began to revive an A4 after 6 years of no use. the engine now runs fine at idle, well almost. my plugs in cyclinders 1 & 4 have a carbon problem, and get encrusted after about 20 minutes of running the engine. I've siphoned most of the old gas out, as much as I could and since added fresh fuel to the tank. along with octane booster and fuel system cleaner. but when testing the engine out after I have done this, I lost power, and now when engaged, it will backfire until it stalls. my plans now are to replace the fuel lines and clean out the fuel system including carb. I'm thinking when I put her in forard or reverse I begin starving the engine of fuel. any other thoughts would be appreciated.

[Bill Bickley]

Pat,

You’re on the right track to suspect the fuel system. When an engine sputters, backfires, and in worst cases quits when you try to apply power, it’s almost always related to a lean fuel mixture. Cleaning up the fuel supply is a good start, but ultimately you will probably have to get into the carburetor and clean out the various jets and nozzles – the main jet being the most important. After sitting idle for approximately 6 years, there is probably a good layer of varnish and other stuff growing on the orifice of the main jet.

All you need to get started is a can of aerosol carburetor cleaner (and perhaps a main bowl gasket, in case the old one self-destructs when you pull the two halves of the carburetor apart). In fact, you might want to try removing the main jet and cleaning it before disassembling the whole carburetor. If you find a bunch of junk in the main jet, it clearlywould be a smoking gun, and you might be able to defer the rest of the carburetor work till a better time. You can access the main jet by removing the ½” hex-headed main passage plug below the float chamber. The plug is oriented horizontally, with the hex-head facing away from the flywheel.

I’d be careful about adding too many additives to the fuel. I’ve had several friends report seeing various things precipitate out of the fuel after mixing certain additives.

Bill