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First check for a leaking head gasket. An A4 will run quite well with one cylinder at low compression and at idle, ... until you apply throttle and load. The head on an A4 should probably be re-torqued every 400-500 hrs. of run time.
Secondly (and quite common), disassemble the carburetor and inspect/clean the emulsion tube ... the emulsion tube (a hollow tube that is approx 1/8" dia X 1.5" long and has several teeny 'holes' bored into the side of the tube) controls the air/fuel ratio for the engine at 'above idle speed' and if one of the tiny 'holes' in the side of the tube becomes clogged with debris will drastically change the fuel/air mix when the engine is 'above idle'. Just a wee bit/spec of crud in one of the 'holes' will result in drastic air/fuel ratio changes when you attempt to apply 'power'; the emulsion tube is NOT 'active' when the engine is at idle it only comes 'on' at above idle.
If you dont already have one, get an adjustable main jet for the carb.
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