Join Date: Jul 2000
Thanked 184 Times in 173 Posts
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Several things to check:
Carburetor fouled ... typically dirt/gum deposits on the carburetors 'emulsion tube'. The emulsion tube is a thin hollow tube with teeny holes. The emulsion tube controls the air/fuel ratio. Any dirt in or on the 'holes' will cause the air/fuel ratio to vary - very common ailment with this updraft carburetor; easy fix - just disassemble and inspect/clean. A gasoline filter housing that is supplying fuel to the carburetor and is 'rusting' in the bottom of the bowl section will usually be the cause of dirt, etc. catching on/in the small 'holes' in the emulsion tube. The emulsion tube is a quite precision device and needs to be 'perfectly clean'.
The A4 is a quite low compression ratio engine and will run adequately at idle even with compression issues and will 'run' with only 2 or 3 cylinders 'operating'. However, ......
A. Head gasket leakage. The usual leak occurs between cylinders #3 & #4 and involves cross cylinder leakage as well as water leakage. The head should be periodically retorqued. One of the symptoms of a head gasket being 'blown' is the engine will idle quite well but as soon as throttle/load is applied, the engine stalls out.
B. Sticky exhaust valves ... depending on the configuration of the wet exhaust system, high humidity in the exhaust circuit can cause the stems of the exhaust valves to begin to rust; the rusts causing friction which leaves the exhaust valve either non-moving or partly closing (reduced compression value/reading). Easy assay and 'fix': open the valve cover on the side of the engine, disconnect the main high tension wire between the ignition coil and the distributor cap to insure that the engine will not 'start', spin the starter as you WATCH the exhaust valves go 'up and down'. If a valve is fopund to be sticking - remove spark plug, 'go into' the combustion chamber with a 'bent' screwdriver (the valves are on the portside of cylinders) and PUSH the offending valve down so that its fully seated ... then spray the valve stem with an anti-rust compound such as "PB
Blaster", etc. let soak a bit, then spin the starter and spray the valve stem until 'repolished' and free of rust .... and the valve 'reliably' returns to 'full stroke'. Sticky exhaust valves are quite common on A4s that 'sit' for long periods ... the cause is usually a pin hole that develops between the gas and water side of the exhaust manifold. A pressure check by blocking one 'end' of the water port or the manifold and applying 40-60 psi water pressure on the water circuit will prove/disprove any 'pin holes': apply water pressure, close the water supply and look for a drop in a gage pressure that is applied to the water supply, etc. Such can be 'brazed' closed.
Get Moyer's "Rebuild and Maintenance Manual" ... and visit the Moyer forum. A4s simply dont 'wear out' they can run almost forever ... the usual thing that kills an A4 is internal rusting of the block and Moyers Manual will describe simple testing that will determine if there is enough 'meat' still left on the inside of the engine. Usually if there enough meat / wall thickness left then the engine can be easily rebuilt, repaired etc.
Last edited by RichH; 09-16-2011 at 12:16 PM.