View single post by Mark Rosenbaum
 Posted: 06-06-2007 09:10 pm
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Mark Rosenbaum



Joined: 03-12-2005
Location: Kingman, Arizona USA
Posts: 532
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 Yes, the vacuum valve (AKA air valve) is the piston-like part.  It has a steel tube that slides up and down inside a mating steel tube in the upper carb casting (the dome or cover).  Normally these two tubes fit together with very little clearance.  When these two parts are assembled separate from the rest of the carburetor, then suspended so the lower part can fall free, it should take a goodly portion of second, to perhaps a second and a half, for complete separation to occur.  (Caution: if you try this with the vacuum valve as the lower part, be certain to catch it before the mixture needle makes contact with anything!)

Free movement of the vacuum valve can be resisted by even tiny burrs or spots of rust on / in either steel tube, or by a bent vacuum valve tube, damaged or corroded damper mechanism or tube passageway, dirty or congealed damper oil, overly stiff or improperly installed rubber diaphragm, bent or incorrectly installed mixture needle or needle adjuster mechanism, an incorrect or missing piston return spring, and possibly a few other things I've forgotten.  Quite often, proper assembly of the parts, or just a moment's application of emory cloth or a small file, will be sufficient to eliminate the problem.