NotTaR of small Gasoline Engines and Rotary Lawn Mowers : Does your 2 stroke engine use reed valve..  
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Does your 2 stroke engine use reed valves?

Since a bad reed valve can be the cause of a variety of problems, knowing if you even have these can be useful - not all 2 stroke engines use reed valves.

(From: John Barry (jb@zedak.keepyourspam.com).)

One easy way to tell is to note where port is behind carburetor. If it just "dumps" into crankcase, like diametrically opposite cylinder, most likely it's got a reed-valve. If the passage from the carb connects to the cylinder at a point closer to the crank center-line than the exhaust port, perpendicular to cylinder axis and perpendicular to crank, it's mostlikely piston-ported. If the carb passage is parallel to the crank eventually (starts out perpendicular on Rotax engines), most likely you have a rotary-valve engine; this is usually the case for serious specific-output.