Small Engines (Lawn Mowers, etc.): Engine sputtering, briggs stratton, bus mechanic

Question
QUESTION: I have a White 42" mower with a Briggs & Stratton 16.5 twin cyl. engine.  At high RPM's the engine will sputter, it starts with no problems and will continue to run, but it just runs a little rough, like it is coughing a bit.  I changed the plugs, filters, gas and while I was at it I replaced the belts too.  I checked the manual and it says my carberator is non-adjustable as far as mixture is concerned.  I also disconnected the kill switches to make sure they weren't causing the engine to sputter.  Engine seems to run a lot smoother at 3/4 RPM, but then I lose power for cutting grass.

ANSWER: I am betting the high speed carb circuit is clogged.  Have you dis-assembled and cleaned the carb?

Let me know.
Eric

---------- FOLLOW-UP ----------

QUESTION: I have not had the carb off yet.  When you say circuit, I am thinking electrical.  Something electrical with the carb?  The reason I am asking, I had a school bus mechanic today tell me he had a similiar problem with a B & S engine and he said there was an electrical box mounted on the side of the engine that he replaced.  He wasn't sure what the box was called though.  

Thanks, Ron ([email protected])

Answer
Formal carb training calls the carburetor passage ways circuits.  There is a high speed passage that meters the fuel during full throttle operation.  You will find what looks like a bolt on the lower portion of the carburetor body.  This is the high speed jet access screw.  If you remove it you will find a brass jet inside.  My bet is this orfice is partially plugged with debris.  Removing and cleaning the carb is the best method to solve the problem.

I know techs who will just remove the jet and blow compressed air through the carb.  This typically works for a while but quite often the problem returns.

Let me know if this helps.
Eric