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 (ron9595@hotmail.com)

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