Small Engines (Lawn Mowers, etc.): Lawnboy runs very slow, wooden dowel, adjustment screws

Question
I have a Lawnboy model 10515 that was given to me.  It is in very good shape and starts well, but only runs at
very low speed regardless of throttle position.  It has a platic carb with no mixture adjustment screws.  
The throttle cable does move the white plastic plate back and forth, but it has no effect on engine speed.
I have disassembled the carb twice, and thoroughly cleaned and inspected everything.  Nothing looks damaged.
Everything is clean.  I'm thinking it might be a problem with the governor, but it is free to operate and the
torsional spring is undamaged.  There is no apparent problem with fuel getting to the carb.

Answer
Remove the muffler and see if the ports are plugged.  There should be 3 holes and you should be able to see the piston.  If the ports are plugged use a wooden dowel to clean the carbon out of the holes.  If this is not the problem, get back to me and i will have you do another step

David R