Hi Dolittle,

You might try running the engine without the cowling to see if there is a change. The regulator diaphragm of the carburetor is subject to the whims of cowling pressure and this could be causing your troubles.

Another carburetor fluke is that some walbros and possibly tillitsons have a governor built into them. When the engine reachs a certain rpm induced harmonic. the check ball in the governor unseats and allows an extra amount of fuel to enter the engine and slows the engine down until the harmonic stops. The governor is what keeps chains saws from overspeeding when the throttle is held WOT.

Some models never see the critcal harmonic and run just fine. Others due to the airframe characteristics hit the key harmonics and go rich in the air no matter how you set the needles on the ground.

Hope these little tidbits help

Carlos G.