Put you some Stabil in the Fuel Tank.
Read the Instruction's on how to adjust the Low Idle Mixture, and the High Speed Mixture. If you haven't messed up the Needle Valve's, by turning them in too much,, then you may be able to get your engine running better.
Over time,, Gas Residue will get in the small opening's where the needle valve is,, and by adjusting them,, while useing Stabil Gas Treatment,, may disloge the Residue,, and let your engine get the correct Fuel/Air that it need's.
Read the Service Manual carefully until you understand what it is saying and the correct way to do this.
When I do my 12 hp single cyl; I first adjust the Idle Speed,, then turn the Idle mixture out, until the Engine Slow's,, then turn it in,, until the engine Slow's,, then turn it back out in the Middle. You have to count the turn's to do this, each way.
Then I open the Throttle all the way,, and adjust the High Speed the same way.
Good Luck