I have a PK 80 from Ozark Cruisers. Has an NT type carb. I read somewhere that I should open up the stock muffler. I cant the tube inside of the muffler down to a nub. I drilled it out and put another hole in the bottom cap as well. Now I have read that when I open up the muffler for less back pressure I should now allow more air to enter the carb? I removed filter from carb and found a black dust like material in the bowl and on the back side of the carb itself. It easily washes off. I am guessing it comes from the air filter as this motor only has a couple or three gallons through it all on clean roads. Should I wash the filter? Should I change it out? Should I replace with one that allows more air to enter? Am also considering trying different sized jets. The motor runs well, but rarely reaches top end. Usually you can feel the motor bag down, or lag perhaps at all throttle. Let off the throttle slightly and she picks up again. Seems to me its not getting enough gas? Maybe not enough Air? When it gets really hot it seems to work better. Advice?