I picked up a good running 064 and put some time on it, then I simply added a couple holes in the side of the muffler, made a little deflector and tig welded it on. I'd have to look at it again and see what size I made the holes, but I figured the area of the existing exhaust hole in the muffler and the approxamate area of the exhaust port at the piston. Then I added the 2 holds to make my total exhaust outlet area roughly 80% the area of the exhaust port at piston. I ran it and re tuned the carb. The saw runs great and pulls verry strong in the cut, but comeing out of the cut it idles fast. If I start the saw and let it idle it will idle like it needs to. I also don't notice any change in the low speed mixture adjustment on the carb unless I really choke it off to probably 1/2 turn from closed, of course than it leans out and dies. The throttle plate idle adjustment is closed all the way and the saw will still idle fast. At times I can adjust the carb and get a good idle, and at other times it just likes to run fast, a random problem. Vaccume leak? or should I close off the additional holes in the muffler and see what happens. My muffler was in tough shape anyway, I took an hour and blasted it, tig welded the cracks and added some metal to make it good rather than buy a new one. I did not do any porting or take anything else apart on the saw, it makes roughly 150-160 PSI compression.