Hi guys, I have a problem. I have a 1981 suzuki fa50, it starts on the first kick every time runs flawless. But it only goes 17 or 18 mph when it hits that speed it changes the sound it makes from a smooth running engine to somewhat of a farting sound. It acts like it's not running as good as it should be and occasionally it will surge for a second or two. So things that I have done to it. Everything. I've done literally everything. Changed every parts and no new part makes a difference in the way it runs. At this point I am questioning if the motor is bad except that it runs and starts so good. I bought a parts fa50 and just started changing parts. Things I've changed so far. Muffler, airbox, both changed and also ran without them on the fa50 no difference. New carburetor, (chinese I assume), changed the 2 jet screws on the side as well from new to old, no change in it running. I've changed the coil, checked the magneto, changed the regulator, the petcock, I've mixed the gas with oil and drained the oil pump to see if it was putting in too much oil, still no change in it running. It also puts oil out the tail pipe not a ton but definitely noticable. Only things I haven't done is change the chain oil and replace the bad battery. It also has a light electrical issue, I think the battery is bad and a couple bulbs are blown. Anyway that's it. Any ideas as to what is wrong? I really like the fa50 but its driving me crazy. (edited)