@ drillpvt, got to do what ya got to do some times, as long as it works.
I just went out and did a few speed tests with my Yerf with the 39T sprocket. The first couple runs were ok, but something was wrong on the top end, I was getting horrible popping and it felt as if I was stepping on the breaks slightly, Top speed was 26mph. I went back into the garage and checked my plug and it was on the white side (lean). So I took out my stash of jets and removed the 125 that was in and replaced it with a 130. Went back out for a few more runs and the engine sounded much better still a slight popping once I got to the very peak of speed but I was able to hit 30mph with no problems. I think I'm going to have to move to a 132.5 jet for now, This is puzzling because I still have the stock muffler on it, I didn't think I would need such a high jet size. My friends have told me that My muffler sounds different from theirs, it has a beefier sound (we all have howhit gy6's with the stock mufflers).
So all in all with the 39T sprocket you can still get decent top speeds with a lot of low end power. Going to the 12G sliders will probably get me 2-3 more mph.
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