Carburetor jetting

You seem to know your stuff. I have that baja sc50, with that odd ball motor Qj139qmb, motor. what’s the trick to getting  to getting the most topend out that motor. I have tried 4.5 to 6.5 gram weight, I stalled a 72cc big bore kit, its got a lot better tork, a new mod Exhaust system. new cdi w/ no rev. No noticeable change in rpm.

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Holy smokes dude, a #125 on a 50?! No wonder your not running right, your drowning the poor little thing. Too much fuel is very detrimental. It must be properly mixed at the stoic ratio of 14.7:1 by weight. If more fuel was better all the time, we’d just eliminate the carb and hook the fuel line up directly to the engine. Engines are air pumps, the more air the better, not the more fuel the better. I’d recommend installing at most a #85 main jet ONLY if you have a high performance air filter. If not, I’d go with the standard #75. You must set the jettin correctly by experiment. Put the main jet in, and run it full throttle long enough to get the plug colored. The best reading on a 4sroke is a light tan, with perhaps some darkness around the edges. Some factory setting are so lean that they are bright white. Your plug is probably black with soot.