The carb settings in the manual are a guide...they won't work exactly for each engine. You want it to be set slightly rich, anyway, since the extra fuel in a slightly rich mixture helps the engine. Here's how I typically set the mixture on my carbs:
Let the engine warm up for 5 minutes or so, then at full throttle, enrich the mixture until you get an rpm drop and/or the engine starts stumbling a bit. Then slowly lean it...you'll notice an RPM increase, then a drop. After the drop, enrich the mixture again such that you're at the high point of the RPM rise. Then enrich it another 1/16 or 1/8 turn. Then let it idle, set the speed, then adjust the idle mixture screw in a similar way. Once you're confident you have that set correctly, quickly move the throttle from idle to wide open. If everything is set right, the engine should respond nearly instantly and run smoothly at full throttle. If you get bangs and flames out of the muffler, the mixture is too lean, and if it bogs down, stumbles, and blows black smoke, it's too rich.
Have you checked the cooling fins and the area under the flywheel shroud for cleanliness?
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