Re: almost closed petcock gives me more power
The higher above sea level you are the less fuel you'll want to deliver to the engine. The air is less dense at higher elevations.
However I wouldn't start playing with jet size until you've checked out a few things. My guess is that you have a blocked float valve due to dirt keeping the needle from seating. We always suggest installing an in-line fuel filter ahead of the carburetor to prevent that and cleaning the fuel tank prior to installation.
The float level might be your problem but unless your engine is mounted at a very odd angle the carburetor is probably close to level. The float bowl is the little cup attached to the bottom of the carb. The float is the white plastic donut inside the float bowl.
You can remove the float bowl and the float and push up gently on the brass tang that the float pushes against with the fuel petcock open. Pushing up on the tang should stop the fuel flow. If it still flows or drips clean the needle valve. When reassembling the float and the bowl make sure the pin the tang rides on is in the proper position and not pinched betwen the carburetor body and the float bowl.
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