To check to see if the fuel tank has an air leak, take the tank off. Fill the sink with water and submerge the tank, have both the fuel line and the pressure line above the water. Blow in the fuel line, but keep a finger over the pressure line. If you see nay bubbles there's an air leak, if not then you dont have one. Also make sure the carb nut is nice and tight, you can also check there for an air leak also. While the engine is running take a soapy water solution in a squirt bottle and spray around the carb's base. If you se any air bubbles or if the engine dies or, the engine's rpm's drop then there's a really good chance you have an air leak. Just take some Heat resistante silicone and place a tab or two around the base of the carb. Then you'll be good.

BUt is the engine tuned right though, if the engine isnt tuned right it will never reach a high enough speed to get it to shift. Hence no top speed, and the hesistation at take off.