Did you use genuine Mikuni rebuild kit, pretty much a waste of money if you didn’t as the Knick offs will cause more problems than good. There is an excellent step by step rebuild thread on Mikidymac’s profile. I would suspect that the engine is sucking air in from somewhere, possibly the fuel selector valve, water seperator, check the o ring on it, you obviously have a gasket between carb and engine, or from the oil seals. A leak down test will determine if the oil seals are leaking, you can run a fuel line from the tank to the carb temporarily to bypass the fuel selector valve. Don’t drive it until this is figured out or the engine could get damaged. Have you taken a compression test?