The hesitation could be a bog caused by the carb - too much gas or too much air. Possibly the accelerator pump or the secondaries.

I think if the issue was with the valves not closing you'd be having major starting issues, you'd be getting reversion, backfiring through the carb and severe dieseling when you shut the motor off.

As for that "smoke generator" idea, you can do the same with a propane torch. Start your engine and open the valve on the torch (DON'T LIGHT IT). Move the torch nozzle around any areas that could potentially be a leak. If there is a leak you will notice an immediate increase in RPM. Don't worry, this is a safe method, just don't forget to turn the torch off when you're done!