[S Bramlitt]

If you have access to compressed air, it is quite easy to place duct tape over the exhaust exit and put a blow gun in the tape and pressurize the exhaust to look for leaks. Get a bottle of soapy water that you can squirt around the head pipe gaskets, header flanges, etc. I use this technique checking for leaks around oxygen sensors on fuel injected cars.

As far as the sputtering? I'd almost guarantee its fuel delivery. I would take the recommendation of simply placing a know good carb on it and see if your problem goes away.