Generally these motors never get air leaks. It would be a surprise to me if it were the problem. I would say the carbs may be dirty or too lean. You will have to pull them apart and have a look. You will be able to see if the bores are an insert or if the cylinder and bore are all one piece. You can put steel bores in but it requires much different carb jetting. I would highly recommend to stick to nickalsil bores even if they are more expensive short term, because steel bores will cost you everytime you blow the motor from them. You see, the steel bores run much hotter, and this engine with the wall thickness of the water jackets was designed to run pretty cool.
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