Possible the powerhead base gasket is failing. A common problem on these engines. Run the engine with the upper cowling off. When these base gaskets fail it is common for them to emit exhaust gas into the cowling area, even for it to get re-ingested into the carb airbox and back into the engine. Sometimes you can get some midsection water leakage too. If that is the problem, you'll want to have it repaired.
