Because my Google Custom Search sent me here, it will others; so I add:
I have a HuaSheng 49cc 4 stroke, stock carb. I discarded the stock air filter and housing, and used a customized after market air filter. Initially, I ignored the crankcase ventilation, and the engine would not run correctly. After modifying the filter to accept the positive crankcase ventilation tube, the engine operates properly. It does make some difference.