I was trying different carburetors on a Ryobi 30cc and was having high speed problems. I ran the engine on a test stand with nothing on the carburetor input. I noticed fuel coming out of the carburetor bore at high speed. It turns out that the prop wash over the carburetor was drawing fuel out (this is an engine with no reed valve and the carb and exhaust on the cylinder sides). I put a cup over the carb to block the direct airflow from the prop, and the problems went away. The original carburetor filter assembly would not fit on the new carb, so I made a baffle to keep the prop wash away from the carb input. The engine runs perfectly now, idle through full throttle. The carb has a 13.47mm bore, much larger than the original. These large bore carbs generally cannot be made to idle well, but now it is fine. I am sure most of you already know to keep the carburetor entry shielded from the airflow, but this is the first time I have had anything but the Ryobi 31cc with the rear mounted carb, so this was new to me. I even tried making a funnel to direct air into the carb as a poor man's super charger, but this made things much worse.

The bottom line is that you need to use the original carburetor shroud with the filter, or make up something to keep the airflow away from the carburetor. There should be static air at the carb input, just like on the original weed eater. Let the engine alone cause the airflow through the carburetor. I hope this helps for some of you (like me) that have limited experience with these engines.