I have a Honda GX200 6.5HP mounted on my McClane lawnmower. Its 18mo old and recently replaced carb because of defective float. I have a problem with engine surging whether or not it is under a load. It does it with or without air filter, although it runs slightly better without it. It runs smoothly at idle. If I remember correctly was having same prob before replacing carb.
HELP!!! - Joe

May need to perform a static governor adjustment on your engine, you can search this site for instructions on how to adjust your governor.