Sounds like you are may be too lean. Try richening it up some and see what happens. If the problem is at idle then richen the low speed needle, if it is at high RPM, richen the high speed needle. If you adjust one, it could affect the other slightly as well so always recheck.

If that isn't the issue then look at the tank and lines. Is it a gasoline or glow engine? Either way, I would make sure your clunk is set up properly in the tank. If it is a gasoline engine, make sure the tank is vented to atmosphere properly. If it is a glow engine, make sure the lines are connected right with the clunk line going to the carb and the vent line going to the muffler and no pinholes or cuts in the lines.<br type="_moz" />