This is puzzling, especially as you have stated that you have flown the plane at a tuning setting that just has the engine breaking from a 4-cylcling state into a 2-cycling state. If that is truly the case and it is staying at that tune throughout the flight, then I wouldn't think you are having problems with holes in tubing or debrie in the carb. Now if the tune is changing, i.e. Going lean, then the tubing/dirt issues others have raised are correct.

I would tune it so it is just breaking into 2-stroke, take it up and fly it vigorously for 3 minute or so and then land. Check that it is still at that rich 2-stroke. Take off and repeat until you are about empty so that you are checking at all tank levels. If the tune is consistent, then I would go with Scott Douglas's thought?

A thorough tear-down, cleaning, and overhaul (new bearings, possibly more) to include carb, may solve your issue.

Are you the original owner? Have you flown with this engine before (in another airplane) without any problems? How much time on the engine?

Lars

PS: I would add an inline filter and never run an engine without one as they are easy insurance.