This is the standard question of many gas fliers. It is not engine, but carb related. The carb needs a reference pressure for the fuel inside the plenum chamber. To that end, it has a vent hole in the plenum membrane cover. The reference pressure can change with plane attitude, which can cause the mixture to go lean or rich. The 58cc engine has provisions to attach a tubing line to that cover just for that purpose. This tubing can be routed to where the problem vanishes. A closed balsa box inside the fuselage gives excellent results. The balsa breathes just enough to stabilize the pressure.