Here is a weird issue my flying partner has with his 912 UL. When he is flying straight and level, every once in a while he gets a little engine blurp. Does it a 2-3 times a flight. Never in a climb. So we decide to pull his cowling off the top of the motor and take a look. Had him start the motor and run it up. It will not go over 3000 RPM with out running rough with the cowl off... I zip tied down all the wires to make sure it is not the thrust moving anything around and causing an electrical issue. Checked and double checked wires. Still does it. He flew it a few times and still blurps in flight. Pulled the top cowl off and no go over 3000 RPM. Runs rough and seems to be carburetion. Has anyone seen this before? Could it be due to the thrust changing the pressure at the carb either at the carb inlet or maybe the float bowl vent? Maybe this is also the issue causing the blurp in flight? I am thinking of checking the jets and trying a size change to see if it is maybe running lean. But the plug colors says it is running perfect! Any ideas for troubleshooting?