I have a 48" span, i.5 kg model with an OS 15LA up front. On a robbe 8x4 prop the engine surges - full revs one moment, about half that the next. On a 7X6 APC prop it works fine till it reaches working temperature, then it loses power or cuts out altogether. Cleaning the tank, fuel tubing and carb hasn't helped. I'd appreciate some ideas on what is going wrong.