i can allmost guarantee someone else will add to my response,,,usually the burbling will occur where the engine transitions from idle to midrange,,,usually if the idle or high speed needle is too rich this will occur,,as far as using a 50 cc in a 35 cc plane ,,not just me ,,,but the manufacturer probably will discourage this,,you may have ground /prop clearance issues,and you could rip the plane in half from over powering,,,read the articles on gassers including this whole thread ,for info,,if youve ever had a weed whacker or gas string trimmer and noticed a lag in engine accelleration at around mid throttle,,thats simmilar to the burbling youve heard of ,usually leaning the needles slightly makes that go away,,also if you have friends that have gassers,,get with them ,,it makes it much easier ,in person,,,ps ,,,once again read this thread starting at page one,,,youll be surprised what youll learn.ps alot of our flying is done at mid throttle,so its important to have this adjusted ,,thats even if it occurs,,