One of the replies mentioned using a "heavy" .91 4-stroke. The Saito .91, an AAC engine, weighs, according to their specs, 18.3 ounces with muffler. The OS .61FX is 19.4 ounces. I would imagine this is without muffler, but it is still an ounce heavier than the "heavy" Saito. Other 4-stroke .91s are much heavier, especially the Thunder Tiger at over 26 ounces.