GI, the one UMS engine I had problems with was the 75. I bought one and Adrian sent me two Walbro carbs with it. One was overly rich at idle and full throttle because you had to with an ultra lean midrange throttle position. The other carb was the opposite with an overly rich mid throttle position. The engine also built up massive carbon on the heads preventing valve closure. The 55 with the Saito carb and the stock 5-100 are basically perfect runners. Adrian took care of me with the 75. He believes in customer satisfaction. -Tom