Hi Guy's, I too am playing with a B&S 40cc, 4-stroke. Sorry I don't have anything new to add yet but I don't think you will get much more performance out of these engines without playing with the valves. Putting more fuel and air into it will require a bigger exhaust valve to realize higher performance I believe. They are not made to be exotic but rather just a work horse of reliability (low rpm). I'll be happy just to have that reliability and 4-stroke sound (for the price) in my model. I think you really need to build your model around these engines instead of building the model and then trying to make these 4-strokes pull them. Just my two cents worth. I think we have learned that the model needs to be less than 20lbs. and preferably 15-18lbs. So far I have spent approx. $160. for this engine and that includes a new and bigger carb. which may have been a wast of money because the cylinder head may need to be modified to take full advantage of the carb. I think it would be prudent to try to find out from B&S what they recommend as maximum RPM for this engine as well.