I guess since no one mentioned it already, a intake stack being about 1 to 2 inches long would work fine. I have been using them on several engines too. The straight cut type with a angled end works well. But one could use a tapered horn like shaped intake stack too. They sell several of the straight intake statcks with a angled cut on the end that work pretty good and you don't have to carve up the cowl too much for them either. Great Planes sells one (a velocity that would work on your 40cc engine OK. The intake or velocity stack helps to stabilize the air fuel vapor cloud that forms at the entrance to the carburetor and it also smooths out the air flow, that varies a lot as the plane is moving around, going past the carb which causes uneven running. Thus the engine tends to run better and any spitting out of the carb is kept inside of the stack instead of getting blwn back over the plane. If you look at the Walbro carbs, you'll observe that the main jet orifice is neat the front entrance to the carb, so it can be sensitive to the fast flowing air going by as the plane is flying.

Here is one on one of my gas engine powered planes shown in this pic.
http://i248.photobucket.com/albums/g..._engine_04.jpg