I have a fairly new evolution 40 cc that I like and has a good amount of power. However, I am having trouble tuning it. It idles good and high speed is good. When I slowly advance from idle to high it goes through a roughness in mid range and then jumps to high speed. My last flight I was going at a mid throttle for a few minutes and when I advanced the throttle quickly the engine quit. Luckily there was little damage. I was able to repeat the problem on the ground. I'm not too familiar with tuning gassers. Any suggestions?

Usually a gas ignition engine doesn't die when idle needle is set richer and throttle is advanced. But it can die if idle needle is set too lean and throttle is advanced a bit quick.

Let me suggest that you try the following on the ground:

Before making any adjustments, verify that your feed line has no air bubbles. If you see any do everything possible to assure a tight, leak free gas feed set-up, otherwise you will chase your tail. Once that's verified, go ahead and start it

Engine to idle for 30 seconds to a minute. Then quickly advance throttle to full. If idle needle is lean, it will stumble and die. Open the LS needle about 1/8 to 1/4 turn and try again. If it's rich, it should stumble briefly and then take the throttle, assuming your HS needle is set okay. Check rpm with a good tach and find the max rpm at full throttle by adjusting the HS needle. You are looking for the steadiest rpm the HS needle setting will give you. If it sags, it is getting too hot and it might be lean or just not getting enough cooling air or load is just too big. Remember that walbro's needles affect each other so any changes to one needle may require a small change to the other.

The 40cc will turn a 20x8 prop when broken in but for now use a 19x8 to avoid overload. If it is still too hot and saggy, duct cooling air around the cylinder and make sure it exits the cowling. Ducts don't need to be elaborate...I use 1/32 balsa, cardboard, even depron and fit the duct pretty close to the fins. On the exits I like hooding the fronts to create a negative pressure there and draw the hot air out...Good luck

Thanks. I'll try this next time out, which may not be too soon with the rain we've been having.