we have a OS 46SF on the club trainer, Avistar, idles fine, full throttle is fine but when flying at about 1/2 throttle it surges real bad, we've already replaced the tank and fuel lines, and can see no air bubbles in the fuel line, any suggestions before I tear into the carb and try to replace the o-rings?

take out the needle valve and blow some fuel through the carb nipple, it might have some debris in the needle jet.....

Also make sure it isn't leaking around the needle in the valve body. I believe OS has a small O-ring in there to prevent that.