I recently installed a venerable and reliable 52fs in a new model and the first flight was fantastic. Loads of power and supremely reliable. The next and subsequent flights were horrible, loads of dead sticks and leaning out in flight even though the needle was set fine on the ground (though now to 1 turn out, usually a bit on the lean side for this motah). I'm using Duraglo 5% and an 11x7 prop.

I attached some fuel tube to the carby nipple, screwed the needle valve fully home and blocked the carby mouth with a spare digit and blew. No air leaks. Tried the same with the needle a turn out and leaking air. When running, waggling the end of the needle valve side to side doesn't change tune.

Is it likely that the needle valve o ring has gone letting air sneak past and making the whole caboosh a bit lean and hence gutless?