Although I've run a lot of CoolPower in ringed engines, it seems to not run great in most non-ringed types. I have used Byrons and PowerMaster with great results in Norvels, both fuels have some castor and are 18% -20% oil. If your head was leaking, it might be helpful to lap it, put a piece of 800 grit on a sheet of glass and wet sand the head button sealing surface until it has a uniform sheen. Reassemble with one stock gasket (they sometimes come with three installed but, at least up to 15% nitro, they seem to give best power on 9x4 combat prop with only one shim. I've seen factory shims from .004 to .008, if i can find one i use a single .004) and tighten the head firmly with the factory wrench. check head clamp torque often. The back plates have been known to leak- remove, wipe clean, reinstall with some gasket sealer (I use Hilomar, a non-hardening sealer for water pumps, etc, available at auto parts stores, easier to deal with than RTV). The OS 25/40 FP needle valve asm will indeed fit the Norvel BM carb and seems easier to adjust, just need to grind the unthreaded tip off. the airbleed is fixed on the BM carb, you can enlarge it with number drills, just go a little at a time, and if you go too far, you can always plug with JB Weld and start over. a few thou in diameter is a lot in added area, just go a few at a time. Every Norvel BM I have is rich in the idle/midrange so opening it up a little won't hurt, I would guess. I would avoid the AME carb, i've seen other guys get the occasional lucky one but I've had several and EVERY one siezed, the carb barrel siezes against the rotor screw due to vibration, and locks the carb up with a big ol' hairy burr on the barrel. I sent em back to Sig and they repaired them but I don't trust them and keep them in a parts box and run BM carbs on my AMEs. Maybe on lower-revving sport planes they will work fine. Because of the rich low end/midrange, the Norvel runs better with a hotter plug, I've tried some colder ones such as the Fox 8 and the low end really went down the toilet. The OS A3 and 8 seem to work well, as does the K&B 1L.