Someone reported on another thread that later Cox fuels contained 15% (standard) and 25% (racing). I believe that person said Cox also dropped down to 18% castor.

I read in a model mag that tests revealed that 20%-22% lube runs better. I try to keep my lube in that range.

I have used 5% nitro and although it will run needle adjustments get much better at 10% and even better at 15%. I would keep it between 15% and 25% unless you are competing, then up to 35% is OK. I usually use 15%.

Another consideration is lube. Cox engines running hot (high nitro) will form varnish if all-castor lube is used. Many of us had to run an occasional run of a "special" fuel containing gun cleaner which, at the time, contained nitro benzine. Occasionally you would need to tear down the engine and clean the piston and cylinder with a brillo pad or gun cleaner. Adding some synthetic keeps it running cleaner. A few use all-synthetic successfully while others (myself included) believe the ball socket needs castor to survive.

If you run them in competition you will probably run them harder and not expect them to last as long. Which brings up the reset tool. You should keep the ball socket running with very little slop. The harder you run, the more often you need to reset.

For sport flying, you may only need to reset once. If this is the case, you might consider finding a friend who has a reset tool to help you. They are hardly worth the cost to reset once or twice. Make sure you learn to do it properly.

George