RE: Magnum XLS-52
I can take a stab at those questions.
1. 18,000 was probably wit ha 9-6 prop which is way too small for practical use.
2. You won't find a rpm vs. prop size chart for any engine. There are too many variables for gas/glow engines. Some of these are fuel used, including % nitro, % oil, type oil..., type of prop, air temperature, humidity, altitude (for pressure), how it was broken in and others.
3. Generally, too lean is when it sags. Again, there are a lot of variables. If you are flying a plane that pulls a lot of g's, the engine will need to be set richer as we only have a suction fuel system and under g's, the fuel draw is harder. If you fly long vertical climbs, you'll need to be richer than is you just fly level. Normally, we rev the engine to full power and hold the nose upwards about 45 degrees and see if it runs OK or sags, then adjust accordingly. A more excat way is to use a tach and tune for max rpm, then back off a few hunderd. Some people say 200-300 rpm, but some engines require 500 rpm depending on how you fly it. Somewhere around an eighth to a quarter turn open.
4. Yes, the engine will break into a 2-cycle long before it gets to max rpm.
And you thought all those batteries had variables.