I have the same problem with a friends 300ex that he brought me to fix. It's getting everything it needs, spark, air, fuel is getting in and out of the carb., valves are in spec, has good compression, new plug, fresh gas, wiring is good, kill switch isn't on. Damn thing still wont start for ****. Some how the carb is getting fuel into it and out of it, the valves are opening and closing yet fuel isn't getting into the cylinder. I took it to a friends to see if he can find out what's wrong with it. I ran out of ideas, other than ether, but that wouldn't help the main problem.