This has been covered before, so I won't get into another long explanation.

Only air goes thru the case and up thru the intake pipe. Fuel is introduced at the valve, but is not injected or timed. When the valve opens, the fuel is there and free to flow at a rate governed by the carb and pump.

The lower end of the engine is lubed by piston blowby, which is the reason that low viscosity, high volume oils are recommended. Most guys run 20 to 30% nitro heli type fuels as these contain the right oil packages.