When you cover the exhaust with your finger, you are priming the engine for start up. Make sure to look at the fuel line and stop ones the fuel reaches the carb, otherwise you will flood your engine. Basically when you do this you are pressurizing the fuel tank and sending fuel to the motor. (make sure glow plug starter/warmer is not on the motor yet)
fuel and air coming out from the fins under the motor.
Are you refering to the cooling head? (but this is on top of motor) Not sure what's happening here. Just make sure that all the bolts on your motor are tight, also the glow plug.
1) How do I stop the car from taking off while in idle?
Make sure your idle speed is set at the lowest setting.
Check your clutch shoes, make sure they are not in reverse position where it engages early at lower rpms.
When I start my car, I open up the throttle a little with the controler trim, as soon as it starts it will try to take off, I just make sure its blocked with something heavy or my foot (the clutch will slip - make sure this only happens for a second or two) or I lift the rear tires so they just spin until I close the throtle to idle setting.
2) How do I keep the temps steady while leaning it?
You cant, when you lean the setting, your engine will run hotter and if smoke starts to disappear - this is bad. Smoke from the exhaust is a good indicator if you are running rich or lean. There should be a trail of white smoke when you run your car. The smoke indicates that the oil content of your fuel did not burned completely in the combustion process and left some to lubricate and cool your engine.
I had temp at 222F. Now while leaning it the temps are rising to high. 254F
I think you are still okay at this temps. As many nitro pros would say, tune with your ears or tune for performance, not for temp.
What does the manual says in terms of operating temp range for the hpi 18ss?