I recently got my first RC airplane which is a Hangar 9 Solo Sports with the OS .40 FX and a Futaba 6EX 2.4 Ghz ( Futaba 3004 servo and 1100Mha Sanyo Rx batteries. ). I have a question about the motor. The way I understand it is the side long needle valve is the adjustment for the full throttle RPM setting and the small needle valve on the opposite side is the mixture control valve. I had some difficulty but I was able to adjust the motor correctly I think. The iddle is 3900 RPM and 12800 at full throttle and no hesitation with any transition and the iddle is smooth. I may be a bit too rich but it's a new motor so I will keep it there for a couple of flights. Now, I want to know what is the purpose of the longer valve on top of the motor ? The manual say : "Throttle stop screw" but they don't explain anything about that one.

Thanks,
Séb

Years ago people would use the throttle stop screw to limit the minimum speed setting of the engine. Obviously yours is screwed in too far because you should be able to close the throttle enough to idle it at 2500. Additionally you should be able to run the trim down on your transmitter to shut the engine off.

The little screw in the center of the throttle arm is the idle mixture screw. The big needle is the high speed mixture screw.

If the idle stop screw vibrates out, the carb barrel will fall out and the barrel spring will be ingested into the engine and ruin it. So don't screw this one ot farther than what is needed to get the engine to shut off.

Thank you for your very informative post. I now understand the purpose of the throttle stop screw. I think I could drop my iddling RPM just be adjusting the iddle mixture screw ( going a bit more lean ) but I may be wrong as I'm new at it. What would be a good range for the iddle RPM and full throttle RPM ? I will do more tests next week and hopefully get in the sky for the first flight

2500 rpm idle and the 12800 high speed would be normal