HI, FROGTWAT! First it's good to know if you have an engine and fuel tank setup that is tuneable. If you prime the engine through the exhaust port and then turn the prop until you are up on compression,that compression should not bleed off in 5 seconds or so. The next check is for air leaks out the back plate, or out the cylinder head. Prime your engine with caster oil, bring it up on compression,seal off the intake with a free finger, then dunk it in water. you should not see any air bubbles coming from the back plate, or from the head. The tank should be sized for the engine you are running. Too big of a tank and the differences in head pressure from full to empty can be too much for a little engine. 6 ounces is a good size for a .40. A pressure line from the muffler to the tank works 99% of the time. make sure that the tank has no other air leaks besides the pressure line. NOW we are ready to tune the engine! I like to set the high speed needle for the worst case scenario, which is when the model is pointing straight up in an extended climb. When you have it set for max rpm, it will be slightly too lean. you should be able to quickly pinch the fuel line without the engine sagging. On an older engine it isn't good to be this aggressive with the high speed needle, what you want to do is creep up on the setting through in flight trial and error. Most engines have only the one needle, and an air bleed screw for the idle mixture. Slow the engine down to the lowest reliable idle that you can manage. If you can't get the plane to idle low enough so that the plane doesn't creep forward on the ground, then we need to figure out which way to turn the airbleed screw. hold the model in your arms and raise the nose. If the speed increases, then you are too rich and need to turn the screw 1/2 turn counter clockwise. If the engine runs worse, then you are letting to much air past the screw threads and you have to turn the screw in.If you have a 2 needle carb, the low speed setting is similar, but you turn the needle clockwise to lean it .Some guys set the high speed needle with a temperature probe, but they are too spendy for me. Hope this gets you going!