The engine's performance when you advance the throttle from idle to high speed is called "transition", because the engine is transitioning from the low speed needle to the high speed needle. If an engine is running fine at high speed, but having problems during transition the problem lies in the low speed needle, because the engine's mixture (% of fuel vs % of air) is controlled at that point by both needles. When tuned correctly your engine should transition smoothly. If it coughs and splutters in transition it's probably running rich (too much fuel, not enough air), if it simply dies or hesitates it's probably running lean (not enough fuel, too much air). If it idles nicely for a minute or so then slowly loses revs and dies it's probably rich - when this happens it's called "loading up".

Setting your low speed needle can be awkward but it's not that difficult when you're used to it. First we'll set the low speed needle roughly so it's close to right. Adjust your throttle stop screw till the opening in the throttle barrel is about the width of a T pin (a bit less than 1mm). Connect a long-ish piece of fuel line to the carburettor. Screw the low speed needle in (lean) all the way till it bottoms out (don't force it, turn it gently). Then start blowing into your fuel tube and turn the low speed needle out (rich) till you can just hear your breath escaping in the carburettor. Connect the engine back to the tank, and start the engine, leaving it to run for a minute or so till it's at operating temperature. Pinch the fuel line, and listen to the engine. If it runs normally for a few seconds then revs increase the low speed needle is rich, turn it in a small amount - about 1/8 of a turn. Let the engine run for a minute to settle and try it again. Repeat this until the engine speed doesn't increase but it just sputters and dies, which shows it's now lean. Adjust the needle back out till it just starts running slightly rich. After this you'll need to adjust your high speed needle again, as it will have been affected by the low speed needle's adjustments. You can use the same pinch technique to test the high speed needle, but it will react much faster as it's drawing more fuel at high revs.

Remember you want both needles to run slightly rich, as the engine will lean a bit out at altitude. When you're happy with the way the engine runs on the ground, have an assistant hold the plane level while it's running, then advance the throttle to full speed and have your assistant hold the plane nose up for about 30 seconds. The engine should either maintain the same speed or increase its revs slightly. If the engine dies in this position do not fly it till you've found the problem, which will probably be a poorly connected pressure line from the muffler to the tank, or possibly the clunk (fuel pickup) may have moved forward in the tank during a hard landing.

While doing this tune up, it's a lot safer to stop your engine before adjusting the low speed needle as the carb is very close to the prop and the low speed needle is often a fiddly screw set into the throttle arm. The other thing is to check your carb over completely before you tune it. I drove myself crazy for days trying to tune an engine which was behaving erratically, only to find the barrel retention screw was loose, allowing the barrel to slop around in the carb. Check all the screws are where they should be, and that the carb is well sealed against the engine's crankcase as an air leak there will make it impossible to tune the low end, even though the high end will run just fine.