There are 2 needles to control mixture on the 55AX, and most other glow engines.

There is a high speed needle, this one's obvious. Large knurled knob to turn, either on the carb, or remote on the backplate. This one tunes full throttle performance. This one is adjusted only at full throttle, to a couple hundred rpm's rich of peak rpm's (a minor, but distinctly audible drop in RPM's). Basically, with the engine running full throttle, you'd first rich this needle to verify that you're on the rich side of peak to start with. The engine should slow as you open the needle slightly (a couple clicks should produce results). Now, slowly, click by click, lean the needle. Engine accelerates with each click, and then stops accelerating. Go any further and it'll quit. Now open the needle a couple clicks until you hear that slight, but distinct reduction in rpm's. I use a tach personnally, and set the rpm's 300 below peak. I find it more repeatable that way. Get used to this part of tuning, it is typically adjusted at the start of each flying session. This is not the needle you need to change to fix acceleration hesitation.

The second needle is the low speed needle. If you look at the throttle arm, you'll see a small flat head screw down inside the arm's center. This is the LS needle. This is the one that you must adjust to get better transition. It affects the mixture at idle thru mid throttle. It is adjusted with the engine OFF (too close to the prop to be safe with a screwdriver). Once correctly set, it won't need changing unless the fuel is changed or the engine is wearing out. To adjust, Start the engine, run it wide open a few seconds, then go to idle. Allow the engine to idle for at least 30 seconds. Whack the throttle wide open. If the engine spits, coughs, sputters and then either dies or finally accelerates, lean the LS needle a tiny bit (screwdriver blade slot width sort of amounts) If the engine dies immediately, no spitting, coughing, no puff of smoke from the exhaust, it's too lean, rich it a tiny bit. Repeat until the engine will accelerate cleanly after idling at least a minute.

The HS mixture is typically tweaked a bit to be sure after the LS is set, just do the HS thing again. The LS needle does have a minor effect on the HS mixture is why.

Once you've successfully completed the above steps, you should have an easy starting engine that doesn't deadstick, idles smoothly all day long, and accelerates eagerly.

Good luck,

J