Mine runs fine w/o an idle bar. It has a cheap TH plug in it.

When an engine HESITATES its tuned to LEAN (not getting enough fuel). When it STUMBLES it's too RICH (getting too much fuel).

Try this... Set the low speed RICH so its operation doesn't intefer with the high speed adjustment (1/8 inch as stated before). Tune the high speed for max rpm then richen it a few clicks (ccw on main needle). (This is because in flight and as fuel levels in the tank decrease (lower) which will cause the engine to lean. A lean engine runs too hot and this will damage you engine.)

Then set your low speed needle. Test it by letting the engine idle a bit (5-15 seconds) then opening the throttle. If it hesitates or dies then it's too lean (open ccw on the low speed). If it stumbles and smokes then it rich. The low speed needle is very sensitive to movement so only 1/8 or smaller turns as necessary. As you get close a 1/64th turn are in order. Once you get the hang of it - it's quite simple and you'll be able to do it in a minutes. Also, a tachometer is better for setting the high speed but max rpm can can be done by listening to the engines pitch. But, be careful to stay a little rich on top (high speed needle)...

Don't fly your airplane until everything is perfect on the ground. Otherwise your risking your investment. As your flying it listen to the engine. An engine in the air has a different amount of load on it. Therefore, your fuel mixture will change slighly.

I have the Tower instructions - they are too big to scan in totality, but if you need the section on tuning just email me. I can't post them as they have a copyright notice on them.

There are other methods but this is what works for me. There was some mention in other posts about transition. If your asking about setting the needles your problem is most likely not a transistion problem. A transistion problem is when you get the high and low speed needles set perfectly but the engine won't run well in mid rpm. My experience with this engine leads me to beleive that once you get the needles set it should transistion fine. Mine has a good carb. on it. I noticed two break-in points. At about 2 tank fulls it ran smooth and then after a gallon it got smoother. Now it ticks over like a Swiss Watch...