Originally Posted by
bruceal
I would say that the low end is too rich or too lean. It depends what it does when it quits. if you try to open the throttle and it sputters, it could be too rich, if it shuts off sharp it could be too lean. Try the pinch test. If you're not sure what that is, it's just pinching the fuel line while the engine is at idle. The rpm's should rise before the engine quits. That's what I would look at first.
First, adjust the high speed needle to achieve peak RPM's at WOT. Do not back it out to richen yet, do the pinch test first. With the pinch test, the rpm's should rise at most a little or not at all and the engine quit after a while. If the engine speed picks up noticeably and runs a long time before quitting, then it is too rich. If it quits soon after pinching, then too lean. Adjust the low speed needle accordingly and re-run the pinch test. When the pinch test is set, do the throttle up test as described above. Once satisfied with the setting, go back and richen the high speed needle a bit. Then re-check the idle, transition, and WOT for good performance.
The carbs on these engines are perfect, unless you have a bad o-ring. The o-ring on the high-speed needle on my 55AX tore up within the first couple of years of use. So check that o-ring and the o-ring at the base of the carb. Replace if they look bad.
By the way, what prop are you using on this engine? Is it within the recommended range by OS? That can also mess things up.