You say that you have fooled around with the low speed needle, which way did you turn it and for what reason? When you say you are trying to richen it or lean it and this does not help, are you still using the low speed needle to make these adjustments? To me it seems like it is still running lean but it is hard to tell at this point.

My suggestion would be to return to factory settings for your particular engine and then tune from there. Always start tuning the high speed needle first.
These are the two methods I would suggest when trying to dial in the low speed needle.

1)
Pinch the fuel line from the carb to the tank. About a 1/4 inch away from the carb. If it dies in less than 1 second +/- a bit, it is too lean. If it is around 2-3 seconds, you are in the "general" area. If it is greater that 3 seconds, it is too rich.

OR/AND

2)
Idle the engine then give it full stick. (WOT) If it dies and sounds like it has been choked it is usually too lean. If it accelerates great then bogs in the high RPM’s it is most likely too lean.(Sounds similar to your problem) If it makes a gurgling noise (loading up with fuel) then dies, it is too rich.

You should get a consistent acceleration when properly tuned. Listen to the type of noise created by the engine as they correspond with the needle setting, during the acceleration.

As for the glow plug, it wouldn’t hurt to take it out and inspect it as this could be a possible cause. Look at it to make sure all the coils are bright orange while your glow starter is attached.

Hope this helps, its great when these machine are lively rather than on the bench

Ryan