So it's surging?
lean running is too much air not enough fuel, or not enough fuel, period.
First i would check for vacuum leaks which are usually caused by failed gaskets that suck extra air into the fuel mixture.. i use WD40 in a trigger spray, saturate the carb/intake mounting area.. if any change in engine speed or it blows white smoke... you have air leaking into the intake.
It can also be caused by a blocked/clogged jet in the carburetor which entails removal, disassembly and a good cleaning.
what engine is on this unit?