Hmm

O.k Is the motor idling well?.motor dies at mid rpm?

Low mixture is too rich. excess fuel loads up in the crankcase even at idle .Even before HSN does it's groove

I highly recommend you turn in the low mixture half a turn maybe more [ leaner ]and try it out after half a turn do in steps of 1/8th.But be sure to adjust the idle screw to have a proper idle.leaner low needle will slow the idling of the motor [ more accelaration] and balance a correct idle but idle screw have to adjusted..

Also make sure the break-in is complete. . Take out the glow plug and give it a turn. if it is still tight on tdc more break-in period is needed. It should feel just snug on TDC with the glow plug off.