Ive had 2 ST 6000 and what was done to even out fuel charges
is to make a new backplate by increasing the volume in rear crankcase ..IE Machine backing plate out of solid alloy to fit rear of motor,check depth as not to crush on con rod,then machine centre as much as possible leaving about 1/4" on outside edge to hold rod to crank,mount to motor ,redrill to fit back of motor and mount.... This work for me as before this mod i had to run the motor very rich as the rear cylinder was lean even if it smoked heavily.