Need help with slip clutch

Hello, not exactly hay, but I do use this machine on hay field verges

 

I have a Farm King 720 3 pt. hitch brush mower, bought it new, it has worked well for 2 years.

Once in a while I've "stalled" the blades on a clump of dirt or a stump, caused the slip clutch to do it's job and save the gearbox and driveline.

When this happens, I don't always notice right away so the clutch will begin to smoke.

 

Lately though, it seems to take less to stall it out and the clutch smokes right away.

 

I have looked through the manual and online, but have not found any info on how to adjust the slip clutch so that it does not slip too easily.

 

Any suggestions?

Need help with slip clutch