I couldn't find anything about how to adjust the special H-head, but I did read that Homelite doesn't offer any advice on adjusting it, and requires that the unit be sent in for servicing,
I would take it in to a local small appliance repair place and see that they say- they may have delt with this problem before, and be able to tell you, or perhaps they will be able to do it for you.
Wish I had something more specific, but it sounds like they really don't want people adjusting it on their own.
Good luck, and hope this helps.