I have a BX 1800. I had the same problem with my tractor. The owners manual wasn't very helpful. It suggested contacting the dealer. Fortunately the adjustment was covered by warranty and I don't know how much it would have cost if it weren't. A creeping machine as powerful as even a BX is a dangerous problem. I know that I haven't been very helpful but since I viewed your question and you seem anxious to get some feed back I thought I'd share what little I know.
thanks again everyone...i have to take it to the dealer, the machine won't even click into position to start. now, i just have to figuer out how i'm going to get a non-starting machine onto my trailer /forums/images/graemlins/confused.gif.