check your pulleys and belt should be able to get over 1000 or 1100 haven't messed with one of these old girls for a while but have had all these problems
you should be able to at least get 1100 out of the fan. there are two nuts on the threaded rod that limit the travel on that rod. when the belt gets worn they may have to be readjusted. If your belt width is too narrow you may have to get another one. good luck
I had a new belt sent with me when I bought the combine. The dealer had "robbed" the belt that was on the machine to give to a customer when they were out of belts. Could I have did something wrong when I intalled the beltIJ Don't see what but you never know