May want to check out the condition of the separator drive sheave on the backside of the lower rotor drive variable pulley. Also the condition of the sheave on the accelerator roll drive and the sheave on the fan drive. I noticed this year on our '75 that these sheaves are getting very worn, especially the driver sheave on the mainshaft. Can't be doing fan speed any good as well as belt wear. I've concluded these machines were originally designed for small grains anyway and never had any extra air flow, and as things wear it just gets a little worse than it already is. As far as speeding up the fan, I'd be a little afraid of that anyway as it may already be being pushed to its outer limits as far as blade strength goes and more speed could cause an explosion. Could also be that more speed would simply not help airflow anyway as it may be running at its most efficient speed. Good luck.