See if the belt will run all the way out even with the edges of the drive pulley when you adjust it to fast.

If it won't, check to see if the secondary counter shaft with the variable sheave (the one you adjust the speed on) has moved to the right side of the machine. Should see a space between the lock collar and the bearing on the right side. If the collars don't hold, the whole shaft moves away instead of the variable sheave closing up.

 

Should be able to get alot more than 800.
I modified a piece to get more fan speed out of a couple 7720's
John Deere - Parts Catalog
I had cut about an inch off of the rod (#12 in picture) and put a bend back in it, this allows you to tighten up the sheave so that the belt will run flush with the outside of the sheave. If memory serves me correct, I think I got top speed of 1040.

 

i can only get 750 rpm on cleaning fan what should i look or do to get more speed

 

A JD mechanic told me to put the pulleys from a 7720 on my 6620 to increase the speed. However, I think I will look into what CCJersey said.

 

On the j hook that connects to the cage of the combine, I used washers to shorten the length of the rod. You could also cut and weld the rod back together to shorten it. I believe the pulleys from a 7720 are bigger and would be your best option.

 

That happens is the back of the pulley half that runs up against the sprockets that run the walkers and the shoe, wears away. Look at the alignment of your belt that runs the fan. I have had to make up a 3/8" collar that sits on the shaft between the sprocket and the pulley. It brings the pulley back in line and restores the cleaning fan speed. Done this to my 7721, and 8820, as well as neighbors 6620.

 

Thanks Chance2.

 

I made the J rod adjustable with a M16 bolt.