I have a new to me 1982 John Deere 6620 turbo combine.  When I have the engine RPM at max and the separator engaged, I can only adjust the cylinder speed from 310 RPM - 650 RPM.  This doesn't seem right to me because the cylinder speed RPM dial range is 0 RPM - 1500 RPM.  Based on the recommended cylinder speeds in the manual, I might be ok for corn and soybeans but I would like to get the advice of others with more experience and knowledge about this than myself.  Does anyone know what would be the cause of me not being able to adjust the cylinder speed below 310 RPM and above 650 RPM?

Thank you in advance. 

Do you have the dual range cylinder ?

I'm not 100% sure if the earlier ones could be outfitted with these or not ?

Dual range has a high and low setting that you need to move a pin to switch from low to high range.

This is what a dual range cylinder looks like.

If I remember correctly a non 2 speed cylinder would look similar to this. This is on a 4420 but I think the 6620 had the same lugs for turning it backwards ?

If you don't have the 2 speed version it could be a problem with the cylinder moving the variable cylinder speed pulley halves in or out or broken spring inside the intermediate pulley.

Below was posted before.

So if it holds true on older models ?

It looks like if his serial number starts with 600 it would be an 83 or if it starts with 595 it is an 82

John Deere made heads and combine serial numbers fairly easy to translate. The breakdown goes like this. All serial numbers you are interested in, end with 6 numbers. The first three will identify year model. I will give you some examples and they break like this. 1989 starts with 630xxx. 1990-635xxx 1991-640xxx and so on. 1998-675xxx, 1999-680xxx, 2000-685xxx, 2001-690xxx, and so on. 5000 numbers available for each year production. Hope this helps.

The breakdown goes as this. All serial numbers you are interested in, end with 6 numbers. The first three will identify year model.

1989 starts with 630xxx.

1990 starts with 635xxx

1991 starts with 640xxx

1992 starts with 645xxx

1993 starts with 650xxx

1994 starts with 655xxx

1995 starts with 660xxx

1996 starts with 665xxx

1997 starts with 670xxx

1998 starts with 675xxx

1999 starts with 680xxx

2000 starts with 685xxx

2001 starts with 690xxx

And so on.

5000 numbers available for each year's production.

Agree check actual speed with hand held tachometer. Does your tach reading remain steady while running or speed up and slow down erratically ? Does the tach speed change on it's own after running for a while ? (leaking adjustment cylinder)

Check the little rubber band that drives the tachometer. There is a hard to get to bearing behind the plastic cylinder tach pulley that can get stiff and affect tach readings. 

Here is the intermediate pulley spring Hud is referring to.

DO NOT I repeat DO NOT tear into this without talking to somebody who knows exactly what they are doing. This spring is under a lot of tension and can seriously hurt you if you are not familiar with how it comes apart and the tension.