Combines 843 cornhead on9400

The machine has the capacity, it's a matter of ground speed and settings.

Is the problem the extra weight of a 843 vs. a 643, both in lift capacity and how the 9400 will handle (not enough weight on back end)IJ

I believe lift capacity is adequate, and the RWDshould counteract the extra weight unless you're going down a steep hill and pull back the hydro too fast, although you'd probably see this with a 643 as well.

There are alot of 7720's with 843 corn heads out there, compare the spec. of the 7720 to a 9400! The spec are at least equal if not higher on the 9400. We run 9400's with 6 row heads and can run 3.5 MPH plus in 180 to 190 bushel corn under 20% moisture. Ground speed is the key with the bigger heads.