How much warmer? Is the motor too hot to touch, or just slightly warmer? Overheating can be caused by many problems: binding in any part of the mechanism (i.e. valve gear, side rods, worm and axle gear too tightly meshed, drivers out of quarter), a weak motor magnet, too much oil on the brushes & commutator, a shorted motor winding, etc. Have you removed the motor to check that the loco is free rolling?
I don't know if there's any difference in the gearing of the two engines, but all things being equal I'd expect the K4 to run a bit warmer under load because of its larger drivers.
Nelson
Ex-Southern 385 Being Hoisted
The most likely cause would be a tighter mechanism. Does the 2-8-0 draw higher current than the 4-6-2? If it does, take the motor out a look for any bind. I remember when I was building my T-1 4-4-4-4, one of the siderods was a little too tight around its screw, and the the screw got hot and nearly welded itself to the siderod. I expanded the hole for it, and everything was much better.
If everything is fine and it's still getting warmer, it could be that the H-9 has a design that traps more heat than the K4.
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