For the F, wide space cylinder bars are thee BEST thing you can do to enhance performance. If you have sidehill sieve problems with grain loss, install 6 cylinder bars so they throw the crop to the center of the machine instead of 4 and 4. I've even done machines so all 8 throw to the center--it has NO effect on threshing, just helps throw the grain towards the high side of the combine when leaning left or right. You lose the ability to reverse the bars for wear when you do this, but the benefits are worth it.Think about it, if bars always throw to the center, on a sidehill they will throw grain clear to the high side because the cylinder is only loaded on the low side! there isn't anything being thrown to the low side.It will hurt flatland performance some, but in rolling or nothing but hills this works great. The other thing on an F or K would be making sure the governor is adjusted properly. Every time the belt is tightened, governor linkage could potentially be changed. High idle should be whatever your op manual says (different yrs means diff rpm spec) and the engine should stay within 100-150 rpm of high idle under load for the governor to be working properly. If high idle no load is 2500 you should always be at 2350 to 2500 under load.