Besides the obvious styling differences: wheels, twin disks (front) and single disk (rear), squarish tank, seat and tailsection, 4-1 exhaust, and black paint on the '77 and '78 engines, they are amazingly similar in character.
The stated intent was to target the F-model to the more "performance oriented" rider. In the process, Honda made some good choices, such as a hotter cam, bigger valves, a stronger cam-chain and drive chain. They lightened the charging system to reduce crank vibration.
Honda also made some poor choices, such as the design of the valve spring retainer, which failed in some cases leading to some top-end troubles when pushed hard. They also leaned-out the carburation to meet emissions and save gas, but the motors run hotter, and the carbs are not as easy to overhaul.
The '77 and later bikes seem to have a heavier frame with longer wheelbase, whether that's the frame or swingarm I can't tell. These bikes are more similar to each other than to any other comparable bike available in the same decade.
My take is that the F-model never caught on with the American Buying Public because there were so many other better or cheaper choices by the late '70's (KZ 900, GS1000, CBX, GL1000) I have both a '78 K and a '78 F. I like the F better, but I've ridden the K more. In general, the K model with the rounded tank and 4 pipes is the bike people fell in love with in the early 70's, and since the later '70s models are not as coveted, they are cheaper - including the F-bikes. Not bad for what is essentially the final stage of development on a motorcycle that defined the term SuperBike.
Six - Minneapolis.
