I kept this on my PC, you may find it useful:
Hondaman says:
The sum differences in the F3 engine vs. the K5 are:
1. The cam has more lift and 5 degrees later intake duration (but same dwell).
2. The F3 has the lightened cam sprocket.
3. The crank is slightly different, but not enough that you will notice unless you try to install a K crank. You work on it just the same, though.
4. There are only 6 rubber drain-hole dowels in the cylinder deck in the F3 instead of 8.
5. There are cap nuts on those same engine studs that have the drain-hole dowels, in the F3. The K5 has normal nuts.
6. The pistons are unique to the F2/3 engines. Earlier pistons don't fit well, because the F3 head also is much larger in the chamber (to fit the F3 valves), so the compression comes out low if you swap pistons with the K engines. You have to find other pistons, like the ones I use: I modify the SOHC750 (1979-1982) to fit, as 1.00mm overbore size instead.
7. The carbs are PD style, more complicated in assembly than the earlier ones. They work the same, except Keihin added an accelerator pump to improve on the throttle lag of the post-1975 bikes. The F0/1 suffer the most from this issue.
8. The F3 clutch is like the F0/1/K7/8 clutch, with a double-steel "slipper" plate in the stack, usually at position 4, and at least one slanted-cut-cork plate to ease the search for Neutral when hot. This is like the last of the K6 bikes, which had F0 clutches.
9. The F2/3 use a 630 size chain and do not have a drive-chain oiler.
10. The head casting will not accept the front cam chain slipper from the earlier engines unless you file away the inner side of the cam chain tunnel to clear the tensioner. If you need a slipper tensioner today, this is the "fix", as the former shorter ones are hard to find. It works fine, I've done many. It takes about 5 minutes.
That's about all the differences there are. It's the same general design, but with less overall spark advance, smog-control carbs, better-breathing (and shorter-lived) valves and guides and a bigger combustion chamber to let the valves breathe more. Oh- and it has a PCV canister under the battery box to keep the oil fumes from exiting the engine breather pipe - so the spigot on the back of the engine is a different size than on the K engines. Overall, it made about 5 HP more than the K0-K1 early engines did.
^^^
And that is the most extreme difference in SOHC CB750. The book is a great guide for rebuilding your bike.
