OOh I got an early F0 in the shop, I was interested in the 76 F/K engine assembly combinations. My brother had mentioned mysteriously faster 750's on occasion.
The fastest ones that I remember had the "F" pistons and cam over a "K" bottom end, with the (nicer and slightly lighter) "F" alternator rotor, running the 086a carbs and 2-outlet petcock on the tank. This gave the earlier, taller primary drive ratio to the extra-power top end, and the top speeds were quite a bit more, closer to the old K0 top ends again (131-136 MPH). The compression numbers could reach 155 PSI on a broken-in engine, hinting at the pistons and cam inside. The really doggy ones had the opposite, with the lower "F" primary drive, taller first gear, "single-outlet petcock, with the "F" head over the flat-topped pistons inside K5 cylinders, which were 1mm taller than the previous versions. These bikes wouldn't reach 100 MPH in most cases, and a compression test would come up with low numbers like 100-102 PSI, even hot.
This stuff is why the F0 and K5/6 parts fiche show(ed) both petcocks in their fuel tank parts schematics. In the early 2000s Honda "sanitized" the many parts schematics and reduced most of the listings to just one type (F or K) in the online listings, so this lore is slipping away. I have old copies of those (from when I wrote My CB750 Book) that I used for research before that happened, though.
Some of you might remember the ads Honda ran about being able to reach 50 MPG with the K4/5 during the gas crisis of that era: what they did was to install 1mm taller cylinders (compression ratio 8.8-ish:1 instead of 9:1) and lower the float bowl levels to about 26.5mm, and installed solid-tipped air screws in the carbs. This caused the mixture to lean out slightly in the 1500-3500 RPM range, and with the US speed limit at 55 MPH and top gear showing 3900+ RPM to get there, plus the slightly oversized Bridgestone rear tire of the K4/5 bikes, it actually could reach that MPG. But, it could not pull it in top gear to much more than 105ish MPH top speed, although few riders noticed it.
