Difference between 750K vs 750F motors?

[ChrisH]

I bought a 76 cb750f out of a junkyard a while back. I realized after I bought it, that it had a k model motor in it. I then stumbled across a F model motor for cheap which appears to be out of a 78 cb750f (motor # 3156256) which I’m going to run in the bike instead.

My question is what are the differences between a K and F model motor? Also, are the carbs interchangeable between the two? The F model motor didn’t come with any, but I have the ones from the K model.

Thanks

 

[dirtdigger]

If it is an f motor the number on the case will start with a cb750FE-**** and a K is cb750E-*** Now a 76 f motor is not the same as the 77 and 78 f motors. The 76 had different pistons and some different internal gearing, different cam, different clutch. The 77,78 F motors had different cylinder head with much bigger valves and bigger ports, bigger combustion chamber, stiffer springs, higher domed pistons, different cam other misc parts. The bottom ends are for the most part in all the sohc 750 are same with couple differences here and there. The k carbs are different design from the later carbs. The k round top carbs are very simple, no accelerator pump, little smaller than the later keyhole carbs. If you have a true 78F2 motor you wont be able to find carb boots that adapt the smaller round tops to the big ports on the F2. carb boots are available though to put the later bigger keyhole carbs on the earlier smaller ported K and f1 head. The later77and 78 head is not a direct swap onto earlier models without some machine work and piston swap.

 

[ChrisH]

Thanks for the detailed reply! The vin on the motor reads - “CB750F-3156256”. Which according to the charts I’ve read, should be a 78 F3 motor? Sounds like I’ll be searching for a set of carbs off a 77 or 78.

 

[dirtdigger]

to verify look on the top fin of the cylinder head in between the two center cylinders intake spigots. There should be a 410 cast into it. We can also verify by measuring the intake port size and maybe post some pics of the engine.

 

[ChrisH]

Got it! Not next to the motor currently, but looking at some old pictures. It appears 410 is cast where you said it is.