CB750KZ 1979 and CB750F 1979 Engine Differences

[X7eater]

I’m trying to understand, what if any, difference there is between the engines on the 1979 CB750K DOHC bike and the 1979 CB750F DOHC bike (called the CB750F-A in some markets).

I was sure that I had read somewhere that the CB750F DOHC bike had different cam profiles and larger valves than the CB750K from the same year. However, I can find no definitive proof of this. Magazine reviews of these bikes from the time show the CB750F outputting a little more HP, but that could well be just as a result of the more efficient 4-into-2 exhaust system.

I actually own both bikes, so cosmetically the differences are obvious across both bikes, so it’s just the engine I’m asking about.

Any thoughts or comments are most welcome.

 

[BAHNST]

I’m trying to understand, what if any, difference there is between the engines on the 1979 CB750K DOHC bike and the 1979 CB750F DOHC bike (called the CB750F-A in some markets).

I was sure that I had read somewhere that the CB750F DOHC bike had different cam profiles and larger valves than the CB750K from the same year. However, I can find no definitive proof of this. Magazine reviews of these bikes from the time show the CB750F outputting a little more HP, but that could well be just as a result of the more efficient 4-into-2 exhaust system.

I actually own both bikes, so cosmetically the differences are obvious across both bikes, so it’s just the engine I’m asking about.

Any thoughts or comments are most welcome.

Hi,
one option may be to look at CSMNL and compare the Cam and valve part nos....you can look up the bikes from Aust/Euro/USA specific markets on that site
Tho the "F" is listed as 1980 here.
https://www.cmsnl.com/https://www.cmsnl.com/honda-cb750k-1979-z-four-australia_model14715/partslist/E03.html#.YY3r-GBByUkhttps://www.cmsnl.com/honda-cb750f-1980-a-australia_model14863/partslist/E03.html#.YY3tAWBByUk

 

[Hugh]

Hi, Im relatively new to 750s. I thought difference was in carbies. With the F running bigger slow and main jets?

 

[X7eater]

Here's the well thought out answer that I received from a very well respected resource from another CB750 DOHC forum:

The head studs for the 750F are different than the other 750s; they are the beefier spiral looking metal like the 900/1000/1100 studs. (All 900/10000/1100 use the same studs) The K, C and SC got the thinner smooth (weaker) studs. The 750F studs can handle 35 ft lbs according to racers. The smooth 750 k/c/sc ones should stick with stock torque, 26-29 ft lbs, some go 30. They say the spiral studs are every bit as good as aftermarket moly studs and most racer/builders in the know don't bother with moly studs for these.

The cams are the same lift and duration for all DOHC 750s. There were some early ones with different sprocket fitments, but it didn't change the cam profile.

750Fs also had a different oil pan and oil path plate on the bottom of the lower case. (The pan and path plate work together as a set so the pickup is in correct position) The oil path plate on the 750F was the same one as the 900/1000s/1100. The 750F oil pan was the same shape as the 900/1000/1100 but the oil cooler line holes were not drilled. If you hold both the K and F oil pans side by side, they are obviously different. The 750F and 900/1000/1100 pans look almost alike, but the 750F pan won't accept oil cooler lines.

And early 750Fs had smaller exhaust studs.

Jetting for the '79 750F was 68/98 (Honda Shop Manual pg 22-16). In 1980 the 750F jetting was changed to 68/100 (Honda Shop Manual pg 23-15) and never changed again as it never was addressed again in the 81 or 82 addenda. (in 82 they changed to screw-in jets, but stayed 68/100) Of course the reason the 750 used leaner jetting than the other 750s was surely because it had 4-2 exhausts, and all the others had 4-4s.

Jetting for all other DOHC 750s (K,C,&SC) was 68/102 for their entire production runs of US bikes.

And once again, here's my cheat sheet for carbs distilled from Honda Shop Manual data.