Topic: CB750 camshaft differences

[fritzer_108]

I have a 1973 CB750K and have a spare R2 camshaft from a 1978 cb750k sitting around. Are the profiles of the camshafts the same for the cb750 K series? I'd be willing to replace the current cam with one that has a slightly hotter profile.

[kmb69]

Cycle-X has about every flavor you can think of.

[bryanj]

Numbers on the cams mean nothing and Honda changed the profile several times between 69 and 78. I believe the hottest cams were 69 and 78F

[Jerry Rxman Griffin aka MuthaF'er]

I'd bet the 78 is hotter than the 73 so you may get a small boost BUT, as KMB says...... there's a lot more out there

[HondaMan]

The cams were broken down into 5 distinct types:
The K0
The K1-K3.
The K4/5/6
The F0/1 and K7/8
The F2/3.

The K0-K6 types will all drop-in for each other, with slightly varying carb jetting: the K0 and #657A carbs use #110 mainjet while the others use #105.
The F0/1-K7/8 uses the leaner mainjets, usually #105. The F0 and F1 with the roundtop carbs use a leaner slide needle because the intake valve opens later in this cam. The later F1 and the K7/8 use the PD41 carbs with #110 mainjets and #38 (or #37.5) pilot jets.
The F2/3 uses the PD42a/b carbs (some K8 also got these carbs toward the end of production). These may have #110 or #105 mainjets, with #37.5 or #35 pilot jets. The PD42 carbs have only one needle slot. Spark timing comes on slower in this cam to prevent spitback up the intake tract from the longer overlap cycle.

There is also the "A" cam from the 750A but it is not a performer cam. It makes the 750 run like a 500 twin, but smooth!

All the cams will inter-fit the other engines, but you may have some tuning to do to make it run well. The later cams (F0 onward) idle better than the earlier ones, especially in hot-day city traffic.

[PeWe]

My CB750 K2 got a K7-1978 cam I found cheap  complete with cam holders, sprocket, rockers. Around $100US for all 5 years ago.
The cam has a ring in its mold  beside tacho gear.

I measured lobes with a caliper. My old worn and rusty K6 cam has a little bit more lift, 0.1mm if I remember correctly (total-base circle). But I doubt it is noticed. Maybe the cams have that variation anyway.

It runs with valve lash of 0.1mm on both in and ex. I do not trust Honda stock 0.05/0.08mm. 5/100 mm can easy become 0 with bad consequences.
57-58whp measured on dyno. This with compression enhancing K7 pistons.

I bought another K7 cam with other stuff for $40US and it look like not much used.
These cams usually cheaper and I cannot see why old K1-K6 cams should be better.

F2 1978 probably an improvement.
CycleX CX-1/Webcam 41 will give more anyway, right?

[HondaMan]

I measured lobes with a caliper. My old worn and rusty K6 cam has a little bit more lift, 0.1mm if I remember correctly (total-base circle). But I doubt it is noticed. Maybe the cams have that variation anyway.

It runs with valve lash of 0.1mm on both in and ex. I do not trust Honda stock 0.05/0.08mm. 5/100 mm can easy become 0 with bad consequences.
57-58whp measured on dyno. This with compression enhancing K7 pistons.

I bought another K7 cam with other stuff for $40US and it look like not much used.
These cams usually cheaper and I cannot see why old K1-K6 cams should be better.

F2 1978 probably an improvement.
CycleX CX-1/Webcam 41 will give more anyway, right?

The main difference in the cam timing is in the post-1976 type: they all open intakes at 0 degrees (with a fresh cam chain) and there is more overlap, due to the lobe having the same duration, while the exhaust remained almost the same as before. They have the same lift height, but a slightly smaller base circle: this was a common 1970s hotrodder's trick for increasing lift with the same camshaft. I was a little surprised to see Honda do it with the F2/3, but that was what we found. The difference in lift wasn't much: instead of 7.8mm as in the earlier engines, it was 8.0mm instead. Combined with the bigger intake valves that does make some difference, though.

This later-opening profile helps boost power at 7000-8000 RPM in the later engines. The carbs got bigger (and more accurate mixing) in the PD series to help support this, too.