Topic: CB750 clutch slipping question

[Garage_guy_chris]

Hello guys

So been having some issues with the clutch on my 74 750. First off its a Cruzin_Image Ebay 836cc build with a ported head and mild can so its making some extra HP,

The looked ok used clutch i put in when build has always slipped at high rpm and load, I had also used some Purple Circlip racing clutch springs. Earlier this summer I changed them out with some Barnett springs and it actually started slipping worse. So I think its time to change out the clutch.

Searching around I found this about early vs. late style clutches and 1.460" vs 1.560" stack height, was thinking possibly I have mixed up early vs late clutch parts? Bike was a basket case with all sorts of mixed parts...




Got around to opening things to check and found:

-Early 41mm clutch hub
-Early U groove banded clutch basket
-7 Friction discs
-6 Steel discs
-Am using a proper MC oil

Friction Discs (all measure at lower spec)
1  .135
2  .135
3  .135
4  .135
5  .134
6  .133
7  .134

Steel Discs
1  .078
2  .078
3  .078
4  .075
5  .078
6  .078

Total thickness of
    1.406" VS ideal 1.460"

Read up on here with suggestions to use the original style  square cut friction disks, decided on a CMSNL kit and the Purple Circlip springs that obviously have some more clamp force then the Barnett springs. Hoping this should clear up my issues...

Kinda surprised .044" of wear is doing the clutch in. hoping im not gonna need an upgrade to handle HP

https://www.cmsnl.com/honda-cb750k4-four-1974-usa_model482/disk-assyclutch-non-oe_02220300000/

[HondaMan]

When your clutch is all assembled, is the face of the top plate even (flush) with the end of the basket's 'fingers'?
And...are the clutch outer's 'fingers' notched wider at the very top?

If so...you need to add a steel plate in the stack. The original K4 clutch usually had a double-steel riveted plate in the stack, most often found 2nd steel from the back of the stack, and the top plate had slant-cut corks on it, and it was thicker than the other plates. If you removed the dual-steel plate, you need to make up that stack-height difference. This is most easily accomplished using an extra steel plate against another steel plate, often at the bottom of the stack. Then you don't need extra-strong clutch springs, which will cause wear on the clutch lifter, the clutch cable, your wrist, and more easily break the pressure plate if you accidentally don't tighten the little bolts REAL evenly (like 1 turn at a time) on the way down.

The stock clutch can manage more than 90 HP, so it seems odd that yours would be slipping unless something was missing.

[HondaMan]

P.S....
There are actually 4 different clutch outers: the Sandcast (9 screws, not 10), the Early (thru K3), the Middle (K4-K6, F0/1, sometimes found in K7/8), and the Late (F2/3, sometimes K7/8). The Late version is also associated with a different clutch cover on the engine in order to accomodate the extra-tall clutch inside. If you install an Early clutch cover over a Late clutch stack, it will not let you find any slack in the cable as the adjuster is too close to the clutch. If you install a Late clutch cover over an Early or Middle clutch, you may run out of clutch adjuster room and the clutch cable will seem to "be too long" as you can barely disengage the clutch.

If you install an Early 10-hole clutch cover over a 9-hole sandcast clutch, you will lose thousands of $$ in value of the bike...

[Garage_guy_chris]

I sorta pieced all this together with the Hondaman book, parts fiche's, forum stuff and cycle X

-I do recall it was pretty close to flush with the end of the basket fingers.
-Fingers are NOT notched bigger at the top, they are even all the way down
-Seem to remember installing a late (tall clutch) cover on this clutch and it had huge freeplay and couldnt be adjusted. Changed it out with a different one off 0454 or 1600 and fixed all the freeplay and disengagement problems   

The engine serial numbers were #2350454 (Damaged cases) the guts got put into #2351600 and both were older K3 design with no knock pins in cylinder / head. I seem to remember up to a certain K4 serial they retained K3 features, up to 2352922
 

-Hub 41mm tall with No angled "B" plate with wire retainer (K1-early K4). also not later 43.5mm later style either
-Reinforcing band around basket and V cut finger ends (K1 - early K4) so not K0 early also not late with straight cut fingers
-No rivited double plate on either clutch. I have seen what your talking about..
-All frictions are square cut NO angle cut plates
-Only issues i could find are frictions worn down to the bottom of specifications and steels looking a little glazed.

[RAFster122s]

I presume you have checked the steels to ensure they are flat, per manual specs. Any warpage can cause engagement issues and bizarre clutch behavior.

[Old Scrambler]

I would install a double-steel at #2 position and use stock springs. Be sure to soak the fibers in oil before assembly. I have had good luck with Barnett RED springs in my race bike, but other weights (colors) may be available...........however; stock springs are best for the street.

[HondaMan]

I would install a double-steel at #2 position and use stock springs. Be sure to soak the fibers in oil before assembly. I have had good luck with Barnett RED springs in my race bike, but other weights (colors) may be available...........however; stock springs are best for the street.

I'd like to echo this....good advice!

[Don R]

 My new friend that had an American turbo pak cb750 K2 years ago fried a clutch so bad it plugged the oil screen and head jets.  Don't do that lol.

[HondaMan]

My new friend that had an American turbo pak cb750 K2 years ago fried a clutch so bad it plugged the oil screen and head jets.  Don't do that lol.

I'd bet $1 that the clutch plates were also not OEM plates: this was a sommon issue with the Barnett plates, particularly when used with stock springs.

[PeWe]

My CB750 K2 has same clutch parts, outer basket and alu parts. Cruzinimage clutch fibers and Barnett springs for CB750.

I have used Cruzinimage clutch with 836 and 80whp, no slip.
Those replaced the very sticky CycleX clutch.

- The wide washer is mounted after the outer steel basket?

Later on K6 and forward that was locked by a circlip on the shaft has a slimmer one.
The outer washer under clutch nut is flipped correctly?
Convex shape outwards so it can act as a spring.