Topic: Need Gearing Help 78 750K

[I Zombie]

Ok, last year I discovered that my bike has major ISSUE. For some reason @ 55-60 mph the motors turning 7000-8000 rpm. On the advise of members here I double checked the tooth count on the sprockets. Turns out the PO or some one changed them to, (if I remember right) 15/42. I need to change the chain anyway, so if I go back to stock what kinda change should I expect? Was considering the CycleX 520 conversion 17/46. Anyone know if that would lower the RPM's that much?
Any help or thoughts would be greatly helpful.

[madmtnmotors]

Your sprocket count 15/42 is only one rear tooth off of stock tooth count of 15/41. This cannot be your problem. You either need to adjust or replace your clutch. There are two adjustments for the clutch, one is the clutch cable adjustment and the other adjustment is to the clutch hub itself. Try adjusting your clutch. If that doesn't work slap a set of friction plates in it. I replaced my friction (fiber) plates for $40.00 (Parts-n-more) leaving the steel plates alone. With the 15/41 tooth count I'm only turning 5000 at 75mph with my CB750 K8. One rear tooth cannot make a 3000 rpm difference, maybe a couple hundred rpm, not 3000. Even when I've got a 16" wheel mounted my rpms will only increase to 5200 at 75mph.

Check out the gearing chart:

http://www.jtsprockets.com/fileadmin/files/jtgearratio.pdf

To try and imagine that your problem is anything other than clutch related is wishful thinking. Denial ain't just a river in Egypt...

[I Zombie]

Thanks for the input. I've played with the clutch adjustmennts & nothing changed. I'm still pretty new to this, so thats why I ask you guys. Being my first bike & all I can't say i feel any clutch slippage. It is 32 years old so whats it gonna hurt to change it out right.
Thanks agian

[KB02]

You can also check out this link:
http://www.gearingcommander.com/

Great site to help figure things out. Even though it has a spot for it, it doesn't take chain size into account, though, so changing from a 630 to 520 chain will make a difference. Still, it's great to get a rough idea of the effect of changes you'll have.

What you have vs. stock vs. what you want to go with (staying in the same pitch chain) are all pretty close to each other in the end result.

[I Zombie]

Looks like I'll be pickin up clutch plates from CycleX along with the chain & sprockets.

[Sam Green Racing]

I think the stock ratio is around 18/48 (2.66) the cycleX 17/46 (2.70) is not that far removed.
Your ratio of 15/42 (2.80) is lower but not that much to cause the problem you are encountering.
In the early days, Honda fitted the smaller sprockets and encountered a lot of chain breakages, smashed gear cases and a lot of warranty replacment crankcases.
Going to the larger sprockets don't alter the gearing that much but it does slow the chain speed down resulting in longer chain life.
It really dose sound like your clutch is slipping and needs adjusting or replacing, other than that, you do know it has 5 gears in the box and not 3. Go with cycleX and new friction plates and springs for your clutch. hope this helps

Sam.

[madmtnmotors]

Here's a site with pictures for adjusting the clutch hub:

http://www.salocal.com/sohc/tech/clutch/cth7.htm

Stock sprockets for the 78' are 15/41 (2.733)

[I Zombie]

you do know it has 5 gears in the box and not 3.
Sam.

Yeppers I keep count on my fingers as I go through em 

I should have done the clutch when I went through the motor. Oh well, that's the fun of on going projects right.

[Sam Green Racing]

Not a big job Zomb, should take about 5 minutes, you can time yourself on your fingers.

Soak the new plates in engine oil for 24 hours or at least overnight.

Good luck.

Sam.

[I Zombie]

Trust me if I have issuses, I'll askin more questions. Thanks again for all the help.