Honda CB750 1980 (KZ) Big knockers

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That got your attention!!

My 1980 CB750 Kz has a shocking clunky knock at tickover which disappears when the motor is revved.
Anyone had one of these (DOHC) motors and had a similar experience?

I've adjusted the clutch, balanced the carbs and adjusted the cam chains F and R.

Scare the beejeezus out of me at tickover, but the motor is sweet as a nut at anyything over 2k RPM???????

Standing by with lump hammer and several cans of Holsten Pils at the ready :D:D:D
 
Has it a mechanical advance and could it be slack springs causing ign knock at low revs?
 
Maybe a weak cam chain tensioner / slipper thing, causing the cam chain to snatch at low revs :nenau

Nick
 
Digger06 said:
does it do it with the clutch pulled in?
Click to expand...

Yep

Not as bad, but still a definite yep.


With the clutch lever out, in neutral, waiting at the lights, this is more like something's alive inside the motor, under or just behind the block and it's trying to find my leg by breaking out of the cases, rather than a discreet tapping of a silver teaspoon an a china cup to attract the attention of the diners at the WRVS monthly dinner.

Rev up and it goes away over about 2k RPM.

I belive there's a primary drive chain on the DOHC motor. I'll have a scan and see if there's a tensioner to fettle.

Goes like a (pretty quiet) train otherwise! :D:D:D
 
There's no primary drive chain tensioner from what I remember. Its a HYVO chain about 4cm wide. The cam chain (also HYVO) that runs up to the DOHC between the middle 2 cylinders is prone to stretching (and breaking, DAMHIK) and that does have a tensioner with a plastic slipper guide thing. At low revs and if worn, this can flap around a bit.
Maybe this is the cause of the noise you hear
 
hyvo tensioner prob worn, but check clutch basket 1st, they can wear , and create backlash, springs could be worn
 
Digger06 said:
hyvo tensioner prob worn, but check clutch basket 1st, they can wear , and create backlash, springs could be worn
Click to expand...

Cheers

I have completely stripped and rebuilt the clutch basket, and miked up all the plates and fibre plates too! Clutch springs are all within service lengths, and the main bearing seems OK.

I've a genuine Honda manual (uselss shite pix!!!) and it shows a tensioner for the primary drive chain, but no adjuster as in the cam chains! Looks like there's a lot of work ahead to get this bugger quiet!:aidan
 
im on about free play between basket and the gear fastened to it?not the plates
 
Another one with a big knocker

Hi

Did you ever resolve this problem with your CB750? I have just taken on the restoration of one with exactly the same issue and wondered if you ever found out what the problem was?
 
Stevie63 said:
Hi

Did you ever resolve this problem with your CB750? I have just taken on the restoration of one with exactly the same issue and wondered if you ever found out what the problem was?
Click to expand...

After a year on the bench, I've 90% rebuilt the bugger. Turns out the primary chain from the crank to the box is tensioned by oil pressure acting on the tensioner. Ive replaced the tensioner and cleaned out all the galleries which were full of shit.

I'll let you know in another year whne I've finally rebuilt it if that was the solution.

Sorry but that's as far as I;ve got :(:D:D
 
Nippy Normans supply Touratech, Wunderlich, Desert Fox and many other makes of accessories.
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