CB750K7 sohc Bottom end rattle

[leonardo269]

Hi,
    I am starting to restore a 1980 CB750k7.

I have heard it said by various people that "all the CB750 sohc engines had noisy primary drives", but I really carnt live with a bike that makes such a racket on tick-over. (sounds like a bag of bolts).  

What have I got to do to shut it up? I am sure that they didn't sound like that when they were new.

I used to have a CB550 that sounded like a sewing machine in comparison!

The last thing I want to do is replace the primary drive chain and find the racket is still there. If anyone has been here before, I would appreciate your advice.

Pic is the bike next to my CBX750.

Cheers,

Ray

[L.A.B.]

The primary drive does get a bit noisy on the SOHC CB750's as they run two single primary chains rather than the more common Morse (also known as Hy-Vo) chain as used on the CB550 and many other types.

Probably a good idea to change the cam chain at the same time if you do decide to strip the engine.

If the carbs are out of syncronisation then the clutch can rattle.

[Allenrc1]

Honda CB750 1969 through K9 had a bad habit of making excessive primary chain noise to fix this you need to replace the primanary chain and the chain tensioner!  Ex Honda  mechanic.

[kayabee]

Start with the cheapest and easiest solution and have the carbs synchronized this can only be achieved using proper vacuum gauges, idling speed can be set a little higher than factory recommendation. If noise still persists then you will have to go down the road of clutch plate or clutch basket replacement

[neale]

A quick way to see if the cause is from the primary drive & gearbox area or from the actual engine ( mains,bigends,cams etc ). Start the engine, then put it into first gear, put the brake on , then slowly release the clutch until it just bites,if the noise stops or greatly reduces,then almost certainly the cause of the rattle is either ;carbs out of balance or primary chains worn.These engines are noisy on tickover,even from new,I used to work on them when they were new!!
       All the best Neale.

[leonardo269]

Thanks for your help everone. The racket continues whether or not the clutch is pulled in and even when it is on the bite, so I think it is the primary chain.  I will get the carbs syncronised before I go tearing the engine apeart though. I have just purchased some gauges, so I intend checking this as soon as the british weather allows (August then!).  

Cheers,

Ray

[tabby]

hi, did you sort the rattle , as I have the same problem on mine, if sorted ,what was it? cheers. Dave.