Can chain

[Dave H]

I’m new to the forum and I’ve recently bought a 78 cb750 which has a rattle whilst running
I’ve checked the cam chain tensioner isn’t stuck but it still got the rattle
Is this a common issue?
Does this now need replacing?
Any help would be appreciated, the bike mechanic I took it too for a service and check over when I bought it doesn’t seem interested in sorting it

 

[UK K6]

It's a common question when you first get one of these bikes. First I highly recommend you download a copy of the workshop manual. It's comprehensive and easy to follow, takes things in steps.

You don't mention how many miles on your engine. If regularly serviced they seem to run for high mileages, but the bigger the mileage, the greater the chance of poor maintenance and wear. Usual engine rattling sounds are primary chains, cam chain and the clutch. The clutch is easy to check. Pull it in, if the rattle goes, there's your answer.

Assuming your cam chain tensioner is free to move, when released (mine wasn't until i took it apart), primary chains are in my experience, the usual source of rattles. My 45,000 mile CB750K6 does it if I set the idle below 1300rpm, even though it is in spec. Increase the idle slightly and the engine runs smoother and doesn't cause the chain to flap as much.

There are quite a few things that affect a smooth idle and increase the chain rattle:
- If your carbs are not properly synch'd, the engine varies It's idling more, causing chain slap.
- A dirty idle circuit in one of the carbs
- Ignition can cause uneven running, poor timing or points gap on one side. A weaker spark on a cylinder (coil, failing plug lead or cap, or dirty / old plug)
- Also I've read about, but not personally experienced, the spindle the points cam (& auto-advance) run on, this can be bent by gorilla use of the large nut on the end when checking timing. Less force on there if you take the plugs out and keep it smooth.
- Poor compression on a cylinder.
- An out of spec valve clearance.

In short, a good service and general tune up and you'll have ruled out the usual suspects. These things are well covered in the workshop manual.

Having done all that on mine, it is still noisy below 1250-1300rpm. So, I set my idle at 1300rpm and jobs a good-'un! If you find YouTube videos of these SOHCs idling, they're either rattling away, or idling north of 1300rpm. So I think that's pretty usual. 1300rpm doesn't noticeably affect engine braking. It may even help things, better oil pressure and battery charging!

 

[dirtdigger]

It is very common to have the timing chain tensioner wheels come apart, they are a rubbery material that gets hard and brittle after 40 some years of running in extreme heat and engine oil. The same thing will happen with the primary chain tensioner wheel. Many people think the clutch is the major noise in the bottom of the 750 but the primary chains make as much noise as anything once the engine gets a couple thousand miles on them. The chains stretch pretty fast and they start to whip make noise at idle especially if tune up is not perfect.

The problem with having a high idle is you are starting to get into the centrifugal ignition advance in the 12 to 1400 rpm area. This will cause even more issues with hunting and hanging idles

You can pull the clutch in with it running and it will probably get quieter which indicates primary chain more than likely getting loose. If you still hear some rattling its probably the timing tensioner wheels chuncking out.

 

[Dave H]

Thanks for the advice
I’ve ordered a workshop manual so I’ll have a read up and try and sort

 

[UK K6]

Dirtdigger, at 1300rpm I'd guess the advancer could be starting to move. I haven't found it causes any hunting or hanging on my 750K6, so maybe those issues arrive if other aspects of the motors tune need attention, e.g. carb synch, running lean...?