Topic: rattle while decelerating in gear

[cb750f-2010656]

Hey, everybody!  It's been a long time since I have posted.  I pulled my '76 CB750F out of storage and have been getting her ready to ride.  So, I couldn't resist any longer and had to scratch the itch and ride her up and down the road today.  I noticed that while decelerating in gear it would rattle somewhere in between idle and like 5000rpms.  honestly do not remember that happening before.  It is normal during idle, acceleration, and cruising.  If I rev it from idle sometimes it will rattle as the rpms come back down but not always.  It has been in storage for about 5 years and I did change the oil and filter.  I have also ran her several times at rest to ensure she was gonna run well.  I was wondering if anyone has had this particular problem before and what it is, or my be.  Thank you so very much in advance for any and all help.  This engine has been solid for as long as I have owned it and am quite puzzled as to why now it decides to act up.

[ekpent]

 I had a rattle on my CB900F once that had me scared and when I really took the time to look for it with my helmet off found out it was the glass in the mirror rattling at a certain RPM    Can you describe better from which area of the engine the rattle is coming from ? Could be lots of things from driveline to engine to other parts of the bike.

[calj737]

I'd be inspecting the chain for proper tension, I'd investigate the exhaust hangars, fender mounts, and of course, Cam Chain tension.

If you say it always rattles at idle, try to locate the source of that first. Unless its the clutch rattle?

[mj1176]

You might have to put it on the center stand in gear and get it to 5k so you can try to pinpoint the rattle without worrying about riding. That's what I had to do when my 550 was making a weird buzz at 6k, turns out it was just a loose screw in the headlight bucket.

[eigenvector]

It might be something as simple as the exhaust or the exhaust mounts.

It depends on the muffler you're using - some of them have some pretty unpleasant notes and who knows - maybe some crud got knocked loose after sitting for a while (like rusty metal baffles)?  I know my EMGO piece of junk sounds like a soup can on heavy compression braking.

Have to pinpoint where it's rattling - front, rear, side, inside your helmet

[cb750f-2010656]

I have had issues before with noises turning out to be non engine related but this is a loud noise that seems to be coming from the engine.  The sound mimics the sound some engines make when the lifter have bled down, but there are no hydraulic lifters in this bike.  I don't have a center stand installed.  Guess I'll go ahead and check the valve lash and adjust the timing chain.  Gonna buy a stethoscope, too.

[Jerry Rxman Griffin aka MuthaF'er]

I'd be inspecting the chain for proper tension, I'd investigate the exhaust hangars, fender mounts, and of course, Cam Chain tension.

If you say it always rattles at idle, try to locate the source of that first. Unless its the clutch rattle?

+1 before I'd blame the engine.

[PeWe]

My bike (CB750 K6) had strange sounding vibrations when acceleratiing around 6000-9000 rpm. A friend thought it was the engine bolts not tightened causing vibrations that made other parts to sing. I was sure that I had tightened them at last resotore. I checked later and one long bolt was not tightened and others could be tightened more. Frame act as a spring when bolts have to force the frame to move closer, squeezing the engine.

Deceleration sound might be a bad drive chain?
Or rear wheel not properly aligned. Check the drive chain adjusters and the marks on swing arm.

[strynboen]

thek the chain not rubs to the senter stand .specily if it are a bit bend..it can get close to the chain

[Scott S]

 Check your drive chain tension, too.

[cb750f-2010656]

Alrighty going over the whole shebang tomorrow. Every nut, every bolt, everything tightened and adjusted out and doing a complete tune-up.  If it still rattles after that I'll get the stethoscope after it but I'll have to try to make it rattle while the bike is  sitting still.  I'm gonna have to make myself a degree wheel.

[Jerry Rxman Griffin aka MuthaF'er]

Why would you need a degree wheel? You don't unless you are installing an after market cam.

[cb750f-2010656]

If memory serves me right, timing chain is adjusted with crankshaft 15 degrees past TDC for cylinder #1.

[Scott S]

 You can just use the post on the advancer mechanism for that. It's covered in the factory manual.

[cb750f-2010656]

I have a Clymer manual.  Somewhere.  I'm not familiar with that method.

[cb750f-2010656]

Alright.  After drive chain adjustment, noise stopped.  I'm  happy.  Thanks guys!

[Don R]

 Thanks for the report, we all learn when we hear the answer.

[evanphi]

You can just use the post on the advancer mechanism for that. It's covered in the factory manual.

[ekpent]

 On a 1976 750K engine I currently have on the bench that had been sitting for a long time I noticed that when I pulled the cam chain tensioner off the back of the block the shaft had a light seize in the housing and would not have worked by just loosening the bolt to auto adjust.  I always pull those on new to me engines and clean and lube the mechanism.

[cb750f-2010656]

When I did the adjustment, I tapped a little on the cam chain adjuster in case it was stuck.  Anyway, the noise is back, and this time if I rev it I can hear it.  Also, it will rattle during acceleration this time.  My co-worker has a stethoscope.  I gotta drive mah Fleetwood this weekend so I can haul my tools to a friends house to fix his 4-wheeler.  Gonna leave the old girl in the shop.  Throttle broke on her on my way to work this morning.  It was 24 degrees out so I just put my hand down there and lifted slide bracket(?) and went on in.  So Monday I will check it out.

[ekpent]

Sounds like she is being a real problem child.

[Whaleman]

http://forums.sohc4.net/index.php/topic,52277.msg1532158.html#msg1532158

Please see a very old post I made. Dan

[PeWe]

On a 1976 750K engine I currently have on the bench that had been sitting for a long time I noticed that when I pulled the cam chain tensioner off the back of the block the shaft had a light seize in the housing and would not have worked by just loosening the bolt to auto adjust.  I always pull those on new to me engines and clean and lube the mechanism.

Auto adjust is mentioned in my manual that I got with the bike 1979. (CB750K6-76). Engine on idle around 1200 rpm, unlock the tensioner shaft and let it adjust. Rattle became worse.

A friend told me to adjust whith engine off. Unlock the tensioner shaft, stick a long bolt or screwdriver into the tensioner adjuster to get contact with shaft. Kick engine step by steT and feel where it will be in the deepest point. tighten and lock.
I guess the spring must be much harder to make the auto adjust to work, or maybe not possible anyway.

[Jerry Rxman Griffin aka MuthaF'er]

After the adjusting bolt has been clamped onto the shaft with death grips a few times it scars the shaft which can prevent the shaft from freely sliding. Remove the shaft and take a little sand paper to it.

[cb750f-2010656]

I just wonder:  All I did was adjust the drive chain and no prob, thought it was fixed.  Then I adjust valves, cam chain, points and timing, and an oil change.  What changed (besides the weather)?  It really isn't as loud as the first time I heard it.  Does the timing chain get loud after it wears or does it just break?  I did ride the hell outta the old girl when I was younger and somewhat less responsible.  Sometimes turning 10,000+ rpms.....