Excessive vibration.

[Magua]

Once or twice before while riding there has been 'excessive vibration', just for a second under load. As of this morning it's back and more pronounced. Noticeable as I pass 4,000rpm and much more noticeable when attempting to tackle a hill.

A quick look at the Clymer service manual says that 'excessive vibration' is usually caused by 'loose engine mounting hardware or worn engine or transmission bearings'. I can borrow a torque wrench to give the engine mounts a good tightening to try to eliminate that line of investigation.

I'm fairly certain that my valve clearance needs adjustment, but could this be related at all? (I don't see how, but I thought I'd ask.)

Can anyone speak from experience?

Edit. I've been doing some further reading. Could be a clutch issue?

1980 cb400n btw.

[Sollyboy]

It could be as simple as one fucked link on the chain , so check each link, is the bike making ok power?

[Magua]

It could be as simple as one fucked link on the chain , so check each link, is the bike making ok power?

That's interesting, I just got a new chain the day before yesterday.

The bike could do with a carburettor balance, but power seems fine. Seems to lack a little at the top end.

[FilthyLuka]

That's interesting, I just got a new chain the day before yesterday.

The bike could do with a carburettor balance, but power seems fine. Seems to lack a little at the top end.

Check the chain tension, new chains stretch out to balls when they are put on. Tighten up the engine mounts, does that bike have rubber mounts? if it does than they may have shat themselves.

If its a transmission problem then.... well...

[Jantar]

A few things that may help with the diagnosis:

Is it just related to rpm?
Is it related to throttle setting?
Is it related to speed?
Or is it a combination of the above?

Try cruising at the speed where it is most pronounced, then back off the throttle and pull in the clutch. Does the vibration disappear completely, or just reduce? Is it affected by holding that speed and changing gear?

The answers to these questions will help to determine the area to look at closely.

[James Deuce]

CB400N's are renowned for vibrating like bastards as they wear. 50,000kms is normally about the time the need everything from the primary drive back to the rear wheel bearings replaced.

Err, that's horrible news isn't it. I wish I could afford to buy you a brand new bike mate.

[Magua]

CB400N's are renowned for vibrating like bastards as they wear. 50,000kms is normally about the time the need everything from the primary drive back to the rear wheel bearings replaced.

Err, that's horrible news isn't it. I wish I could afford to buy you a brand new bike mate.

Well I'm only at 34,000, so hopefully the news isn't all bad.

[Magua]

A few things that may help with the diagnosis:

Is it just related to rpm?
Is it related to throttle setting?
Is it related to speed?
Or is it a combination of the above?

Try cruising at the speed where it is most pronounced, then back off the throttle and pull in the clutch. Does the vibration disappear completely, or just reduce? Is it affected by holding that speed and changing gear?

The answers to these questions will help to determine the area to look at closely.

Okie dokie, I just went for a ride.

Rpm, yes and no: It happens around 3500 to just below 4,000, but only if in a higher gear than I'm meant to be. I tried it in 5 and 6th, much more noticeable. It doesn't seem to happen if I'm accelerating hard, or at least I don't notice it. Then again, yesterday while carrying a pillion, it was happening without being in a higher gear.

Throttle setting: if I pin it open in a higher gear, no change. Lower gear, throttle open it's not noticeable.

Speed: throttle, rpm and gear seem to be the key. A combination.

By throttling off and clutching in the vibrations disappear.
By changing down a gear the vibrations disappear.

[Jantar]

OK, The good news is that you just eliminated the simple causes like wheel balance, chain/sprockets etc.

The bad news is that it appears to be internal. Maybe someone with more knowledge on your specific bike can help pin it down further.

[Magua]

OK, The good news is that you just eliminated the simple causes like wheel balance, chain/sprockets etc.

The bad news is that it appears to be internal. Maybe someone with more knowledge on your specific bike can help pin it down further.

Dooooh. Not good news.

Thanks for the help though. _b

[Magua]

'Mind you, could also be the carbs need balancing. A broken/cracked engine mount is possible.'

'Don't those engines also have a chain driven balancer that's realtively famous for screwing up somehow? Could this have come out of synch? I vaguely recall something about it not having a tensioner for said chain.'

'Stinkwheel, you read my mind. I was thinking carb balance or maybe the balancer shaft chain is loose (if it's chain driven, I never owned one of those new fangled 'N' series bikes).'

'Snap...was also thinking carb balance, it's amazing what a difference getting the carbs balanced can make.''


Some feedback from another forum. Maybe it's not all doom and gloom.

[Max Preload]

Sounds more like a misfire - do you know what that feels like?

[Magua]

Sounds more like a misfire - do you know what that feels like?

Nope. Could you enlighten me?

[xwhatsit]

Superdream motors had shitty chain-driven balancers. The 250/400T was especially bad, but I thought it was improved by the time the N came around. Adjusting the balancer on my CB250RS is a bitch; you have to take off all kinds of bits and pieces to get at it.

Does the manual show you how to adjust the balancer chain?

It would be nice if it were just the balancer, as that's not very important, really, and means no damage has taken place. Heard of people dropping the balancers out of certain motorcycles as you get more horsepower (and really numb nuts), but that sounds a good way to shake your cycle to bits.

[Magua]

There's something I can unscrew to gain access to the Balancer chain adjuster nut. If the adjuster fails to self adjust when said nut is undone then I have to remove the right crank case cover to do it by hand. So I've been putting it off, heh.