GSX750F banging noise?

[sammcj]

Hello and Happy New Year to all the wise KB'ers out there!

I've purchased myself a 98 Suzuki GSX750F Katana from 200BUSA here on KB.
Really nice machine and BUSA was very helpful.

Here lies the problem (I thought I'd pick everyone's brain at once to see if this has happened to someone before):

There is a noise that sounds a bit like something making a thumping / thudding noise against the tank between 2.5 - 5K RPM.
Obviously it's not something banging against the tank and it's something echoing up through the tank / faring.

The noise is:

-Regular
-Not there with the clutch in
-Worst under load, but travels evenly with the rev's
-Can't hear it after around 4-5K (Probably just due to engine noise)

BUSA and I thought it was the worn front sprocket, perhaps with one tooth that was extra worn / chipped BUT:

-The front sprocket IS worn and needs to be replaced but as T.W.R. wisely said not worn enough to be making a noise like that.
-I had a think about it and if it was the sprocket surely it would be faster in a high gear at the same RPM - but it's not.
-The engine / gear box runs mint, very smooth and rev's nicely.

The owner before BUSA must have put some horrid greasy rubbish on the chain which has covered anything near the chains path in grime which I've cleaned off, I've cleaned the chain (which is much newer than the sprockets are is in good order) with meths and will lube properly when it's back on.
I am replacing the sprockets tonight as I have already bought them and pulled the damn thing apart.
I've googled the hell out of the problem / noise and have come back with nothing.
I'm guessing the sprocket isn't going to fix it and I'm quite worried as I don't have the funds for anything big at the moment.

Any ideas O wise ones?

[MSTRS:Me]

More info reqd. Can you feel it as a vibration? Does it in/de-crease with revs? Is it only when moving?
The clutch in could be a clue.

[CookMySock]

Engine mount loose?

Steve

[sammcj]

More info reqd. Can you feel it as a vibration? Does it in/de-crease with revs? Is it only when moving?
The clutch in could be a clue.

I could probably feel it every so slightly if I think about it, not really though?
It slowly gets faster with the revs, starting just a bit faster than once per second and moving up to 2-3.5x per second.
It's only when moving but haven't tried it on the stand with the clutch engaged (probably not a good idea?)

[sammcj]

Engine mount loose?
Steve

Very good point, might be a bugger to find, I'll look over tonight after the sprockets are back on (wish us luck!)

[one fast tl1ooo]

Maybe 200busa left a little guy under the tank with a little hammer.

[CookMySock]

If you don't find anything external, I'd be really tempted to whip its covers off and make sure you don't have a sticky valve stem. It's a shot in the dark, but one that would pay dividends if you were right.

Steve

[T.W.R]

Just reading what you've said in PMs and posted in the thread, personally I suspect it's elsewhere in the transmission unfortunately.

Especially when you say it disappears when the clutch is engaged (I presume you've done this while on the move?)

Chains & sprocket make noise when they're poked: tight spots, unlubed, broken or damaged teeth (easily spotted...apart from the obvious the damaged teeth scar the chain links, collapsed or disintergrated rollers, damaged chain: twisted etc

If the noise is being made by the external parts of the transmission (sprockets & chain) then it'd be constant regardless of the clutch being engaged whilst moving. IMHO I'd be looking at the main driveshaft bearing (that's why i said to check for any weeps around the seal) It isn't a major job to replace the bearing just awkward to remove the old one. seeing you've just got the bike from 200BUSA I'd be at him to get it fixed (he'd have suspected that it was the bearing I guarantee).

With the chain removed you can manually turn the drive sprocket/main shaft and you'll feel if it's notchy or not.

[MSTRS:Me]

I could probably feel it every so slightly if I think about it, not really though?
It slowly gets faster with the revs, starting just a bit faster than once per second and moving up to 2-3.5x per second.
It's only when moving but haven't tried it on the stand with the clutch engaged (probably not a good idea?)

Only when moving narrows it down a bit. Now you have the problem of further narrowing...(it could be something in the front wheel or discs - but unlikely).
A tight spot on a chain can make a noise, but is more usually a thumping vibration through the frame. The tight spot is not always the chain, but a sprocket out of round. The front sprocklet is probably too small for that to be a problem unless your chain is adjusted too tight to start with.
If the back wheel is off the ground when on the main stand, give it a go.

[sammcj]

Just reading what you've said in PMs and posted in the thread, personally I suspect it's elsewhere in the transmission unfortunately.
IMHO I'd be looking at the main driveshaft bearing (that's why i said to check for any weeps around the seal) It isn't a major job to replace the bearing just awkward to remove the old one. seeing you've just got the bike from 200BUSA I'd be at him to get it fixed (he'd have suspected that it was the bearing I guarantee).

BUSA seemed like a decent sort, he said he was 90% sure it was the sprocket and if there were any problems at all to bring it back for him to have a look at so I don't think he'll have any qualms looking into this.

[sammcj]

Only when moving narrows it down a bit. Now you have the problem of further narrowing...(it could be something in the front wheel or discs - but unlikely).
A tight spot on a chain can make a noise, but is more usually a thumping vibration through the frame. The tight spot is not always the chain, but a sprocket out of round. The front sprocklet is probably too small for that to be a problem unless your chain is adjusted too tight to start with.
If the back wheel is off the ground when on the main stand, give it a go.

The rear sprocket is worn looking like it's teeth are almost point in towards the bike.
The noise actually is a thump or a thud, I guess I should update my post to say that. - pretty hard to explain in words!

[MSTRS:Me]

The rear sprocket is worn looking like it's teeth are almost point in towards the bike.
The noise actually is a thump or a thud, I guess I should update my post to say that. - pretty hard to explain in words!

New sprockets! And chances are the chain will also need replacing.
Always replace all 3 bits at the same time...they wear together.

[sammcj]

New sprockets! And chances are the chain will also need replacing.
Always replace all 3 bits at the same time...they wear together.

Yep, One of the few rules I do know... but.. (I always have a but!)

It looks like the chain is in good condition, I think it's been replaced recently and that's whats caused the sprockets to die really fast, I've been though every link to make sure none are more stiff than the next and to make sure every roller is spinning freely without dents or wear, I had a mate look over it yesterday and he thought it'd be a waste to replace it since it's in good condition and is very expensive.

[sammcj]

Especially when you say it disappears when the clutch is engaged (I presume you've done this while on the move?)

I just re-read that post.
It disappears when the clutch is IN, i.e. when you're not in gear

[MSTRS:Me]

I just re-read that post.
It disappears when the clutch is IN, i.e. when you're not in gear

You mean when the gearbox is not turning?
Can you put the bike on its main stand, start the engine, put the bike in first and let the clutch out. Now the wheel is spinning, what do you see/hear? Pull the clutch in and see/hear what happens.