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I had a first ride on a bike I have been working on today. (51000 kilometer, L model) I am worried about a whining noise that I think is coming from the engine when it is under load. Possible gearbox??
The problem is just when you are riding the bike, when the engine is under load, not when it is in neutral on the drive.
When driving down the road this noise starts as soon as you apply a bit of throttle. If you roll off it goes away. If you pull the clutch in it goes away.
If I blip the throttle, with the clutch pulled in, the engine (top end) sounds fine, but as soon as you let the clutch out and apply a bit of throttle, it whines again.
It is there in every gear, so not an individual gear problem.

The engine work I have done is to fit a new cam chain and valve clearances. I removed and reinstalled the cam chain tensioner, as I know over tensioning a cam chain can make a whining/whirring noise.
Also I fitted a new drive chain and rear sprocket (with the old, worn, front sprocket left on for now. Still waiting for delivery of the new front sprocket.) - I know, could be the problem right there.
Oil and filter are new and up to three quarters on the sight glass.

I have just spoken to the lad whose bike it is and he says it has always been like it. But he is coming over later to test it, to make sure we are talking about the same thing.
When I rode the bike before the work the exhaust was blowing so bad I may not have heard this whining.

I had a TZR250 once where the bearing behind the front sprocket failed and that was making a noise.??
Maybe its just rebuild paranoia!! Every noise is a disaster.
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And another thing......these engine don't have primary drive chains do they?? (A chain between crank and clutch that some big four strokes have.)
When the bike is ticking over, and you blip the throttle from low revs, I can hear a snatching sound from inside the engine as the revs pick up.
Its not there when you blip the throttle at higher revs, just from very low revs. Sounds just like a slack primary drive chain.

Well, the lad came and rode his bike. He said the bike was whining like that before. In fact he said he liked it like that!

So he's taken his bike and that's the end of that.

I know this is no longer your problem but I thought it worthwhile posting up for the help of future searchers on this topic. The whining is because the chain is too tight, all the symptoms you describe are spot on and I've had it myself exactly as you state. Another thing that can be heard with a tight chain is that as you push the bike around you will get a click-clicking as the chain engages with each tooth of the front sprocket. You don't need me to tell you that a tight chain will wear out both itself and the sprockets early, and if severely tight will bugger the output shaft bearing.

The easy way to get the right tension is as follows -

1. Find the tight spot by moving the bike along, i.e. the point at which there is the least amount of slack. Feel the slack by pushing up on the lower run of the chain just below the point where the rubber sleeve which protects the swingarm ends. You shouldn't be pushing hard, just the gentle pressure of an index finger,

2. You are looking to have an amount of slack in which will let the chain just touch the rubber sleeve as it starts to feel that is tightening up and you are going to have to push harder to move it further. Adjust as necessary. Trying to muck about actually measuring the slack with rulers etc. is a PITA, this rule of thumb (finger? ) gives a slack within the recommended setting.

3. Make cup of tea and apply for job as race mechanic