Need help!. About a month ago while riding my 2010 660z , I rode over a speed bump (maybe 10-20kmph, luckily) when there was a noise in the engine and the engine locked up. Mileage = 22000km.
First thought it had seized, then I noticed that I could turn the motor backwards to a point and it would jam up and then forwards again to where it jammed again (Est about 60 degrees) from one point to other. But couldn't turn it over a complete revolution.
A hell of a lot of stripping later, engine out, head and barrel off, I thought I found the problem and that cam chain seemed to be catching on something around bottom sprocket. Because after guiding it and turning the engine slowly, suddenly the engine turned over freely again.
Has this ever happened to anyone else?
To cut a long story short - I reassembled everything, motor turned freely by hand, started bike, it idled a few minutes and did same thing again.
I'm totally baffled and frustrated!
Yesterday I have stripped out magneto to check cam chain and lower sprocket behind it. Only to find out chain isn't fowling anything as per my original thoughts. Chain and sprocket is fine.
I removed timing chain completely and tried turning over motor with no joy. crank is still turning backwards and forwards as before between 2 points. Any thoughts? Can only think its a bearing failure.

A few questions for you which might help diagnose the problem:

(1) When you are trying to turn the engine over and it locks up I assume you're doing this with it in neutral? If you pull the clutch lever in (clutch disengaged), can you turn the engine over freely?

(2) Have you/did you notice any difficulty in selecting gears before the problem occurred - stiff action, false neutrals, jumping out of gear etc.

(3) You say your bike is 2010, is there any chance it could be 2009 model that was registered in 2010? Have a look at the VIN plate under the seat and see if it says "Yamaha Motor Italia S.p.A" on the first line.

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Hi Pleiades,

Answers to your questions -
1. I am turning the motor over by hand at magneto nut with spanner, yes its in neutral, Still jams even with clutch pulled in.
First thing I looked at after this occurred was clutch boss nut and it was tight.
Was just turning it over now and the crank turning about 340-350 degrees, just not completing a full revolution.

2 Haven't had any problems with false neutrals, jumping out of gear or any transmission hassles.

3. Yes it does say 'Yamaha Motor Italia S.p.A'

OK then, the good news first: We can rule out the gearbox being at fault as it does it clutch in or out. Yours would appear to be a 2009 model and we have historically had a few gearbox problems with these which is why I asked about it's age.

Next port of call is probably to (as Pepsi intimated) examine the primary drive gear and in particular check that the the crank nut is still tight. You may want to read this thread, rather than me explain it all again!

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I have stripped clutch side of motor. Clutch completely out, Gear cluster on crank are taken off.
Balance shaft turns freely. But crank is still only turning between the 2 points. Nothing else is connected to the crank in the engine casing.
The head and barrel is still on but I've had that off and all is fine there.There is no visual evidence of bearing problems.

Suppose next job is to drop engine again and split the crank casing.

I'm afraid so. If you have disconnected the gearbox by removing the primary drive gears and clutch then it must be something in the engine itself. There are several things it could be from a dropped valve to a piece debris jamming things up. It might be a broken dog from the gearbox, after all it is a 2009 model. I wouldn't like to guess or speculate though; the only way to find out is to open things up and have a look.

You could check the valves/top end though before getting heavily involved in splitting the crankcase.

Some further reading that may be of use:

Read this thread regarding broken gearbox dogs, but as you'll see from reading it, there are almost always symptoms of difficult gear changing prior to the problem manifesting itself.

Also read this thread about premature main bearing wear caused by loose crank nuts. Again though, there are warning sounds (knocking etc.) prior to disaster.

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Piece of a gear by the looks of things, lying under crank shaft, every time big end passed the bottom of its stroke it hit this object.
Not to much damage, counter weights on crank got a couple of dings in and inside crank casing has some marks and gouges in. Obviously this piece of gear went round a couple of times.
I haven't split crankcase as yet, just got head, barrel and piston off. Once It was all off I could see into bottom of crank case and noticed it.
Use a magnet to retrieve it.

I can't seem to post photos otherwise I would.

Still have to split crankcase to check where this piece can from.

Pepsi and Pleiades you both seem to be spot on.
Piece looks just like piece of gear dog as in link.

It's more than likely a piece of the 3rd gear dog that has broken away. I guess it broke off a while back and just sat harmlessly in the sump and when you went over the speed bump it got dislodged and trapped. Luckily you were going slowly at the time.

You will have to replace the offending gear and check the stopper lever arm and its bearing which are the root cause of this particular problem. What happens is the positive stop gets sloppy allowing two gears to be momentarily selected at once causing the weakest part to break - namely a dog on 3rd or 4th gear (usually 3rd).

You're quite lucky as in quite a few cases the crankcase has burst open.

Anyway I hope it all goes well.

As for photos, upload to something like Photobucket then paste the direct link to the stored image in the [ IMG ] tags from the "insert image" button of tool bar when creating a post.

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SEE PHOTOS BELOW

Yep. That is the classic 2009 gearbox issue.

It's hard to tell from the photos of the damage to the counterweights and inside of the casing, but you might get away with it?

Can you do me a favour Rob and have a look at this thread and send me the details by PM. I need as much evidence that I can get.

*I've embedded the images for you. Click on the edit post button and you'll be able to see what I've done/how to do it in future.

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