Causes of apparent piston seizure?

[Sully60]

Starter motor? One way clutch or sprag clutch, don't know what they use, might of destroyed itself.

Yeah, it sounds more like a transmission, drive, starter type thing than the engine. Two gears meshing that shouldn't can give the one way only impression.

[Tumbles]

Starter motor? One way clutch or sprag clutch, don't know what they use, might of destroyed itself.

I see where you're going here, I'll check it out.

[Tumbles]

Yeah, it sounds more like a transmission, drive, starter type thing than the engine. Two gears meshing that shouldn't can give the one way only impression.

Yeah agreed. BTW another signal was she rolls forward in neutral sweet as. Its just when its geared it doesn't. Also just selecting gears feels completely normal. There is none you can't select and no unusual clunkiness. Therefore I think I might check the flywheel/starter side of things first. Besides, getting to the gears on these means splitting the crankcase so I'm keen to make that the LAST option.

[Ixion]

Ok, I shot up to Palmy ast night, grabbed my tools, came back down to Welly this morning, and got stuck into it.

The cam chain and guides look fine.
The engine can be turned over using the flywheel nut in reverse only. When you got to turn it forwards, it locks INSTANTLY It doesn't lock when you turn it back to where you started from, it locks up straight away.
Exactly the same results are gained when turning the her over with both cylinders removed (the whole things down to the trans).
Valves, cylinders, pistons all look fine.

So: Its definitely not anything to do with cams or pistons. This leaves!: The bottom! Yay! I'm so happy right now!

So. Anyone got any more ideas on where I should be looking first? I'll be attacking this tomorrow night.

Can you turn it back indefanately (ie more than one revolution)?

The only thing that locks in one direction only by design is the starter drive. Though sometimes a loose wotnot can jam in such a way that it forms a sprag . I doubt it's transmission.

[Tumbles]

And that's exactly what it turned out to be.

The centrifugal clutch designed to let the starter motor drive the crank shaft but not let the crank shaft drive the starter motor has failed, meaning that the engine was constantly spinning the starter motor for a period of time. This fragged the starter motor to the point where the starter motor seized, siezing the whole engine.

Once I realised the starter motor had siezed, I removed it and took it apart to check the damage. I had to bang apart the start motor with a hammer and I estimate 10-15% of its total mass came out as dust. Results below.

So now its off to TSS for a new (second hand if avail) starter motor and to borrow their flywheel remover. I'll keep ya all posted if anything obvious comes out of the centrifugal clutch inspection.

[Ixion]

Check trademe . There's one of the wreckers puts up an XV250 starter (should be identical) every so often. (Never an XV750, alas)

EDIT. Found it. Bucket-n-bits, down your way.

[Tumbles]

Check trademe . There's one of the wreckers puts up an XV250 starter (should be identical) every so often. (Never an XV750, alas)

EDIT. Found it. Bucket-n-bits, down your way.

Cheers mate, I have asked them previously and didn't get a reply. I'll try and make contact again.

[FJRider]

Cheers mate, I have asked them previously and didn't get a reply. I'll try and make contact again.

Check starter button switch for continued contact, dirty or wet sometimes does it. OR broken spring in button

[Tumbles]

Update:

Well, I've taken the flywheel of and the one way starter clutch at the back of it looks fine. Turns out its the gear behind this, turns the starter clutch during start, has locked itself onto the driveshaft effectively bypassing the clutch. Despite my best efforts with a bearing puller this afternoon, I have been unable to remove this gear from the driveshaft. Therefore its time for TSS Red Baron to do what I cannot and fix my bike.

Also managed to pick up a starter motor from Buckets and Bits. Cost me $200 though which is gutting.

Thanks to TSS again for letting me borrow their flywheel puller and bearing puller no questions asked. You guys rock.

[Tumbles]

Well, turns out the gear can't be saved, the copper(?) bush had welded itself to the crank. A new gear costs $360 so seeing if Buckets and Bits can do me a second hand one for a bit less than that. Anyone got any advice on how that could happen? I'm worried that it means the oil isn't tracking through the engine properly. Any ideas on how to check that?