250 MK3 drive issue

[Glawster]

Hi Guys,
I don't post here much but I would value some help. I bought a 1972 250 MK3 3 years ago and did a complete refurbishment of the bike, but as the engine was running perfectly well I have not disturbed it. The only issue I have is what I'd describe as a "clonk" only occasionally when the throttle is closed or rolled on. In my mind I put this down as possibly the clutch basket being loose so I thought I'd have a look. My observations so far..
- Both primary drive and clutch basket nuts were tight.
- there is a perceptible backlash between the primary and clutch basket gears. Not huge but there is definitely some play. Is this normal?
- The teeth on both gears seem OK. There is a spacer but no shims behind the clutch basket and it's sitting slightly behind the primary gear (see pic). Is this correct?
- The clutch centre has some damage from the plain plates. I have a spare clutch centre and it's similar. Can these ridges be dressed out?
- When in gear there is maybe 5-8mm backlash between the clutch centre and the output sprocket. Is this normal?

The plain clutch plates are burred, but the friction plates seem fine. I have many spare plates / springs / clutch parts so would plan to select the best ones for refitting. What I can't really find is a plausible cause of the clonk I started looking for in the first place. Your opinions appreciated.
Best regards,
Derek

[Jordan]

It's difficult to diagnose noises without being there, but I wouldn't think the culprit is backlash in the primary gears or clutch.
In the gearbox, the selector dogs have some clearance as normal - partly from the needs of the tapered dog faces.
Could it be possible the noise comes from farther back - rear hub?

[ducwiz]

Hi,

this "clonk" cannot be identified remotely. I can only offer more ideas about it.

- excessive play in swing arm bushings?
- incorrect chain tension?
- loose fuel tank?
- crankshaft has noticable axial play (> a few mm/100)?
- worn big end bearing?

Your clutch center is OK, looks like all the used ones I had my hands on. No easy "dressing out" is possible, but also not necessary.
The backlash between primary gears as well as between clutch centre and sprocket is typical, no problems here.

Finally: congratulations, she is a real beauty !
But please, don't take her on the road with this silencer and the rubber manifold. These will choke the engine drastically. Use i. e. a pattern Conti muffler, and an open bellmouth or a K&N clamp-on filter.

seasons greetings Hans

[Glawster]

Thanks for the feedback on this forum and also to Kevin (Machten) by Messenger.
I've check out all the chassis suggestions and all seems OK. I also compared the level of backlash at the rear wheel with my neighbour's 450 and they are similar. I have a spare clutch basket which seems to have marginally less backlash and I'll order a shim to align the basket to the primary gear and a tab washer from Nigel. Interestingly the tab washer removed did not have a tab to lock it.
I dressed out the ridges on the clutch centre and the burrs on the plain plates. Will reassemble and give it another try to see if anything's changed.
Many thanks
Derek

Oh, and Ducwiz - yes, you're right, the restrictive exhaust and airbox do limit performance. I imagine it's also fitted with the softest cam available. Certainly it doesn't like to rev. I have considered letting it breathe a bit more easily, but it's easy to start, ticks over perfectly and is lovely to ride if you're not in a hurry. I've got other stuff to ride if I need to go faster!