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Have you tried turning the engine by hand and seeing if it feels like it’s spinning smoothly. Sounds like there is a lot of friction somewhere.

Its a good suggestion, maybe I will try and replace the crank just for the craic - but I fear its beyond my patience at the moment. Plus I think you need a press which I don't have. They haynes manual suggests you can get by with a heat gun and a freezer but I dunno - maybe that worked with the nice brand new engine they took apart. Throw in a little warpage on something and I'm not sure I would be so lucky. I have sourced an engine with 10k miles for £250 with effectively a months warranty on it. If I tried repairing the engine there is a very good chance: a) I would lose something important b) I would still be at it in a months time c) 'something else' would present itself as broken costing more time and money New engine should be here this week and hopefully be back up and running by the weekend - just seems like a cleaner solution. Interestingly you can buy a brand new 'JSM' engine for ~£450. Looks like a carbed version of the Yam. I swithered about that. I mean all that would be different would be the intake side (which I've got and works fine) and possibly the stator / crankshaft which is easily swapped?

pff torqueing is for amateurs. Bolt tensioning is where its at ... But seriously, need to be careful if you are applying grease or not. VAG specifically state not to great wheel bolts for example and to torque to ~ 120kNm. If you apply any grease you need to reduce the torque applied otherwise you can yield the bolt. The difference between a dry bolt or a greased one could be between 30 to 50% less torque to generate the same clamping force.

Yup, radial clearance seems fine and lateral play was within spec but it felt rough turning and totally jammed at certain points of travel. I suspect a previous owner was less than careful with oil change. It’s quite sad because other than that issue the rest of the engine and gearbox was actually in great nic for 30k miles. At least I’m getting a crash course in motorbike mechanics …

To bring some closure… the barrel and piston were fine, the bore scope just really magnified some minor scratches. The bearing between the connecting rod and crankshaft is very rough and sticks in some places. Sadly it’s new engine time as I don’t have the enthusiasm to swap out the crankshaft.

Edit. Put the bores ope in again with a different angle. This doesn’t look good … feeling more positive that a new barrel and cylinder might be the cure. cylinder is at the bottom of travel in the picture. when stuck it was at the very top of travel.

Right sit rep, was all free and loose, took it out for a few laps, all going well for about 30 mins then there was a clunk and she stalled. Just a clunk when I hit the starter. Pushed it home and put a socket on the crank and it was well seized. Worked it back and forth and it broke loose. Started up for about 10 seconds and died again. I suspect I can loosen it off and when cold it will be ok. Until it’s hot again… I would have a go at changing the piston and barrel if I was confident that would solve the problem. I put a bore scope down the spark plug hole but nothing jumped out as being damaged. No smoke when she’s running. There is some light cross scoring of the barrel mind you. If it’s bottom end I’d rather just get a new engine.

thanks, didn’t realise turning the engine the wrong way would cause damage. It feels like how you describe, very easy to turn then a bit of resistance and then a bit of a release. I think changing the oil made a lot of difference, first time even though it was turning there was a bit of drag but after oil change and run it was much easier. Don’t know why it would jam up though … or maybe it didn’t and it’s the starter itself that jammed up and turning the engine just freed something.

Cheers, what was your problem? Not charging?

Cheers

Hello, I bought a ybr125 with 29k miles. On my first outing it conked out a few times and eventually just wouldn’t restart. Big fat clunk and that’s it. Fortunately I wasn’t too far from home and pushed it back. Battery voltage seemed ok so didn’t immediately think that was a problem. Though occasionally it would weakly try and turn the engine but mostly just a clunk. At the same time I read that I should be able to turn the rear wheel (and thus engine) in 4th - but I couldn’t. Became convinced the engine was seized and proper fubarred. I stuck a ratchet on the crankshaft and did manage to turn that, backwards and forwards. By now the battery was flat, lots of clicking from the relay but nothing else. So I stuck a new battery on it and she started up again no bother - but would cut out hot or cold periodically for no apparent reason. Cleaned out the throttle body and that seemed to help a lot. Noticed with the engine running the battery wasn’t getting much voltage, around 12v. Measured the resistance across the coils at the connector and all were very low, around 0.08 ohms. Clearly something dodge there, so ordered a new stator coil. By now I was starting to wonder if it really had been seized and all my problems were electrical and poor general maintenance. Feeling confident it was running reliably - at least while the battery was charged - I changed the oil, ordered a new spark and air filter and took it for another spin. This time I made it round the block with no mishap, switched it off in the drive feeling reasonably happy, went to do another lap, hit the starter and … clunk. Nada. into the garage, stuck the socket on the crankshaft and loads of resistance turning left, turned clockwise and it moved, tried left again and turned no problem. Actually much easier than before, feeling a bit of resistance I guess at the top of travel but otherwise quite loose. Hit the starter button and away she went… I can hear an occasional clatter. It doesn’t sound great to be honest but having listened to a few YouTube vids none of them do. at this point I don’t know what’s wrong with it. Valve clearance is ok. Seems to have compression, a little white smoke on startup but ok when warm. I have a nasty feeling there is something loose and occasionally jamming but I tend to jump to the worst case conclusion. If it’s bottom end I guess it’s f**ked? Would a new piston and barrel help? Doesn’t seem like that’s too difficult or expensive to try? sorry for the long post, grateful for and advice.

Hello, recently decided to take up motor biking at the grand old age of 39 thinking it would be less trouble than a mistress but now I dunno … say my CBT a month ago and bought a ybr125 to putter about on before my direct access. I have found car forums great in the past and sadly am joining this forum for technical advice. Will post in the right section.