Gearbox issues, what could it be?

[mudguard]

Just asking for some info from you guys on an issue i had with my YZ125K while riding it at Biddaddaba yesterday. Had completed a few laps when going into a slow corner, dropping it back to 1st gear when the bike lost momentum, just felt like the clutch was pulled in, but definitely in 1st gear, and rolled to a stop. Selected 2nd gear and got back to the pits. Rode bike again an hour later got part way around track and same thing happened whilst in 3rd gear. Got going again for about 200 meters and lost 2nd gear.
It just feels like when you hit neutral when riding or if the clutch is pulled in, no crunching noises but a low noise like something is spinning and not engaging. Anyone have a guess at what it could be??

[Viper666]

1st 2nd & 3rd all rely on different gears dogs to work. Have you still got 4th, 5th & 6th?

2nd & 3rd rely on 5th pinion, 1st need 4th.

There is no ONE common component.

Unfortunately you will have to pull her apart.

Drain the oil & see if there is any metal in it. If you can poke a magnetic pick through the hole & in the oil.

[GMC]

Sounds a bit like worn dogs, if they get rounded off they will throw straight out of gear again. Although it is odd for it to happen to several gears at the same time, while all dogs will wear a bit it, it is usually only one gear that will get worn enough for you to notice.
If replacement gears are hard to find then there a a few places that can regrind the dogs back to spec.

3rd gear is common for worn dogs as it is the most used gear.

Being that it happens on several gears it may be something NQR with the shift drum

[KB171]

I'd be checking the clutch first, good luck with it.
Cheers KB

[matcho mick]

i'm with KB,mainshaft nut ,

[mudguard]

Changed the gearbox oil last week, after a full season of racing last year. Was surprised how clean it was and only a couple of miniscule specs of metal there. Just checked the gears again with the bike on bike stand. Without using the clutch, it selects all gears with engine not running. So maybe something to do with the clutch??

[Montynut]

Clutch hub failed. Is the basket rivetted onto the gear?

[whitey 43]

Normally they are riveted...

[Viper666]

I'd be checking the clutch first, good luck with it.

i'm with KB,mainshaft nut

Clutch hub failed. Is the basket rivetted onto the gear?

This would mean no gears.

Either dogs, but that would mean more than one gear as previously explained, or serious shift drum/shift fork malfunction. Possibly where the shift shaft connects to shift mechanism. May have broken the small spring that centres the hook.

[Simo63]

Changed the gearbox oil last week, after a full season of racing last year. Was surprised how clean it was and only a couple of miniscule specs of metal there. Just checked the gears again with the bike on bike stand. Without using the clutch, it selects all gears with engine not running. So maybe something to do with the clutch??

Sounds like a slipping clutch .. you're saying it selects gears but then feels like it's hit neutral.  And sitting on the stand in the shed (cold) it's fine but at that point there is no pressure or heat applied.

I always look for the simple things first and you did say you changed the gearbox oil a week ago?  What gearbox oil did you put in when you drained it?  Could it simply be a slipping clutch when hot?

[Viper666]

I always look for the simple things first and you did say you changed the gearbox oil a week ago?  What gearbox oil did you put in when you drained it?  Could it simply be a slipping clutch when hot?

Totally agree but he said he lost particular gears & was able to ride back to the pits, if the clutch was slipping I would think it would slip in all gears.
But the clutch side is the easiest to get to so start there.

A while ago I did a complete rebuild on a YZ250F & used aftermarket bearings for the gearbox. The clutch shaft bearing had a steel ring around it to provide a mounting point.
Everything went fine with the rebuild, adjusted clutch, internally then externally. Went for a ride and all good till I put it under load. (Running in) The more I pushed it the more the clutch slipped. Strange as new primary drive (Clutch basket), new friction & steel plates, new springs. I found if I adjusted the cable so far off as not to work (When disengaged) it was fine but adjusted up (As per specs) slipped.

Solution, after many a sidecover off, was the ring around the clutch shaft (Aftermarket) bearing (All that holds it in place) was mild steel, compared to hardened steel of the OEM, was flexing (Only about 2-3mm) when I was adjusting it but then the helical (Angled) cut gear on the primary drive would try and "Push" the clutch towards the gearbox under load taking up the gap & this would then push the top plate against the clutch rod thus disengaging the clutch. Took the hardened one off the OEM & put that on, problem fixed. Not saying that is what is wrong here but just proving small things can create big problems.

[mudguard]

Thanks for the info guys. I will attempt to remove the enging case this arvo, to check out the clutch area first. Will give you some feedback on this.

Neale, The service manual diagrams are great.  Can you post a service manual diagram on this post to show the engine case and clutch parts area of the engine? Or send me a link as to where I can get the diagrams?

Craig, The gear oil I use is Motul Transoil SAE 10W30. This was in the bike for last years vmx season and I had just drained it and refreshed with 800mls of new Motul Transoil before last weekends ride. Rode the bike for less than 10 minutes when this issue started.

[PCMAX]

For what it's worth I bought an MX250 last year which had 4 good gears and then when you shifted into 5th it was a neutral. It shifted there firmly but no drive.
I pulled the gear clusters/drum/shift forks etc out and could not see anything obviously wrong. After 3 stubbies and a lot of staring I found that one of the little hardened steel cylindrical thingies (item number 8 in Viper666 reply) that locate in the shift drum had a lot of play. The hole that it fits in on the fork was elongated.
Anyhow I bought a new shift fork complete with item #8, put the motor back together and i now have 5th gear restored.

[Viper666]

Mudguard go here

http://www.starmotorcycles.com/star/myyamaha/prompt/L3BhcnR2aWV3ZXIvP2xzPXN0YXI=/Parts%20Catalog/starthere.aspx

Proceed to parts catalogue & follow your nose

[Viper666]

Mudguard,

What did it end up being?