YZ125D/E gearboxes - what am I doing wrong?

[Nathan S]

The story so far:
I freshened up my YZ125D just before the Nationals last year - new crank and new bearings and not much else.
When I took it for a test ride in the backyard, I discovered that under load in first gear, it would slip/skip - feels like a gear with missing teeth. The rest of the box works fine.
So in a mad panic, I swapped in my spare 125E gearset, and took the bike to Coffs.
It still does it.

That bike has been parked in since then, but I has a closer look at the original D gearset, and it's clear that it isn't engaging first gear properly - the ends of the dogs have been rubbing, rather than engaging.
NB: The dogs are NOT rounded, although they're now probably a bit shorter than they should be.
Where the dogs are supposed to engage about 4mm, they're obviously only just engaging (call it 0.5mm).

So the 'YZ175D' project bike was fired up today, using the original 125D gearset. The first few laps of the back-yard were all good, but then it started doing the same thing.

Pissed Off is an understatment...

What am I doing wrong?
Something that can be shimmed out?
Something to do with the shifter mechanism?
Etc.

(My shitter 125E got slapped together using a DT first gear, no attention paid to shimming or anything, and it shifts beautifully. Ditto several other engines I've slapped together).

[DJRacing]

Nathan, Did you change the selector forks when you changed the gearbox? How much play do the cogs(1 gear) have on the shaft? Is the shifter(#2) moving the selector drum correctly?

[TM BILL]

DJ clocks have COGs  transmissions have gears  Geez your not originaly from WA are you 

[Lozza]

Worn shift drum, you won't pick it by eye either.When swapping the gears did you swap the drum also or leave that in place?

[Doc]

I'm with Lozza, sounds like a worn shift drum/selectors. Having the dogs on th offending gears backcut would make things more positive as well.

[vandy010]

i've been reading this one with a bit of interest but?
can you explain "backcut" for me Doc?

[Lozza]

Tis simple really  the 'dogs' on the gear mesh with holes(or other dogs) on it's mating gear and provide the drive.When HP numbers creep up or the dogs get worn you can undercut the dog to make the leading engagement face slope backwards,on the corresponding hole the leading edge(trailing if it's a dog) is sloping forwards.So when the fork  pushes the 2 gears together they slide down the  'ramps' and the more force on the gear the more they force themselves together.
However the heat treatment on each dog face is thin and the material of gears is like a 'marshmellows' inside below the treatment, so it isn't a sunday afternnon job.
Also because the gears are heat treated already you cannot simply reheat treat the face again. 
Even to very competant machinists gear cutting is  voodoo magic.
HTH

[eno]

I find the most irritating thing about gearbox setup is that when the cases are put back together you need to be able to see what the gear dogs are engaging like. Maybe one of those little camera things on a bendy stick or failing that cut a hole in the back of the cases...Doh!!

[Doc]

thanks Loz, the Institute of Greater Terminology explains it so much better than me 
I'd have said the lumpy dog bits that go in the corrosponding holes or slots in the gears become rounded and don't grip, when the dogs are back cut woof woof the edges are all nice and sharp again and maybe slightly angled back so as to lock the gears positively when engaged. Noel Harker at Albion did my 400 back when Noah was a boy and he must have done a swell job as it's never been apart since 

[Nathan S]

I swapped the whole gearbox in one go - both shafts, all the shift forks and their shafts, and the drum. The 125E was a low-use bike with a known history, so I seriously doubt that its bits are too worn.
Forks definitely look happy in both gearboxes.

The dogs are simply not engaging far enough.

[crossedup2]

Nathan

I could be off the track here but the USA Yamaha parts pages have the Tech Bulletins within them. I seem to remember a reading of a mod in one of them to elongate the shifter claw to allow additional rotation of the shift drum in one direction. This may be to get the drum to rotate the smidgen more to totally engage 1st gear.

https://www.yamaha-motor.com/sport/parts/home.aspx?sb=2   (Under yz125D)

Might help! At least you don't have to split the box "again".......
Other alternative is to check the box side float. Another bulletin talks about the correct shims and that can be between the gears that are not meshing!

Good luck, one screw won't fix this.

Peter B