Working on a 1975 125. I have replaced my armature and stator assembly from one from a AT1. Will the governor be the same and or work from the 1975 set up? It bolts up fine. I have discovered my crank bearing is bad on left side and I can’t get it in time. Is this going to be a tear down and bearing replace? It was running fine until I changed out the stator and armature. Can’t get it in time and it won’t crank now. Any advise?

I'm struggling to understand how a crank bearing would make it impossible to time?

(especially when it was running fine)

Can you explain why the bearing is bad?

If it bolts up & moves to advance & back i suppose it should work fine. If the crank bearing is Very Bad it might make perfect timing a bit of an educated guess but if it was running ok before then it should work now. On second thoughts / re-reading, so you're using the "75 advance on the 70--ish stator. I'd think the locating tab for the advance unit would be in the same place so has the key disappeared in the crank to locate the armature so as to be able to time it. Be sure you're holding the advance weights fully open to set your 1.8mm BTDC timing. Sorry if i misunderstood--or didn't read it properly.

Just looking on Partzilla there's two starter/generators shown for 1975 but only one for 1970. I'm going crosseyed looking at it all so you check it out & see what ya make of it incase they aren't all interchangeable.
www.partzilla.com/catalog/yamaha/motorcycle/1975/dt125b

www.partzilla.com/catalog/yamaha/motorcycle/1970/atm1b

If the crankshaft bearing is indeed bad, then yes the cases must be split to replace.

As for the swapping of ignition - parts - if the part numbers between both bikes are the same ( on all the swapped out parts ) that should not present a problem.

There's a general rule that bikes that have bad bearings do not really run fine. Something's got to give.

Do a part number analysis -

a

Ok I found the problems. I have the wrong governor. They are not interchangeable from 71 to a 75. 180 degrees out. So maybe that will help someone else in the future. As for the crank bearing it has so much play That when the lobe on the governor is opening up the points you can move it and the points would close. I now have the motor apart waiting on crank bearings. I also need a governor for a 69-71 AT1 if anyone has one.

That's real good info to have but just wondering if someone has had the advancer apart to lube it & assembled the cam 180 out in the weight slots as think that can be done--without having one in front of me. I'm loosing track of the story but if its the can you used before & the armature has changed to earlier then what you say must be right--just unusual for Yamaha to make changes like that. Anyway--Stuffed LH main, so Bugger!!. Just be careful the main hasn't been spinning on the crank as was common on small motors but not so bigger ones. Thing is, if it's been spinning, a new bearing will only half fix it--hope it's not that as turns into a drama.

It turns out that the armature is bigger in the 69-71 years. Also the key way alignment in relation to the key way for the governor is 180 degree different. But it will bolt up no problem you Just can’t get it in time that way. It is riveted together and couldn’t be easily changed if at all.

I need a governor for a 1972 AT2. I believe the part number is 173-81153-10-00. Any suggestions? eBay has one for $99.00, yikes!

what is wrong with yours?

Mine came off a parts bike from 1974? And I believe it is incorrect. I think it’s why my timing is off and I can’t get it dialed in. All I get are big backfires. One shot out the exhaust baffle!