I have one other issue with my 71 AT1 I want to solve before race season. At the beginning of a race the bike shifts perfectly but after 20 or 30 minutes of hard riding almost every time I downshift from third to second I hit a false neutral which requires me to downshift to first then back up to second....really irritating! I’ve meticulously adjusted the shift fingers that actuate the drum and checked the operation which looks perfect. Any thoughts? This issue caused me a nervous moment in my last race when I was approaching a short steep hill climb, shifted down to second and hit my neutral.....should have known better. At that point I was at the bottom and was committed. Luckily some quick footwork got me over the top!

Under the little cover held on by three screws is a retainer on the end on the shift drum. Under that retainer should be a shim. Sometimes that shim gets left out and drum has too much end play which can cause random issues.

I've seen shim installed inside the cases which is incorrect as well.




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A thrown chain can smack off the end of the drum where MarkT has highlighted--the stickyout bit the highlighted parts fit on. But--you'd be having trouble from the start not 20 minutes in. Apart from rounded dogs & things i wonder about wear of the drum tracks at the high point of each gear. & i've seen severe wear in the roller wheel on the arm holding it in gear. The wear happens in the middle where it rolls so looking at it you'd never see it but if you remove the roller arm & move the roller it'll move up & down heaps in a bad case. Maybe we just had a batch of bikes with 'that bit' not hardened. Wasn't AT1 but they all run that system from 100 to 175. Number 24 in the link, but unlikely cause. Other thing is on my DT1 i've broken the roller on the selector pin 5 & was a reason they did away with that & went solid pins. I see the AT1 had those rollers so maybe one has shattered. Hope you find a definite cause as nothing worse than how ya describing catching a false neutral. None of my ideas make sense in the 20 minute thing though. Be great to see a video of you racing it.

www.partzilla.com/catalog/yamaha/motorcycle/1970/at1b/shifter-1

There’s no damage from the chain, I’m going to have it apart to put new clutch plates in soon so I’ll look for wear. I did split the cases to replace the crank bearings a long time ago, I hope I got everything back together right. I’m going to check the possibility I forgot that shim and it’s accessible without splitting the cases. I did make a race video, I’ll post a link on a new post. Thanks for the insight!

Shim is outside under the little cover.

Unless it was put inside.

Hopefully you find it outside.

My new clutch has arrived so I tore into the bike last night. I checked to see if the aforementioned shim was installed correctly and it is. I pulled the clutch basket off and pulled all the shifting related components out short of splitting the cases and inspected each part.
I did find one suspect part, the roller on the spring loaded index arm has quite a bit of slop in it. I found a good one in my parts pile and will use that one. The index pins all look good but will replace those as well for good measure. Hopefully this'll take care of my shifting issue, I'll post my results after I get it back together.

No sign that someone's filed the stop slot in the arm to pull it further into gear?. I did tricks like that on my 250-360 but just caused me grief & false neutrals etc. Actually what i did showed up more changing down gears. Problem in the end was just internal issues, dogs & drum high point wear etc. Roller arm wheel wear is a good place to start though.

That's an important part. Yamaha had a service bulletin on using a slightly stronger spring from a different model on the roller arm for more positive shifting.

Also, the later YZ/IT used a nice ball bearing roller to improve shifting. If I remember correctly... the whole arm will swap into the early bikes.

I found a bulletin talking about using the 315 shift drum to improve shifting on the earlier models.

Might be worth looking into next time you split the cases.







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Thanks for the info, I found the service bulletin that recommends using the stronger stopper arm spring from the Yamaha YR1part #90506-09008-00. I've located it on eBay and will get it ordered. I really don't want to split the cases but if this doesn't work I may have to.
The slop I found on the stopper arm wheel was only in one direction and not the other...the pin the wheel rides on must be worn out of round I'd guess theres maybe .030"-.050" wear