Replacing a few gears in my new to me 70 250 and the parts list has this note, 
replacement parts for 214-17131-10(3rd gear for DT1C) are275-17131-12 and 
308-17151-10
Is that the gear next to it that it engages? I have what I think is one but the center of the gear has cut outs like for oiling or something. No other gears have that cut out.

I wouldn't doubt the Yamaha engineers designed those gears for oiling, but the tabs are locking gears. 

What I believe happens is this:

When the gear you reference on the main axle, 3rd gear pinion, is moved by drum towards drive sprocket direction,
3rd gear wheel next to it on main axle is locked. At the same time, the 3r gear pinion on the drive axle is in the center position and is locked to the drive axle. 

Also, 3rd Gear Wheel (30T) on main shaft, 3rd Gear Pinion on drive shaft (23T), 1.30:1 ratio.  3rd Gear pinion on main axle that locks the wheel is 23T also but when locking 3rd is aligned with nothing on drive axle
I just changed that exact gear on my CT1.    

I'm not familiar with the note you're referencing but there were several transmission revisions.  Perhaps these bulletins might help?



 

Hello My Grandson is having a hard time finding suppliers for ignition parts for his 1968 DT1 250 We live in Ontario Canada, has the group any suggestions where he can order such items. Thank you Bernie

 from near Salisbury, UK.

If you haven't done so already, see what manuals (parts, service etc) are available in the Tech section here - see top toolbar.

My first port of call for a parts enquiry is the CMSNL website. Whilst not every part is still available, if you select a part (available or not) and scroll down, you will see which other models used that part. Also, if a part number has changed (referred to as a supersession), then the new part number is usually shown as well.

If a part is shown as being available on CMSNL, there is a chance that it might still be available from Yamaha - I always use my local dealer.

If you go down the non genuine parts route then, for ignition parts, use a trusted repro provider such as Daiichi (genuine and Daiichi parts will have a logo on them, cheapo crap ones won't). Buy spark plugs from a trusted source, as there a lot of fakes out there.

Ebay is often a good source for parts. If possible, use CMSNL and the internet in general to find pictures of what a part you are after looks like. A lot of parts are accidently incorrectly listed. Likewise, do quite a wide search for any items you need, as they might be listed for, say, a DT250 instead of DT1 - knowing what the part you're after here looks like is a great help.

KDI (see link on the right side of the screen) offer a range of repro parts. Likewise Yambits here in the UK, but their quality is very hit and miss.

Change both sets of brake shoes, unless already done - current part numbers are shown on CMSNL.

Partzilla in the US offer similar info to CMSNL, but I've found that their library of photos and model cross referincing isn't as good.

Any of the marketplaces in Canada (Craigslist?) might throw up parts as well.

Thank you for your early reply, I have been able to source what we need from ebay as a source that you mentioned I took. a look at CMLS site and was very impressed, however we went with e-bay for the cost and delivery We will go for CMLS parts when we are sure that he can get the DT running consistently, all the info you forwarded has been a great help now and for in the future. We will let you know how we get on. Cheers Bernie