Please describe this neutral switch you have mentioned. When I look at my engine, I see exactly the same thing I have seen on other AT-1CMX models. Here is another one with a 4 serial number and the engine case and other components look like mine.

www.mecum.com/lots/494771/

Aside from things I have changed, this machine looks identical to mine with one exception: the front fender bracket has round holes and mine has two elongated slots. That is why I posted the fender bracket photo from mine earlier.

The Mecum one has the wire connected to the neutral switch with the boot over it.  I swear I've seen that on an early AT1M....  (time to start looking for pictures of AT1M's with verified US AT1M VIN's.)  The bike I think I saw could have been a "converted" Enduro too...  There's not too many "real" AT1M's out there in original condition.

What Enduronut says about the alloy plug also makes a lot of sense.  I'm pretty sure my AT3MX has a plug and not a switch.

Keep in mind the Mecum bike also has no "provenance"...  it could have been brought here from another country and it could have started life as an Enduro in that country...  really impossible to tell. 

(Plus "restored" bikes by big name shops where they have no provenance listed tend to be poor examples of "originality" a lot of times....  I'd guess either you or that bike has the correct front fender bracket...  not likely there were two Yamaha versions the same year.  Lots of aftermarket versions though.  And neither has the cool rear plate holder.)

Even DT1MX by memory [that we sold new] still had the full DT1 mag with lighting coils.
So if thats the case them maybe AT1MX had similar with the stator loom incorporating the light blue neutral wire which had to go somewhere so fitted the switch rather than blank it off & have the wire left hanging.
I dunno, just hypothersizing. That's a big word.
 

RT325, Babbitt's diagram shows Part #: 136-82540-00-00 (original) or 136-82540-03-00 (replacement) as being on both Enduro and MX AT1's. It is named the Neutral Switch Assembly. Other sellers have the blind/straight plug (part # 90340-20004) listed for Yamaha Snowmobiles. Babbitt's shows only the Neutral Switch Assembly, as stated above.

Had had a look at that Ebay Carb.
Blimey that'd be wishfull thinking at that price but goodluck to them.
Anyway my guess is someone has round filed that step so it would fit on a typical honda 125 185 200 inlet direct or to the manifold which has offset holes to the inlet hole.
Not an ideal situation for sure & very much weakened, big thick washer job.
Sorry answering on the previous page--now catch up on this page lol.

RT325, I agree with your assessment of the ebay carb.

I got called away shortly after my last post, but after further study, I agree .... with everyone who has said the AT1MX appears to have the neutral switch. I was basing my original comments off both Partzilla's web sight, and my original Yamaha microfiche, both of which show and say that it had a plug type bolt, and included the part number.

Just a couple weeks ago, I sold a nos AT1MX stator assembly (part # 248-81300-10), and it had the lighting coil and blue neutral switch wire w/cap. Makes sense that Yamaha would not leave that wire to flop around, but needed an attachment point.

So . . . yet another Yamaha parts mistake found that doesn't seem to match reality.

Anyhow, I'm not arguing that it is, or isn't an mx model.
However, I will argue that this bike was not originally slated for the US market place . . . how it got here is anyone's guess.

Seems we still have to answer why RT325's bike is an enduro, and the other two 4XXXXX series bikes appear to be MX ???
We may never know.

PS . . . I'd still like to see those carb stampings if you find the time

Mikuni Photos

i.imgur.com/Vn1DNik.jpg
i.imgur.com/cocLriQ.jpg
i.imgur.com/nfTwwOw.jpg
i.imgur.com/tXs1kTh.jpg
i.imgur.com/R7wKdaj.jpg
i.imgur.com/DISf8Qk.jpg
i.imgur.com/R88dy2q.jpg
i.imgur.com/DDGnabx.jpg
i.imgur.com/IzucL4t.jpg
i.imgur.com/mEk9va7.jpg

Hey. . .  thanks for those pictures
The mystery deepens . . . 

I blew up the photos ( something like X10) , and enhanced them with both more contrast, and sharper imagery.
I believe it's a Mikuni VM26SH carb that was originally slated for use on a "late production" 1968 SL351 snowmobile . . .  which used codes 802  & E2

The code was crossed out at some point, and a new code "248 M2" was stamped  (correct for 71 AT1MX)
But oddly enough, the numbers are upside down ....  as in, facing the wrong way . . . differing from every Yamaha motorcycle carb that I've ever seen.

This isn't the first time I've seen Yamaha numbers crossed out, and what looks to be official Yamaha replacement numbers stamped.
Then this thought crossed my mind .... Could it be that Yamaha had some kind of unadvertised / unknown program that they used to get rid of overstock or excessive parts and bikes?  Just saying . . . what you have does not follow standard protocol / procedure, nor does it seem to be recorded in any official Yamaha literature that I've ever seen ???????????????????????????





 

First of all, in general, I think you have put a bit too much faith in early Japanese record keeping. As I mentioned in my opening post, I am the author of The Tiddler Invasion, meaning that I have had contact with a number of enthusiasts and mechanics, etc., of small Japanese motorcycles of the '60's. The Japanese worship efficiency. They don't care about American marketed model years or the exact changing of models. They use up the supplies of individual parts within the manufacturing schedule. I suspect that (a) the company was caught off guard a bit by the extreme sales success of the early Enduros. Yes, a lot of early Enduros were GYT-Kitted, but in America, convenience always sells. The sales of the MX models caught the company off guard as (a) the popularity of motocross was just beginning to really build in the USA and (b) they had a lot of Enduros scheduled, but they suddenly needed to up the MX production to meet the final demand before the reed valve models were ready. This whole affair coagulated to produce many running changes in the final first generation models.

See the next post for the detailed version resulting from this issue.