I'm no expert but from the parts lists the 1975 MX400, 1976 IT400, and 1976 YZ400 all used the same cylinder and head.  The differences as I remember them described were in the pipe, airbox, and carb tuning.   

The YZ400C came with an "off road" kit that made it "green sticker" legal (California, USA) so you could legally trail ride it on public land.  Kit had a new quieter muffler (with required spark arrestor), airbox restrictors, and jetting specs that quieted the sound and toned down the power.  Friends I knew that had the YZ400 got the green sticker and only used the new muffler/spark arrestor.  The other mods really killed the power and it was rare that rangers did anything but check for spark arrestor...  though occasionally the rangers would set up a sound meter and check your noise level back in those days.

Probably not the best power delivery for street use... I've ridden an IT400 that was made street legal and it was fun, but also liked to wheelie a lot unless short shifted and care with throttle/rpm was used.  I expect it might get tiring if used for daily commuting?

 

    I've been told this before, about the MX, IT and YZ all using the same cylinders.  But I've also compared a '75 MX400 cylinder (or at least one that was purported to be off of an MX400) to a '76 YZ400 cylinder (or at least one that was purported to be off of a YZ400C) and there were differences in the ports.  After all this time, it may not be possible to know for sure which cylinder is which anymore.  There must be port maps out there somewhere?  I know that the YZ400C actually had a few LESS hp than the MX it replaced, but the power delivery was better.
     My YZ400C was bought (as a moldy old wreck) in Columbia, South Carolina while I was a poor college student living there.  It still had the stock silencer, which was a surprisingly large, (especially for the time) non-rebuildable, steel bodied affair, (I cut it open and rebuilt it anyway) and no spark arrester.  When I first got it running it was a rocket with a very ON/OFF power delivery which I found extremely thrilling, but very hard to control.  I retarded the timing by just a few degrees and then I had a big bore 2 stroke with terrific low down torque that was fun to ride in the woods.  I should probably add here that I also stuffed the crank balance holes when I rebuilt the crank and polished the ports and installed Boyesen 2 stage reeds originally intended for a  much newer YZ125.  In it's final form it had a flatslide Mikuni that I took off of a '87 RM250--this was actually a couple mm smaller than the original YZ roundslide carb, but made it even easier to ride.  I sold that bike in Gray, TN in 2000 or 2001 and have regretted it ever since!  If I ever do build my DT with an IT or YZ cylinder, I will probably try to replicate a  lot of this combination (though I'm not sure yet what I'll do about the expansion chamber) and then only ride it when I needed a thrill, so to speak.  Certainly not as a daily commuter. 

1975-77 and beyond were CRAZY years here in the US for motocross bikes. 

New models and/or mods were coming out every 6 months or less from my memory of the period. Much of that is undocumented.  But I remember seeing bikes at dealerships...  same year model...  but not the same.  The 1976 Husqvarna I got had a rare Ohlins-designed "snake pipe" expansion chamber versus the classic Husky side pipe.  It also had different porting and jetting than the earlier 1976 250CR models....  allegedly engine was an exact copy of the one Mikkola used to win the World Championship.  

Anyway, back to Yamaha.  You are correct and I remember reading the MX400B and YZ400C porting were supposedly NOT the same.  MX400B had more high rpm power by quite a bit from dyno tests of the day.  They "detuned" the YZ400C to have more mid-low power at the cost of a few HP on top.  There is also a "-01" revision code listed in the YZ400C part number (MX400B part was originally a -00 revision as far as I can tell) which indicates some sort of change....  

 

Reminds me of my mx250b new in "75 which went better than my yz250c in "76 & it annoyed the hell out of me. Probably the same story except it had a retarding "at high revs" cdi system on the "76 where "75 stayed constant. But it was more than 'that' causing the lack of very top end. "75 barked & got into it right to the last rev. Sorry, i'm wandering off track again.

Here's a dyno chart comparing 75 MX400b and 76 YZ400c

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It looks like the MX400 had more power everywhere...  you can't ride a dyno though and it could be they detuned to smooth power delivery and/or reduce wheelspin?  Sometimes less = more and a bike that is easier to ride might have faster lap times than one with more and/or different power delivery.

Mark wherever do you find this info. Great to compare. Thanks.

If my memory is correct, the YZ 400C had two different rods. One was longer or shorter than the other. I just can't remember the details from when I restored mine.

I'm still kickin.

I've seen this, I believe it is taken from a Cycle World article?  I remember reading the article, in fact I just saw it again recently, but don't remember where.  If I can find it again, I'll post a link--the place had loads of old Yamaha articles starting in the late 1960's, IIRC.  A lot of them were in French, but probably most were in English.

RT325, I have a few old magazines from that era still...  My first-ever new bike was that 1976 Husky and I read everything I could find about it and the other 250's of the time when I was dreaming of someday getting a new bike.

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Great to see you back here Scott!

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Junglebiker, it was in a 1976 issue of "Cycle" I think?  I put the box away. 

"Cycle World" was a different magazine and not my favorite of the time...  I think they catered more to street bikes but they did have some GREAT tech people like Jennings writing articles so I do have a few issues of Cycle World too.