Wow, I had no idea!  I remember reading "replace with known good" but assumed it was good if it checked good.  I'll try the air filter and spark arrestor suggestion first.  How would I go about finding a member who might loan (or rent) me a known good CDI?  Post an inquiry here on the forum?  Might be better to send mine to someone who would be willing to try it on their 360.  Again, thank you so much Mark for the superb advice!

This is a 175 video I made a few years ago with 3 CDI's.  Two of the three won't rev right.  One (Terry's) sounds really "rich".  In this case, I swapped around the 3 CDI boxes on the good CDI stator and it ran the same with each CDI box.  

Not so lucky with the stators!  All had meter readings within factory spec.  Terry's CDI had a new source coil installed by a shop and it made no difference.  I tried swapping parts around and measuring with a scope but never did figure out why the two bad CDI stators didn't work right.  

We replaced his CDI with a points system and Terry's bike ran great again.


 

Interesting!  What do you think of this:   fireplug-cdi-1975-76-yamaha-dt400-motorcycles ?

The fact that you got that bike new and still have it is amazing. The any throttle setting rpm limit, as prev mentioned, makes me question things. Besides that, I would think a 170 jet at that altitude would be a good start, then check plug health. I also recommend contacting the local Yamaha dealer to see what locals are doing for same.
Cross check petcock bowl, but a a limiter at 3800 sounds like an electronic one.

I also have a working CDI box for that bike. When I bought my bike, I rode it 1 or 2 times, then put a new VAPE system on, so I know it worked ok. If you want to use it to test your bike, you are welcome to.

I believe the guys at CDIBOX.com are/were members here.  Several years ago they were branching out from snowmobiles.  

You might try contacting them.  As I mentioned though, often it seems to be something is wrong with the stator and NOT the CDI box. 

I wouldn't waste money on a new CDI box is what I'm saying... but they also rewind stators so maybe they can fix your ignition if you send it to them and/or verify it's working properly.  If I remember correctly, they seemed to have a lot of knowledge of the old Yamaha CDI's.

Amazing how much "trial and error" is involved.  Seems like the protocol for bench checking CDIs must be missing something important.  Thanks for the vid Mark!  I would like to send you a short video of my 360 but I've been unable to on the forum.

PABDT - I bought the 360 at the Iwakuni MCAS Px while stationed there in 74-75 as a Marine pilot flying off the USS Midway (now a museum in SD).  With the exception of about 4 years (while my sons "cared" for it), the bike has been licensed, insured, and trouble free for 51 years!  It is very kind and generous of you to offer a loan of your spare CDI box and I may take you up on it.  Just want to try a couple of things MarkT suggested first.  Many thanks, Pat

 "I would like to send you a short video of my 360 but I've been unable to on the forum."

Photos, and maybe videos, can be posted to the forum via a third party host such as Imgur.

"PABDT - I bought the 360 at the Iwakuni MCAS Px while stationed there in 74-75 as a Marine pilot flying off the USS Midway (now a museum in SD)."

Absolutely bloody terrifying! 

Nearest I can get to that is the badge on the replacement tacho I got for my RT2! 

Video works great if you upload to YouTube and post link here.