Turbodan, I anxiously await the item number for the bolt-on kit! :-)

Well I put the 52.5 pilot jet in this morning. Probably no surprise to anyone but me but sorry to say it didn't make much difference. Might, maybe, possibly, have smoothed out the cruising just a bit but can't really say for sure. Not nearly what I'm looking for l and it definitely didn't help at all with the off throttle surging. 

After a few laps around the field and once through the woods, I took it on a 20 mile round-trip on the blacktop, the farthest I've driven it at one time since I got er running. I stopped and made adjustments to the air screw several times along the way but found no magic spot. Oddly there were times when it seemed to cruise along pretty smooth but other times under the same conditions it did the surging thing. Still pulls good as long as I'm on the throttle, even a little bit is all it takes. Pretty strange. 
 

Great thread!

I’m no expert or overly experienced but I’ve been reading and researching the surging scenario on the assumption that my 400 would surge. It’s not that bad, however, one change is replacing the standard 3.0 slide for a 2.5.

I think this entire thread is fascinating...I found myself looking for reed blocks and a SC500 carb...but the thought of tearing down my Green New Deal which I am quite attached to stopped me

could it be that the Yamaha engineers, once they discovered they had created an oddball (turd too harsh). more or less optimized certain bad ratios and settings and sizes to avoid scrapping and accepting no payback on what was to become a one-year program? 

No way to know for certain, but the single year of service for this design is something that seems to support that conclusion.  The only things that really changed in 75 were the bore size and case volume.  To my knowledge the first year DT400's were not known for seizing and holing pistons like the DT360.

I do like the improved clutch release design on the 74 360.  Its easier to fix what I believe to be flawed in the 360 motor than it is to find a better clutch release actuator for the RT motor.

I would probably pass on the SC500 carb.  It is bigger than stock but other than that there are better options like the Keihin PWK or Mikuni TMX.  Hard to beat a 35-38mm flat slide. 

Something in the back of my mind is telling me that the maximum cc for the competitions that these bikes ran in was increased from 360 to 400 between 1974 and 1975. If this is correct, then I guess that Yamaha took the opportunity to sort out SOME of the issues on the DT360A (not the cdi pulser, though!).

There was never a 360cc competition limit that I'm aware of.  After 250cc came the 500cc class which was sometimes called the "Open" class.

Suzuki, Husqvarna, CZ, and many others had "production" competition bikes over 360cc long before 1974...  I vaguely remember that when the two strokes were making more power in the mid to late 1960's, "less is more" became the trend.  An off road race bike around 360cc had more than enough power to be competitive even at the International level considering the chassis and suspension technology.  More displacement didn't always give a proportional increase in power.  And the lighter weight of the smaller engine made for a faster overall package.  About 360cc seemed to be the "sweet spot".  Only the best of the best could handle the power of a full-on 360cc open class MX bike in the primitive chassis of the day anyway..

One of the things that Yamaha has always done is maximize the use of common parts and castings... My guess is what funded the MX bikes was the use of the parts for mass production of the Enduro aka "DT" series. 

For example, the 1974 Enduro bikes were heavily based on the 1973 MX bikes.  A lot of talk has been about the DT360 being based on the 73 SC500...  but the same basic engine architecture was used for the 73 MX360.  I'm guessing any compromises in design were made so Yamaha could have the "bragging rights" of a 500cc MX bike...  probably for the American market as we tend to go for the "bigger is better" marketing. 

One of the things that makes Yamaha great...  sharing of basic designs and many parts over a few models and model years...  is probably responsible for any compromises in the DT360 (if there truly are any). 

It also might have been a rare misstep for Yamaha...  The SC500 was not a great MX bike. I can imagine the Yamaha engineers were scrambling to get back on track...  The DT400 debut coincided with the MX400 debut in 1975...  Curiously the YZ had to wait until 1976 to get the new 400 design...  probably had to use up parts somehow.



 

Thanks for clarifying, Mark. 

Thinking about "500 is better than 360" bragging rights made me think of one of my favorite movie scenes... 




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Probably not a huge amount of help here, but here are pictures of the larger than stock carb on the smooth running RT2 I had. "80" stamped on the upper half of the carb and as you can see, mounted with a spigot to flange adapter and fits on the bike with hardly any extra space.