I had to 3-D print my own carb to airbox boot/adapter, that worked great and if anyone needs one, happy to print and ship one.I drilled and tapped for the genuine Mikuni oil injector barb, was very careful, and that went quite well.So now I'm tuning, first, the bike runs much, much better. My original carb was just shot through. She now responds to the air mixture, idle set, etc. Unfortunately I failed to take note of what jets are in the VM by default... sigh. Also, the needle comes on notch two - I now have it at notch 3 (4 seemed worse). My symptom is that at 3000 RPMs, she stops being smooth.. at best I'd say it becomes a bit choppy. I'm not certain if it's lean or rich, just don't have the background/skills to know. I think.. it's going lean so I'm thinking I'll go back to notch four and then focus on the air screw.FWIW this bike is all original (except for this new carb), and in really great shape.Question: Anyone have any experience with this particular upgrade/install? any advice on jetting? I have a complete assortment too to facilitate tuning.

You can get a small shim / washer and slip it under the needle clip , to effectively have a 3.5 clip position .
Niche cycle or other mikuni distributors will likely have one .
It may , or may not , solve entirely your issue , but it's worth a try .
From memory it was .5mm thick .

Makes sense, question though - are these adjustments; the needle position, air screw, etc., are they that sensitive? The bike seems so much more crude than that.

It's all about finding a balance . It's not a major to give it a try , and you might find a happy medium .
If you don't try , you'll never know .

Yeh" have the same problem,[dt400E 1978] That roughness [ 4stroking] @3k rpm, is annoyimg , i dropped the needle, it made a wee bit difference , i screwed the Air screw in a half turn [ to stop the bucking on overun], this makes it rich too, i think the needle is worn, anyway i have a new VM30, that fits the intake boot [the Vm34 size does not] its smoother, its got an 3.0 cuttaway slide fitted,[ oem carb is 2.5, maybe this is the answer ,

radadirat, according to the Yamaha parts listing (legends yamaha docs) and partzilla that bike had a 80 idle jet and between a 170 and 190 main.

I would rather start with jetting from the new carb than put the old jet sizes in. Its all relative to the new one anyway. The old VM34SS for example, 80/190 and maybe the VM34 you just got 35/260??

I experienced similar w/VM30 transition to a newer model. Just giving you a heads up.

Also, not sure that 2.5 to 3.0 slide change will matter much since its on 2 total different carbs. If it had been a changed on the original carb, then would have expected leaner part throttle transition to the needle.

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BTW, I have a 360 that gave me similar grief and kind of still does to an extent. In MY case, I :
put 606 Boyesen♦ reeds that require inc fuel low end and less on top. I went from a 180 to a 170 jet and went to the 4th position. It was probably ok on 3. Its not perfect, but better than before.

♦there are varying opinions on these. in some cases could add another layer of tuning difficulty. lol, thats no fun.

A slick way to fix this is add less gear so the engine loads up a little.