Someone will correct me if I'm wrong but Turbodan mentioned "four stroking" which is a "rich" condition.  His advice will lean out the 1/4 to 3/4 throttle range.

Msavitt's advice is talking about richening the idle to just off idle throttle setting with a larger pilot jet...  turning your air screw in a little is also richening the idle/off idle throttle opening range.

Not taking sides here...  but it is kind of opposite advice and needle clip adjustment is mostly effective at 1/4 to 3/4 throttle which is likely not where throttle is held when problem occurs. 

What I'm trying to say is take it one step at a time...  one change...  since the air screw adjustment was favorable, I'd try the larger pilot first.  Just don't do more than one thing at once. 

I’ve made some contributions to this topic here over the years since I put my DT400C together a few years ago. Here are my top (4) changes I did to greatly reduce the bucking and surging. BTW early cycle reviews from 74/75 when testers got their hands on the first 400 bikes used the words “bucking and surging “ to describe the ride. Back to what I was saying: change the slide from the stock 3.0 cutaway to 2.5 cutaway. Then lower the idle to as low as it will go and still idle. Next up the pilot several sizes. Finally, turn the air screw in as far as you can and have it keep running. I spent a whole summer trying all kinds of combinations of things. These worked for me for about a 90% reduction in bucking and surging. You’ll note all my changes richened the mixture. It runs pretty blubbery when cold but straightens out after a couple miles down the county road. Good luck.

Edit…as Mark T says…make one change at a time. I made lots of other comments on the topic but one more important one was changing timing didn’t affect it all. So I put it back to 2.9 BTDC.

Ya want a manual decomp lever to operate the original that the kick start bring on. Then activate it on a closed throttle on the highway which might halve the [call it bucking] & soften the blow. Ok, just another silly idea. You could likely stop it totally by leaning off the pilot & needle very lean, plus the slide cutaway. But dangers in doing that on those big singles so not recommending. Might work just idling around the village. I'm not aware that suzy ts400's did that, so if not, what was their secret, different carb or just different everything.

Suzuki TS400's do not do it.

I do suggest leaning out the needle.  Best part about that suggestion is that it costs nothing and can easily be reversed if the results are not what was desired.

The needle clip has the greatest effect in the under 1/4 throttle range.  Lot of overlap here with that and the pilot jet, slide cutaway and air screw.

Without hearing the bike I could only guess if it is a rich condition like four stroking or a lean condition like surging.  If it's lean, adjusting the clip in the other direction may resolve it just as well.

My experience with jet needle on a Mikuni VM is it mostly affects the 1/4 to 3/4 opening range...  which coincides with the Official Mikuni tuning and operation guides. (which can be found in Tech Library and on Internet)

 

Hmm, my take on leaning the needle too far is ya asking for it to seize if for instance you've been going balls out down the highway for a good distance then roll it back to cruise into a built up area & Bam--nips up. Of course there's lean--& way too lean--so if ya do that just be careful. If you go too far you may feel it gasp a bit if you take the throttle up little by little on the road. Anyway, jusy my thought right or wrong.

There is a significant difference between needle taper and needle clip position. Needle taper is primarily in play in the mid throttle range. Clip positioning has the greatest effect at low throttle openings. 

 

Turbodan...  We're talking about Mikuni VM.  What exact carb is that chart from?  Rochester Quadrajet?    (just kidding).  In what context do they mean "effectiveness"?  Can you post the source and written description of that chart?

Anyway, the "shunting" problem being discussed here happens with closed or nearly closed throttle.  In my experience throttle position knowledge is CRITICAL when tuning a Mikuni VM.  Not "RPM" or anything else.  Once you know the throttle position, then, and only then, you can tune the corresponding carb circuit(s). 

The Mikuni VM manual states in the operation description that during low throttle openings there isn't enough pressure differential to draw fuel from the main nozzle.  All or nearly all of the fuel comes from pilot circuit at low throttle settings.  This can be confirmed and observed by looking into the carb throat with the engine running. 

Talking about clip position, you can completely remove the slide needle from the VM carb and it will idle normally.  As the throttle is opened, it will start to blubber and flood but that doesn't start to happen until around 1/8th throttle in my experience. 

You can actually ride the bike that way...  say if you lost the clip while making an adjustment out on the trail and need to ride slowly back to camp without a needle installed. 

And again, my experience agrees with the Mikuni operation manual descriptions.  The calibrations that are going to affect the 0 to 1/4 throttle opening the most are the pilot jet, the air screw, and the slide cutaway.  Messing with the needle would only be useful if the symptoms occurred up closer to 1/4 throttle.

Mikuni VM tuning manual actually has a chart showing fuel flow shift when changing clip from the #3 (middle) to the #2 and #4 positions. The chart clearly shows a plus or minus 5% to 8% shift in fuel flow at 50% throttle when you move the clip one groove either way and less of a shift at lower (about 3% to 6% more/less fuel) and higher (about 1% to 2% more/less fuel) throttle openings.

 

To add to RT325's caution about "nipping up"....  I have a friend with a 250 that was rich...  easy fix in his mind, swap the main jet in side of carb bowl.  He went down a few sizes and thought it was getting better.  Then he went for a full throttle run and (lightly) seized the engine.  His richness was at mid-throttle and he was adjusting the wrong carb circuit.  Someone had raised the needle all the way by putting the clip in the bottom groove.  Lucky the damage was minor and putting needle clip back to stock groove fixed the rich issue.  And of course, re-installed correct main jet. 

P.S.  Mikuni manual also has a chart showing fuel flow shift when changing needle jet (main nozzle) diameter...  the long brass tube.  This chart shows the fuel flow shift is far greater at under 50% throttle opening.  So if you had a mixture problem between 1/2 and 3/4 throttle opening, changing the main nozzle would not be very effective.  None of this info is very useful.  Unless you are chasing an issue or trying to fine tune.  Then the charts and information in the VM manual is very helpful. 

Yes mark it is from a Rochester Quadrajet.  Nailed it.
 
For all the time spent going back and forth on this he could have tried raising and lowering the clip position to see how it responded.  I always like to try what I have on hand and order parts if a satisfactory result can't be achieved with the adjustments available.

The VM is not unique in it's principles of operation.  Like most modern carbs the pilot circuit responds to vacuum, the main responds to flow through the venturi and the needle blends the transition between them.  My experience with jetting several VM round slides is not much different than jetting a modern TMX or PWK.  The needle, clip position, pilot jet and slide cutaway are fully in effect at a low throttle cruise.