To me, adjusting the needle clip on Mikuni VM to fix a "light throttle" position symptom makes about as much sense as trying to fix the shunting by lowering the air pressure in the tires...   but that's just my experience and maybe a little fear of damaging the engine by leaning out the midrange too much by lowering the needle.  That's just my opinion though and worth what you paid for it. 

And maybe I trust the tuning manual from the company that designed and built the carb a little too much...   to each his own. 

P.S.  I know it's easy to get advice from strangers on a forum but I was wondering if anyone was going to post this little gem from the Mikuni Tuning Manual:

 

With a torquey big bore 400 I'd venture to guess the above advice applies to a higher speed range.

 

Very interested in this, my 76 DT400 is/was doing the same thing when I had it running (currently not operational), I thought maybe an air leak so I changed out the crank seals, both left and right.  It had new head and barrel gaskets which are not leaking, I checked for leaks around the intake but found none.  So I guess it's carburation and from what I'm hearing the main jet is too small, is this correct?

Hi all -

Love the stir this topic creates, it is very interesting.

Lots of opinions from much more experienced mechanics than me that says this 4-stroking is due to too rich, probably so.

My DT360 was fresh and air tight when I had the 4 stroking chug thing going on at low to no load medium RPMs. The timing was spot on and the carb stock, exhaust clean, really spot on.

The one thing that reduced, but didn't eliminate the chug chug, (which I believe is just a reality of the design) was to move the pilot jet up to 55, so yes richer.  This came with an air screw setting that was significantly (perhaps a 1/2 to 3/4 of a turn or more out from stock 1.5 turns setting, meaning more air, leaning out the increased fuel flow with the 55 pilot jet at this very low throttle condition.  But is my bike really running richer with the 55 pilot and the air screw opened up as described?  Is the net fuel to air ratio still richer, leaner or close to same but perhaps with more overall flow?  I don't know, but it definitely tamed my bike a bit.

I love the bike and have learned to live with and to appreciate this 1970s trait!
 

Needle jets [emulsion tube] do wear out with the slide needle sawing away for 50 years. Often [well, on our old MX bikes] a new needle & needle jet [E/tube] will sort it. Needle is the only thing moving to do with jetting [except for the slide]. Different story i know but when i worked in the suzuki shop on LTF250 & 300 quads i'd do probably one needle & jet combo a week--come in blubbering at about 1/4--1/3 throttle because i guess that's where they mostly operate around the farm following stock. Mind you i'm talking cv [vacuum] style of slide operation so maybe constantly varying & juggling around. They overcame that problem by going to silver looking jets or stainless maybe. You could eyball the jet holding it up to the light & see the elongated wear in the metering section at the top. Sorry--was that a bedtime story. Don't fall asleep now!!.

"Shunting" or "surging" at light throttle cruise on these models is not a classic rich "four stroking" condition in my opinion and experience.  If it was, going richer would make it worse, not better.

Not the main jet.

 

As I mentioned earlier in this thread, my 400 does this exact same thing. I simply ride thru it. It doesn’t really bother me anymore. It is one of my easiest starting bikes, and it’s a monster when you really get into it. But I have to ask…. Is it harmful to leave it as is?

We've become used to silky smooth fuel injection and computer controls...  I wouldn't worry about it unless it really bothered you.  The few Yamaha big bores I've ridden that did this have never bothered me...  part of the "character" of the beast. 

Some bikes are worse than others though.

Perhaps you mean.. a bit more un-refined? (:
Sneezles61

I had a thread on this topic about 6 months ago. my fix for my 400b thanks to a lot of contributions from the members on here was needle position standard clip, all jets to standard, idle mix screw 1/2 turn out, up the gearing to 16 front 40 rear. I have no shunting or overrun any more.
the bottom end is rich n blubbery to 1/8 throttle then clean for the full rev range
I can hold 30mph in top gear without being up n down on the throttle, the higher gearing makes the engine work a little harder and gets me past the point on the throttle were the bike starts bucking and wanting more throttle. the main thing for me was curing the overrun when you close the throttle, now when you close the throttle it closes and no bucking or overrun etc