Mark T what is the procedure to remove the tube please
Gary
 

As i said, "With main jet and washer removed, needle jet (aka main nozzle, emulsion tube) pushes out from bottom to top".  Not sure how to say it better. 

 

Except your bike may have the carb changed to one with the main jet in the side of the float bowl? 

Then it's the "jet setter" and washer you remove. 

Pretty sure 400b had main jet threaded right into needle jet though.

Garydean, you'll find a lot of threads on this site regarding your issue. I bought my first DT400 (1976) a few years ago and rebuilt the bike. When I got it together with tuning per the Yamaha manual I couldn't believe how unhappy it seemed at low load cruising. I figured I did something wrong so off came the carb for another cleaning...and I'm pretty anal about that stuff. Made no change. I bet I had the carb off 20 times that summer, including rebuilding a second carb to try. No change. To make a long story short...I posted my final results a couple years ago. The bucking and surging can be greatly reduced by several things: (1) Change the slide to a 2.5 cutaway. Stock is 3.0. I got mine at Summit Racing. (2) Up the pilot jet considerably. I am now running a 60, up from the 52.5 I put in a couple years ago. (3) Turn the air screw in as far as you can and get the idle good and blubbery. (4) Turn your idle screw out to lower the idle as much as possible (1300-1400 RPMs).  I ride my DT400 now quite a bit. Have about 1400 miles on it now. A final comment after doing all the things I mentioned...learn what the bike likes and ride it that way...that's what I do at least. Good luck!

Edit...been awhile since that first summer with the bike.  Other things I tried with no change to the bucking and surging...changed the timing to and fro from 2.9 BTDC, changed heat range of spark plug, changed plug gap, bought an NOS cdi unit, probably others that I'm forgetting. All of these things made no difference.

my 1971 RT1B Enduro and my 1975 DT400B has this problem. I basically have turned in the pilot jet screw until it was almost closed and this eliminated the buck and jerk at closed throttle on both bikes, probably could use a throttle slide with a smaller cut away or a UFO Throttle slide mod or a little bigger pilot jet. my 1970 RT1M does not have this problem with makes me think it's due to a lean  condition with the throttle slide cut away while decelerating with a closed throttle.

you can try finding a 2.5 throttle slide for a 1975-76 DT400.  stock is 3.0.

Rich at Rich's Taylored porting can do the UFO throttle slide mod which I plan for my 75 DT400B and 71 RT1B
richstayloredporting.com/

 

this is the UFO slide mod I talked about. this makes your Mikuni round slide carb perform better than a modern flat slide and it can also solve the lean condition decelerating with the throttle closed. planning some mods to my 1975 DT400 perform better than stock and of course try to get a better mid range and top end pull while keeping the bottom end power the bike currently makes.  I already have the sleeper pipe which helps the bottom end and midrange plus a little more top end.

 

Hi, thanks for the info from all. Were can I find 2.5 throttle slide for a 1975-76 DT400 in the UK?

Try Steve at Motocarb.

Hi Guys
I finally got the bike out today after a number of weeks (mother died) rode the bike hard for 15 mins, had the over run etc as previous post, I screwed the idle jet in 1 1/4 turns and set the tick over screw to a 1500 rpm, pick up was good, then had a break 30 mins later took 6 kicks to start, I need choke to start otherwise it wont, then rode home and had no over run or bucking, rpm dropped pretty quick when clutch it and revving, before it would hang quite a while so I think my problem is sorted, was a much better enjoyable  ride home. I took the bike to 85 mph but I could not see with my eyes viberating LOL but if I lift my but off the seat all is clear, so a lot of harmonics going on

More padding in the seat? 
Compare to my Harley... My 4 stroker vibrates... but not as vehemently.. but its definitely there... Top end on my RT2 is about 45 and @ 5500 RPM... My eyes are all over too!  (:
Sneezles61