Great news

Hi Airbornesilva , I am running a B9ES witch is correct for the 400B, the B8ES are for the DT 250B. I checked with Yambits and all of my manuals just to confirm it. I have also fitted a 12 volt Rexs ignician system, as the old 6 volt cdi and coils were corroded, the bike is a USA barn find, 3 years have now past and  now fully restored she fires up first kick hot or cold.

Thanks P.Wells, after learning on here what I had thought was right I too run a B9ES, don't know why the shop gave me a B8ES... I have the Vape 12Volt system in mine, the little it ran before my lower rod bearing went out it ran like a bat outta hell! Will be getting a new rod and bearings in it fairly soon, I'm also going to do all the painting while the lower end is in the shop, then I want to deal with the shunting issue, lot's of good information on here for that...

Hi P.wells, that’s fantastic news. 
Can I ask you what part number you got from Allens Performance for the needle jet / needle jet tube?
Was it 6F9 or 6DH2?
Thanks…

Hi JammyYammy, I purchased the 6F9 as stated in the service manual, the old one was also a 6F9.

Many thanks.
Sorry for all the questions, but Allensperformance want me to check the ‘series number’ of the needle jet: 

“Could you please check the series number in the is it 166 we have a bit of information on the DT400 it will be on the needle jet 166-0-8 are  £16.31 each, we dont have stock of 6F09 - £8.42 they are back order postage £4.14 . Just check the series number for me please. I can make you a sale order up and give you ring when we have stock”.

My needle jet only has ‘0-8’ stamped on it. I’m not too sure where to find the ‘series number’. 
Is 166-0-8 correct?
Thanks…

I've never seen a genuine Mikuni needle jet that didn't have the size, ie "O-8", the series, ie "166", along with the Mikuni "square in the corner or a square" trademark symbol stamped in it.

The 166 is similar to a 159 series.  159 is on the right.  Notice that the 159 has a thicker rim near the top and a slightly taller shroud. My understanding is that the 159 series is used in a wide range of carb sizes (30 to 36 or 38) and the 166 variation is designed to fit the smaller size carbs (up to 34) better and not stick out so far. 

yamaha-enduros.com/index.php/forums/attachment/15870

Partzilla lists a 159 P-0 for the DT400 in the current parts list (#18)...  which is a slightly richer needle jet than the O-8 originally specified. 
www.partzilla.com/catalog/yamaha/motorcy...0c/carburetor-400b-c