www.aliexpress.com/item/1005003396111073...00025590168117%21sea

Surprised that the needle was so worn. Usually, the anodize protects it.
As for Needle Jet, Yes 1st to open up from vibration.

cliff

Hello again GR8UNCLEAL, Thank you so much for suggesting Allens, I now have a new Mikuni needle jet and needle on its way to me. When I have fitted them, I will update the post to let all of you know the outcome.

Thanks again RT325, New needle jet and needle on its way to me, GR8UNCLEAL recommended Allens and they came up trumps.

 
I don’t want to dampen your enthusiasm, but IF the new needle jet / needle don’t fix the ‘shunting’, you may want to try what others on the forum (Pedalcrazy etc) have suggested - increasing the size of the pilot jet considerably and putting in a new slide with a 2.5 cut away (I think the monoshock 400s have a 2.5 cut away) etc etc etc.
I made a spacer out of an ‘old’ slide to sit on top of the 2.5 slide (as the new slides are shorter than the OEM slides) and bought a range of bigger pilot jets (all from motocarb). I had a quick test run and it seems to be much better, but real life is getting in the way of me doing more testing and to be honest with you, I’m not very good at ‘tuning’…
I hope to do some more test runs shortly.

JammyYammy, how tall is the spacer? might need to try it on my DT400C when I get it going again. I'm going to try the other suggestions first and see what I get but would like to have this option if I need it. Thanks!

Measure the height of your 3.0 and subtract the height of your 2.5.  That difference is what you want for your spacer. It's been long enough ago for me to recall a measurement but just measure your stuff so the 2.5 plus spacer equals the height of your 3.0.

Thanks Pedalcrazy, don't know why I didn't think of that

My spacer is exactly 12mm high. I made it out of a cheap old eBay slide of the same diameter as the existing one. It floats on top of the new ‘short’ 2.5 slide. Fiddly to make without a lathe, but simple to fit.
When I get my fuel tank back from the fixers, and I fit my new air filter, and the sun comes out, I’ll do some more test runs. The one test run (a few months back now) was very promising, but I still need to play about with jets etc.

Here's a 2.5 slide I found on Amazon, is this correct?  It says for a VM32/34 so unless I'm mistaken I have a VM32 on my DT400...

2.5 slide