Thanks Gr8uncleal, I’ve sent e-mails to both Motocarb and Allens to see if they can get me a 2.5 slide.

Hi Hatrick1960,
Did you manage to get a 2.5 slide of the correct height?  Did that, and your new pilot jet, make your bike perform better?
Thanks…

I’ve made some progress. I made a 12mm spacer (from an unused slide) to sit on top of my new 2.5 slide (which is 12mm shorter than the OEM ‘tall’ 3.0 slide).

  

Hopefully, some test runs next week. Unfortunately, I’m not very good at ‘tuning’…
 

I am in the process of rebuilding a DT400B carb, the idle jet was seized or you could say it was bonded to the aluminium after 47 years. To make matters even a little more challenging the previous owner broke a small drill bit in the centre of the jet. I was able to drill through the bit , and re-tap the thread. There was no remnants of the original jet left to identify if this carburetor uses the VM22 series or the BS30/96 series. Obviously I don't know what size of jet was installed as well.
Your comments would be appreciated. Thank you.

Hi, the standard pilot jet is #40.
The manual I have states the carb is a VM32SS (hopefully, someone with a bit more knowledge/info can confirm).
Steve at Motocarb (if you’re in the UK) is very helpful/knowledgeable. He has jets for the DT400B in stock.
Have you got the standard carb? It should have the ID number 50061 stamped on it.

THANK YOU FOR THIS!!! Always wanted a DT when I was a kid but could only afford gimmie bikes! Now with this 76 DT400 I’m desperately trying to make it rideable and the bucking is just phenomenal! Mine at 3000 rpms, and just trying to hold that speed, is just terrible. I bought the bike out of Utah (I’m in Mn) and it has a 40 pilot and 160 main. I put a 180 main in it and richened up the air screw for the pilot to help richen the idle. The main seemed too rich, so I put it back down to the 160 and it revved better but it sure doesn’t make shit for power after about 56-5700 rpm’s. Tomorrow I’m going to try this setup you mentioned here and lord almighty I hope it works!!!!! Thanks again!!! 

welcome from NZ. The very first thing in my book for anything to do with lack of performance & not making power or wanting to rev freely--is exhaust & in particular the tail muffler & down deep in 'that' tail muffler is the spark arrestor & is non removable so ideally needs a GOOD burn out so everything is hot enough to burn away. Ok i do go on, but see how 'that' pans out before going too far off the beaten track with carburation. Then again i could be wrong. If it's erratic rich running at 3000 it could be just too rich on the needle which you can drop the needle 'raise the clip in other words'. Those big motors do wear out the needle jets in the metering part & also wear the needle but both those things are 'sort of obvious' if you look closely at the jet with it out of the carb & up to the light. They go elongated. not sure if its the pulses of the big motor flapping the needle in the metering very top part of the jet. Anyway see how ya go & if ya standing still on full song it should fair rev its nuts off in my book. See what others come up with.

That's what I've found too. Good gas and if it pings, richen up the air screw. I richened mine until the pinging went away and then it rides and runs like you have to get used to a 400. 
 

Very interesting outcome on the other DT400 post with rich running symptoms where seat had been restricting the filter box intake. Have a look.

might be worth checking the reed valves next time you have the carby out.