DT 400B 1974 for 1975 American market.
Engine has had a total rebuild (1st size overbore). Carb been through the ultrasonic bath 3 or 4 times. All new genuine jets fitted.

Having a new problem to me.
Bike starts first or second kick with choke full on, idle screw backed right off, air screw at one and a half turns out.
When the bike is warm I can get a good idle (1300rpm) and this is improved with the air screw being screwed 1/8th turn in, crisp throttle with no hanging.

With the bike warm/hot it continues to fire first kick and run well.  Haven't taken it for a ride yet waiting for UK registration documents.

Leave the bike to cool down and it will not start with these settings (choke on or off), I have to totally back off the idle screw and add the 1/8 turn on the air screw and use full choke so that I am back where I started!

What am I missing?

(FWIW I am not a newbie to carbs since I have a DR800 and XL600LMF that both have 'interesting' twin carb set-up's)

A better description of exactly what you mean by "idle screw backed right off" would be helpful.  And also specifics of exactly what idle stop screw changes you're making to get a 1300 rpm idle when when warm. 

My first wild guess is you might have a restricted starter jet in the bottom of the float bowl...  or maybe a cracked (or even missing) starting enrichment draw tube?  (When you drop the slide all the way it increases "suction" on the starting circuit.)

The idle screw is wound right out to the point that the tip is in the carb body so the slide is fully down.
FWIW I have checked that the slide is fully down on the bench with the screw in this position.
To get a 1300 idle when warm this idle screw is screwed in five and a half full turns from this fully out position.

On the starter jet that is a possibility, I have had a magnifying glass on it the last time I had the carb apart and it looked good but no effort to pull the carb and check again.
The starting draw tube looks good.

When I first tried to start the motor I didn't have enough suction which I traced to a combination of the new O ring on the crank leaking and then the passageway through the carb was obviously slightly restricted but this appears to have been fully cleared with the last trip into the ultrasonic (hour and a half at 70 degrees) I have managed to pass a fine wire through the full length of the passageway and it meets no restrictions and comes out perfectly clean.

When I last did a leakdown test there was minimal loss, a couple of psi over half an hour but mentally I have made the connection between the slide position and my starting issue.

Yes, definitely check the starting jet.  Slide down all the way definitely increases the pull of fuel through the start enrichment circuit.

If you alter the idle mixture screw while idling, do you get a response from rich to lean.
Rich being screwed right home so should go lumpy & rich if the jet is big enough & lean is out past two.
I guess air leaks somewhere or crank seals mag side in particular but all highly unlikely.
Idle jet has a progression hole straight up from the pilot 'think thats right'. & the idle hole is on the angle pointing forward. Unlikely to be blocked though.
But one thing i had on my 250 where bike had sat for i dunno a year or two & our fuel is terrible crap in nz.
So it grew fuel residue up the needle jet inside where the needle runs & closely encompased the needle.
Took me a few goes to find that as didn't expect it & never seen it before or since.
Starting was much like you decribed but ran ok when warm then a problem again when left for the afternoon.
Anyway, was just a thought--lots of thoughts.
One more--reed petals, but unlikely unless one is broken or missing a piece.
Or bent so are 'very' open, not just daylight which is ok as blown shut on the piston down stroke.

Thnks for your thoughts.
The idle mixture screw seems to be working fine when the engine is warm.
1/8th turn in (richer) smooths the idle, if I go 1/2 turn out (leaner) then the motor 'hangs' if I blip the throttle which sounds right. With the 1/8 in from the starting position (1 and 1/2 out) I have a nice crisp throttle that just returns briskly to 1300rpm.

Air leaks are possible but I ruled them out when I last performed a leak down test (will probably do another just in case something has changed)
Reeds whilst original, so nearly 50 years old, are in good condition (bike only has 5000 miles on it), no light visible, no marks, no chips.

Needle jet and needle are new Mikuni so should be OK .........
Jets are std sizes.
Petrol, well I can get 99 octane with no ethanol at present so that is what is in this bike.

Time to pull the carb again!

Got to say pulling the carb on this bike is a joy, on my old thumpers removing the carbs means removing airbox, suspension and unbolting subframes!

Well after a couple of days off to do some powdercoating I got stuck back in and I had my DT400 running this afternoon.

A quick rundown of what I did in case it is of use to others in the future

Carried out a leak down test (8psi max) which wasn't consistent. Some times lost everything in a couple minutes, other times held 8psi for half an hour. Traced this to the auto decompressor. A light touch of the mechanism and air escaped. Got the grinding paste onto the valve and that seemed to solve the issue.

Question, is there a preferred setting? The WS manual doesn't say anything so I have just gone for a very small opening.

Pulled the carb 3 or 4 more times, twice more in the ultrasonic and 2 more 'sanity checks'!
All good in there.

Tested the spark with a tester. I can get the spark to jump 4.6mm, this seems low, my DR800 hits 8mm. Note the DT has a Rex's 12v conversion.

Kicking. Well using full 'choke' (enrichment) and no throttle tends to wet the plug with a couple of kicks ..... the bike will not start. But on one of the carb refits I forgot to connect the oil feed (yikes!) but it started 2nd kick with full choke. Shoved the pipe on and the motor died instantly. Since this missing pipe gave a clear air passage into the carb it got me thinking that a lack of air was my problem.

I'm tending to think that full choke and no throttle is 'flooding' the motor. Note the idle adjuster is fully backed off so the slide is fully down so little air will get in.

With a dry plug and no choke the bike has let out a couple of backfires and then a couple of kicks later with just a touch of throttle it starts. (tried this several times)

So whilst I have got it running today I'm wondering if it will start again tomorrow.

Any thoughts? Am I onto anything or am I just going round in circles confusing myself!

What carb is on it?

My first thought is that you're playing with idle too much. Get the bike started by whatever means you can, warm it up, and dial the air/fuel screw + idle until you get it crisp on the snap and idling warmed up.

Then you don't need to touch the idle screw. Let the bike cool down and start to diagnose cold-starts next. You should be able to play with just the air/fuel mixture screw to get it to start along with whatever choke position it likes. If you find you're adding a lot of air or fuel for starts, it will indicate either a possible leak when cold or that your cold-compression and hot-compression are wildly different as the engine warms up.

From here, I'd need to see what carb you're using. Some choke via extra fuel and some via air restrictions (more traditional). There's a chance you might need to adjust the choke in order to get cold starts to comply.

Loosing 8 psi in 30 minutes is still a pretty big leak.  

(Personally, I wouldn't go over 5 psi max, it's possible to blow a seal out of the case)

Air leaks tend to get worse when hot or over time...  It's very possible to not lose any pressure overnight...  on my bikes if I'm losing more than one psi an hour, I keep looking to find the leak(s).  

Also, have you actually RIDDEN the bike yet?  Several times I've helped friends with starting/idling issues on a bike they've just been starting repeatedly and fiddling with in the garage... never actually riding it (sometimes for good reason, like yours of not having registration)  After a good warm up ride, everything is fine.  

Hope you figure it out soon!  

The carb is the std Mikuni with standard jets (all replaced with genuine Mikuni)

The only reason I have been playing with the idle screw was because previously the bike would not start unless it was backed right out.
When I got it running I set the idle speed after a slight change on the fuel screw (1/16 turn richer)

On the leak down it now holds 7 or 8psi for half an hour without any loss, I'm relatively happy that I have no air loss but haven't left it over night.

Waiting for tomorrow to see if it will start from cold again with these settings, there will certainly be more air getting in.

Spoke with our vehicle licensing authority yesterday, fired off more photos and I think it will be registered within a week, never had this amount of problems all because Illinois use their vehicle licensing number and call it a 'VIN' number which of course doesn't tie up with any of the numbers on the bike.
Get that and I should be able to take it for a spin and really get everything bedded in properly!