1976 DT400C had two carb specs...  500061 and 500063

Main 170
Needle Jet O-8
Slide needle 6F9-4  (-4 means clip in 4th groove from top)
Slide cutaway 3.0
Pilot jet 40
Air jet 2.5 (often 2.5mm hole drilled into body, so not removable)
Air screw 1.5 turns out
Starter jet (pressed into well in bottom of float bowl, not removable)  100 for carb 50061 and 70 for 50063 (this is only difference listed)

You listed some strange pilot and main jet numbers...  so you might have "rebuild kit" jets and not genuine Mikuni?  If so, they likely don't flow the same as Mikuni.  But you said your bike runs great so who cares?  The only caveat is if you ever change those jets, you don't know what you have so for example putting a genuine Mikuni 45 pilot jet in might actually still be leaner than the aftermarket jet marked "40"...  Or a Mikuni 35 might be richer than than aftermarket one marked "40".  You just can't compare the calibration directly. 
 

Mark - many thanks for digging out the information which I have duly noted.

Yes the bike certainly seems to go pretty well, but having never ridden one before I did not know whether it is going as well as it should or not, hence wondering if the carb was jetted correctly. I think I shall leave it well alone except for fitting the new float bowl gasket.

Although I gave some odd numbers for the jets, they do carry the Mikuni mark, so the assumption is that it has the stock jetting.

Thanks again.

Hi Folks. I was about to ask the same questions about standard carb spec, jet sizes, settings etc. for my recently acquired  rough running 1976 DT400C which I'm just getting around to sorting out, when I found this thread which is just what I was after. I want to start with a good base line and take it from there. I'm also going to make sure the exhaust system isn't blocked or gummed up. Thanks. I'll be back....probably :-)