I have two carbs one has an identification of 498 and the other 501

What is the difference, they both seem to be identical?

They both work fine on my DT400

Thanks

498 is the model code for the 1975 DT250B, US, Canada and "others".

501 is the model code for the 1975 DT400J, Japan only.

Are they completely identical, with same needle, jets etc? 

Yes, everything appears to be the same.

Did the 250 and 400 have the same carb?

My 250B and 400B do not have the same carb. Unfortunately I am out of town or I could send photos.

They might look similar externally but not likely they are the same internally..

400 carb should be a larger internal venturi diameter and not likely jets are all the same either.  But who knows.  Not much info on Japanese models and/or someone could have machined and jetted a 250 carb to match 400 specs. 

If the 498 carb runs ok on the 400 then I guess it must have been jetted to suit?

Not necessarily. 

Carb from a smaller engine often runs okay on a larger engine in my experience. 

Reading between the lines I think what you're saying is you haven't actually checked the jets or venturi diameters? 

Jets are the same. I measured the airbox side and reed valve side both the same.

Venturi is the bore inside the carb.  But the 250 should have a smaller OD on the spigot if I remember correctly.

Anyway, without pictures or something more who knows what you actually have. 

On those later models Yamaha usually had a 5 digit carb number stamped (i.e. 49861 or 50061) in the USA.  You might have carbs that are not original Yamaha at all or that were used in other countries?

If it runs good and they are identical, who cares.