Excuse my ignorance but reading about setting my carb on my DT400, one of the running recommendations is changing the “slide” from a 3.0 to a 2.5.

Is the slide the throttle valve, if so where is the difference, it can’t be the diameter? 

Yes.

I think that it is the angle of the cut away slope at the back.

Right, ok, I see your in the UK @Gr8uncleal (and you have a few RD’s  ) any recommendations to Mikuni parts suppliers?

If you have a Yamaha part number, then it might still be available through your local dealer, or maybe Fowlers on the net or fleabay.

I think that there are companies called Motocarb and Allens that supply Mikuni parts here in the UK. In the US, I think that Sudco are quite well regarded. 

If you buy from overseas, it isn't through fleabay, they don't have a UK VAT registration (which I think CMSNL do) and has a value of less than £135, then you probably won't pay VAT. Worth factoring into calculations.

I think the problem with the stock DT400 slides (throttle valve) is that they are taller and no longer made by Mikuni.  So you have to buy used or new old stock (NOS) from CMSNL or somewhere else.  Good luck finding a 2.5.  They are around to be found!

You'd need a good reason to need to change the slide, but going to a 2.5 richens the mixture just up off idle--well a bit further & overlapping to make the needle jet such harder. Something like that. Just go ride it. Yes it's the amount of cutaway, so 2.5 has less. Classic with RD twins when the customer was playing around with needle clip position then put the slides back in the wrong side which put the cutaway to the front effectively having no cutaway & running drowning rich. 5 minute fix & customer thought we were magic lol.