i think the 1974 DT250 came with a 28 mm carb,i would either rebuild the one you have or buy a good used one and rebuild that.here is one for your bike on Ebay.
http://www.ebay.com/itm/CARBURETOR-1974 ... bc&vxp=mtrthis one is from a 1976 DT250 and it would be one i would also use.it may require rebuilding and possibly rejetting to make it right.
http://www.ebay.com/itm/1976-76-Yamaha- ... 79&vxp=mtrthe jet setter o ring is still available and it is not expensive.this is a link to parts still available for the 1974 DT250 carb.
http://www.partzilla.com/parts/search/Y ... parts.htmlthis is a link to the master parts list @ Partzilla.
http://find.partzilla.com/?q=*:*&r=t&s= ... 22DT250%22this is for the 1976 carb i left a link to.
http://www.partzilla.com/parts/search/Y ... parts.html