21 mm carb on a 300? Yikes.
My XR-75 in 1974 had a 19.5 mm carb.
The Del'Orto site (
http://dellorto.it/en/products/carburetors/) shows that carb, but here are the details:
Engine: 50-100cc, 2 stroke
Application: Motorcycles
Production place: Made in Italy
Venturi diam.: 15 - 16 - 17 - 17,5 - 18 - 19 - 19,5 - 20 - 21mm
21 mm would never flow enough air to satisfy a 300 cc 2-stroke.
I opened the manual, and it shows the 21 mm carb. wow. the drawing matches the part number above. Crazy!