How can you tell the year model of a Yamaha YZ 250 dirt bike serial number 1W3-001435?

you should look on the fram of the bike right in front of the gas tand on the side and it should tell u. 1977 250 YZ250D 1W3-000101 6520 NOTE: This Is The First Year Of The Plastic Tank, Used On All Bikes Except The YZ80, YZ100. It Was Also A Completely Different Frame/Monoshock/Engine Design For The 125/250/400 Models. Bob (Hurricane) Hannah Raced An Almost Completely Stock YZ250D In His Sucessful 1977 Season. From this very helpful YZ identification web site. Hope this helps

Go here to look up serial numbers