So you didn't flush your oil tank before hooking it back up?
You should remove the tank & do that step, make sure your lines are clear & somehow back flush your pump.
No ideas on how you might do that, Deet is the guy to help with that issue. Please post how you cleared the pump when done. Thanks!

Good morning everyone, 
after reading this post, I'm wondering about the battery connections, I notice on the new harness at the back of the bike 3 red wires , one red is more toward the front of the harness right next to a black , I'm was assuming these 2 wires red and black right next to each other, coming out of the main harness were for the positive and negative of the battery,  but after reading the post 3 Red Wires On Dt250a , I'm scratching my head again , I'm going to try inserting a Pic of the fuse plugged into 2 reds , more than likely I've got it wrong,  I'm hoping you guys can shed a little more light on this for me. Also I was reading a post that , I think MarkT mentioned there is a black coming out of the harness up near the coil that is for Checking points? and not to hook it up because there would be no spark it it was hooked up Not sure .. would it be the black wire with the open ring connector in the picture near the coil? I take back what I said about this being simple.
I appreciate all your help
thank you
John

Thank you to Ericb1318 for sharing this

Wire connections and harness layout are in the service manual.

Vintage Yamaha Enduro Fans - 74 DT250-DT360 Factory Service Manual (yamaha-enduros.com)

The "black wire for checking points" is not on the 1974 DT250.

"Ring connector" is the frame ground.