Aloha everyone, I have a 1975 Yamaha DT 250 just acquired about six months ago been riding it and all of a sudden the clutch started slipping took it apart. Everything looks good clutch plates are within specifications. Any ideas? Any help would be greatly appreciated. Thank you

Welcome from NZ downunder.
Slipping all of a sudden doesn't make sense.
So thinking outside the square, by memory those have the cluch mechanism underneath the motor.
Have you landed on something that might've bent the arm to catch on the case or can the sump guard get bent against it.
Or has someone filled it to the top with oil--when you weren't looking.
Has the chain derailed & bunched up damaging the actuator part of the case.
Has anyone tried adjusting the clutch on the screw that lifts or lowers the arm which increase or decreases your cable freeplay.
If that accentric screw is turned in past its useful lifting or lowering it'll bind by bottoming out in the groove.
Effective movement is all within half a turn each way when it's far enough in to be safe in the groove.
Does the cable move freely & feel like it's free at rest & not holding against the rod.
Sorry lot of questions from me turnng a short answer into an essay lol.
Hope you enjoy your time here.

Greetings from Tennessee.