Hello everyone! 

I’m fairly new to fixing bikes, recently I bought a 1978 DT400. I managed to get the bike running but I have 2 major problems. 

First, the clutch doesn’t seem to go past 1st gear. Would this be something that would be easy to fix, I don’t mind having to disassemble the engine.

Second, immediately when I start the bike the engine revs to about 4000 rpm on idle. I don’t think it’s the carburetor because I did adjust the screws but it doesn’t seem to make much of a difference.

Also I’m happy to see there are people that love to keep old bikes alive. I’ve been wanting to work on an old enduro for the longest time and I’m glad I finally got the opportunity!

Sounds like you have an air/vacuum leak for the high idle. I could be wrong but I don't think the clutch has anything to do with not being able to go past 1st gear, sounds like a problem in your trans, either you clutch works or it doesn't as far as I know. Someone smarter than me will come along to let you know.

Greetings from Tennessee 
  Is your carb slide stuck not going all the way down have you taken the carb apart? Or air leak as airborne suggested 

Sounds like you've had the slide out & put it in not aligned with the guide pin. The needle hasd to find its way down the spout of the jet too but i think you have that 'in' & slide likely crosswaus so its resting on the guide pin but in the cutaway side otherwise it'd be reving even harder if say half a turn out.
Regarding gearchange, my guess is the right cover has been off & the change shaft has moved over an inch then gone back on a tooth out of mesh with the change mechanism. especialy if you had the gear lever off at that time so it can move--with right cover off. Link one meshes equaly wth 19 in second link. All a wild guess at the moment.
www.partzilla.com/catalog/yamaha/motorcy...8/dt400e/shift-shaft
www.partzilla.com/catalog/yamaha/motorcy...t400e/shift-cam-fork

I really do appreciate the response I’ll check it out this weekend and see what I can find.

Be sure your lower throttle cable has free play when throttle is closed. Flick the throttle & see if the slide returns to bump the stop-idle speed screw. Then back out the speed screw & see if the slide bottoms out in the carb. Just be sure its all operating freely, & that the oil pump cable isn't hanging things up as that cable can fray going through the adjuster into the pump.

Yes check the slide in the carb, Should get a nice thud when you let go of the throttle. The throttle situation must be looked at and you need to be sure all is good throughout from top to bottom.. A stuck throttle is really scary. Good luck!