1980 DT 125 rebuilt/bored top end. 1.50 over. New carb. Engine starts 1 kick, but runs wide open. Adjusted air/fuel mix to factory setting. Adjusted slide screw all the way out. Choke is off, slide moves freely, not binding. Got me stumped. Anyone have any advise or have had similar problems and have found a remedy? Thanks in advance.

LH crank seal, carb o ring to mounting flange there? If '80 is a spigot mount, any cracks from age on the rubber spigot?

Any chance the slide is in backwards?

Yeah my first thought too--slide not located. Wouldn't operate freely-- well ok would very freely but!! only lifting & dropping a very little bit as 9/10 wide open. You might be just feeling oil pump return spring pulling the cable back mainly.
Needle has to find its way down the jet too while locating the slide on the guide pin. Cutaway on the slide faces back towards the air fliter.

If slide is closing properly and completely...  with a nice "clunk"...

My guess is air leak from crank seal behind magneto not holding vacuum.  Use 77-79 IT175 magneto crank seal as it's better. 

My mistake, it is a '78. Replaced all seals with rebuild, orientation of the slide is correct. I don't remember if I changed out the slide and needle when I replaced carb with the one that came with the "project" have to check my Yamaha bin. Thanks for all of the advise.