Recently got a 75 MX400B. It sat easily 20 years. Working on the fouled stock 38mm Mikuni carb but thought about a flat slide carb. Mikuni TM/TMX or Keihin PWK.  What's some thoughts on the conversion. The engine is going in a Champion frame, with low pipe/ oversize air filter.  Not a racer though. Thanks

PWK gets my vote over the TM series, should be able to make smooth power with the kehin 

You could go either way. The PWK seems to be more responsive. The TMX is easier to get jetted crisply and makes better top end on some bikes.

I've always used PWK's on mine, getting them to run crisply at a cruise has always been a challenge.

Thanks for the suggestions and inputs. Surprisingly, able to get a good cleaning of the stock MX400 VM38 Mikuni. Reinstalled and ran the engine briefly last night, a good thing. Didn't know the engine condition, sounded fairly tight, no bearing noise! Horrible silencer/spark arrestor-loud. I need to repair/replace the intake manifold-excessive cracking. Will probably clean it then seal temporarily to continue motor testing. Any suggestions on a replacement or another option? Has anyone done a 3D printed replacement? I have found NOS manifolds-$100+. This engine is going into a Champion YAM3 chassis so have a little flexibility.