New to me DT400 is in really bad condition, everything stripped to a pure dirt bike, but was announced as "running".
In deed, I started the bike and motor in idle run fine without bad noises. But then smoke appeared under the front of the gas tank, so I stopped the engine.
Didn't find the source, nothing visible on the wiring loom or surroundings. And bike don't have spark, now.
My question now is, can I try to start this CDI bike also just by connecting the black wire with the coil,,as it is possible with the points bike DT250?
I know the CDI is not reliable and I will convert the ignition  later. For time being,  I just want to find out in a short ride, how engine is running and shifting, before a possible rebuild starts.
Mabe just top end rebuild and new seals or enough?
Thank you in advance for your help. 

Not possible to do that on a CDI bike.

Smoke aint good but an easy way to find the short. But is the smoke coming from a worn decompressor & smoke is wafting up there under the tank. Ok not that. Nothing is battery powered in the cdi but i guess a short to ground or short together could melt the wire casing. Have a close look.

The smike must come out of the wiring loom.
 It appeared after motor was running for 1-2 minutes..I think you are,right,  must be a short 
But where? I will have a closer look..Thank you both 
 

Most common "smoke under the tank" cause I've seen is the rectifier and/or the rectifier wires (white and red)...  generally caused by a shorted battery or no battery and the red touched ground. 

Technically, that won't make it stop running on its own...  but what can happen is adjacent CDI wires get melted/shorted in the harness.