23 ktm 350sxf smoking out vent tube

You might just be a little over full on oil. How do you measure your oil on your services? Sight glass or measure it out and pour? Always best to measure out the CC’s and pour in especially on KTM/HSQ/GG. 

I use 1,000cc when doing oil and filter. I only do 900cc when not doing the filter. You’re always going to have 100cc or so of residual oil left in the motor. Is it just a little hazy or is it a cloud coming out of that thing like the dudes van that parks in the back of the pits? 

Go off of the sight glass, it is there for a reason. As you've figured out the hard way, measuring a precise amount of oil has caused you to overfill. You're never going to drain all of the old oil out or even precise amount each time. 

Mine did that when I would put in the recommended 1L of oil.  I started dropping down to 900cc of oil and that stopped it and the oil level is in the middle of the site glass.  The problem I found out recently is that the manual tells you not to remove the 3rd oil plug that drains an additional 100cc of oil out of the case.  So you're overfilled when you put the recommended amount in the case. They corrected this mistake on later copies of the manual.  There's another thread on here about it.

https://www.vitalmx.com/forums/tech-helprace-shop/23-ktm-350-sxf-oil-change-drain-bolt-question

or you could be like me, 16 250sxf and 3 out of the 4 oil jets to the cam lobes were plugged causing the cams to get nice and hot. doesnt hurt to pull the tank, plug, and valve cover and just crank it with the starter to verify the cam lobes are getting oil.

or he has gotten contaminates in his oil and the jets got plugged. it doesnt take a lot to plug them. if the oil level goes down in the sight glass when pumping hes getting oil

its shitty but you have to pull the head and take the valves and springs out. you can get them out but you will never get them back in because the valve spring is in the way. once they are out they are equivalent to carb jets. hold them to the light and you should see through them. personally since this is an engine jet i just replace them. but ultrasonic/carbclean/wire will do the job. 

also before you even do this did you hold the starter for at least 15-25 seconds to build enough oil pressure for them to squirt out? these bikes dont have a whole lot of oil pressure at idle. its like 5psi at idle and 25-30 at limiter, says so in the manual. 

Don't know if you figured this out, but I had something similiar once on my 250 Sxf.  Turns out I had put straight anti freeze in when I thought it was already mixed.  Changed your coolant lately?