For some reasons I keep getting a leaking drain line after buying a new carburator. Why does gas come from this line? I thought it only came from that line is when you open the drain line screw. Any suggestions or repair ideas?

What are you working on?  None of the old enduros have a drain line or drain line screw screw.  Hard to find a "new" carb.

Floats out of adjustment, bad float needle and seat, or rarely a hairline crack in the brass "standpipe" overflow tube in the float bowl are main reasons I can think of for an old Mikuni to leak out of the overflow line attached to bottom of bowl.

Possibly the float is out of adjustment , because the carburetors aren't that old.

I have a 1975 Yamaha DT125B Enduro , yes they have a drain line on the bowl a screw there on the bottom as well.

I stand by my original statement. 

A DT125B has a jet holder in the side of the bowl.  It can be used to drain the bowl.  But the fuel will come out around the holder as you remove it.  The hose on the bowl is an overflow hose and not a drain.

Contrast the above with later model carbs...  maybe around the 1990's Yamaha two strokes like the RT180...  where they sometimes have a small screw near a hose in the bottom of the bowl.  As you described in your first post, you loosen the little screw and the fuel drains out the drain hose.  

And you keep saying the carbs are "not that old" so maybe you are buying the cheap (usually Chinese) knockoff carbs?  If so, that could be your problem right there.  Many of those are dubious quality.   But we'd need to see pictures of what you have to know for sure. 

Photo please if possible, or even a video would be better. Video of it leaking that is.

Yes this is a tempory carb. the China made crab. I ordered new parts for my orginal carburator parts aren't cheap . Well I did fix the problem cleaned the needle valve , which probaly wasn't the problem this time the float was out of adjustment. Its no longer leaking and yes that jet/drain valve is a dual purpose bolt.

Me again…
I had fuel dripping from my overflow pipe. The brass overflow pipe in the bowl was cracked so I replaced the bowl with a new old stock item. I have been through three float valves. It has the washer. The floats are not leaking. I thought i had fixed it for last two weeks but it has started again
Last time i rode it, it ran badly for 10-15 seconds like it was going onto reserve, but then cleared up.
Since then i have fitted an inline fuel filter. And after fitting the filter (and not starting it) it is overflowing again! Drip drip drip….
So either the float valve is dirty and / or I have set the float height incorrectly. Maybe both. Or there is another problem. I measured the float height with the carb at an angle so the float tang just touched the needle at 21mm for a 1978 mx.
Even so, I am doubting myself that i am setting the float height correctly:
> I am measuring to the top lip of the channel the gasket sits in (and not to the bottom of the channel). I thought that was right… is it?!
> The floats are different shapes but it looks to me like either can be used for the measurement. Is that right too?
Could there be any other reason why I have this over flow problem?
Thank you.
Mike

Float arms should be parallel to gasket surface with needle valve closed if you've done it correctly.

 

Ok… thanks for the tip; will check for that.
Mike