So, yesterday I decided to reinstall the old carburetor that originally came with my DT1E onto the bike (Mikuni VM28?32?30?... unsure, but it's not a 26). Somehow, I managed to pull out the small brass tube that connects the gas tank to the carburetor from the carb body without any effort.

Now, I'm faced with three options: either drill the body, tap it, and install a new oversized threaded brass tube (which I'm not too keen on due to the hassle, plus my lathe buried under a mountain of spiders), or attempt to braze/silver solder the old tube back into the carb body.  Would JB Weld work if I coated the original tube and pressed it in with, say, a C-clamp?

On a side note after reinserting the tube into the hole I pulled it from, I tightened it finger-smoosh-tight and kicked the bike over, only for it to go wild—full throttle for about 10 seconds—until I pulled the spark plug wire and gave myself a little wake-up jolt. This bike hasn't done this before, but I've been messing with the carb and could have introduced an air leak somewhere. I'm unsure if this is related to the issue at hand. Any thoughts would be greatly appreciated.

 

If you tap the brass fitting in it might go tighter when near fully home & not leak--i hope. Or a drop of locktite & tap it in.
With the run away engine, on those early carbs the flange gets bent & tightening it fully tight can maked the throttle stick open.
If there's an Oring set into the flange be sure its no too fat as causes problems trying to compress it with not enough room to compress.
Can check sticking just by pulling up on the cable at the carb & see if it stick right up on full throttle. If it does then back off the flange nuts half a turn & see if it fixes it.
If it does fix the sticking then check the flange for flatness & you could file it if needed.
The insulating spacer block might not be flat either.
Just thinking about the bent flange possibility, if slightly bent & you file it flat, i'm not sure why it doesn't still try to pull on tightening & carry on sticking but i've done that successfully although might need a thinner Oring in the flange groove if too much alloy is removed. Terrible matterial those early carbs
Alternatively, is the slide just misplaced & sitting on the guide pin which is about 3/4 throttle.
& has the needle gone down into the needle jet because its very easy to have both those problems--slide missed guide pin & needle missed jet.
Hope ya get it safe as pretty scary when they 'run away'.

I've pulled the brass fitting out of a fuel tap trying to remove an old hard fuel line .
I tapped it back into place , and it hasn't leaked . I don't see any reason why you shouldn't be able to do the same with your carb .

I too pulled the brass fuel inlet nipple out of my mikuni carb.
Tapped it back in place with hammer and
wood drift, and had been fine since.
MikeK

Yep, what everyone said, tap it back in, it will stay in place, common issue with some carbs and petcocks, 

(don't know what happened to the first response I wrote ??)

I agree with all the others, had the same thing happen to me, just tap it back in and all is good...