There are 3 tubes that go into my XR100's carb. I took it apart for cleaning, and I don't know where one of the tubes go. One of course goes to the tank, the other is the overflow on the bottom, but there is a third one that i have no idea..

Also, I've tried adjusting my airscrew but it doesn't seem to do much. The bike seems to idle rough. The old owner had a Big Gun pipe on it with a 105main and 38pilot. I put a 95 main back in it. It also sometimes revs up by itself while idling. Any ideas?

 

Do you have a vacumn operated fuel valve? if so , thats where it would go. If you have a pri, marked on the valve, then its a vacumn operated type.
Look for vacumn leaks, for the reving up problem and rough idle.( Intake flange, cap on top of slide.)
If the idle goes up by turning the handlebars, check the cable adjustment. There needs to be some slack in the cable.

 

The third tube is just another vent. There is a little clip on the airbox that it slides into. Just follow the big air tube back to the airbox and you'll see the clip.

Hope this helps,