...yeah, we dont konw what all "...back together and working..." indicates.
BUT, if the bike is 'in good working order', then your primary symptom is that the engine is not receiving enuff fuel mixture.
those xr's are so reliable that they can be about to hand-grenade from wear, and yet still start and run durn good, so i would venture to say that your float-bowl jet is stopped up, providing that your gas IS turned on, right? <g>
If u know of another xr100 owner, swap carbs right quick and see if that is the prob right off.
Also:
1. if you pull it with a 4-wheeler/etc will it start that way?
2. loosen the float-bowl or fully remove it, then turn the fuel petcock on to see if fuel is even reaching the float bowl or not.
3. if it is, then check the float-bowl (buy some carb-cleaner or get some compressed air, and blow all of the carb orifices out (everywhere you see a hole). Wear GLASSES as most of these will spew carb cleaner right back at you if you dont get the spray-nozzle in the hole really tightly...
4. spend the $18 and buy Eric Gorr's motocross repair book, filled w/lots of clear pictures and clear directions on troubleshooting.
ttfn,
jks