As shade said. I had this experience with my quad, rebuilt carb so many times. Turns out since the petcock was on a slight hill in the tank, I wasn't getting much gas.
Fill up tank and check fuel line, be sure it isn't pinched anywhere. Be sure petcock is on.
Also if you can turn gas flow on (petcock) and open up draining screw at bottom of carb. There will be a continuous flow for like 4 seconds (float bowl full of recent gas) but wait to be sure it continues for multiple seconds later. -put something under the hole to catch the gas so you won't be wasting- if all that check out then you might need to look at your jets. Did you replace them in the rebuild?
