My dirt bike will run but it seems like its not gettig gas?

When I take it out its will Run and rev up in neutral and stay cranked when I change gears and when I take off its starts sputtering I'm not sure what's wrong I first Checked the gas and oil but that's all fine it's a Honda crf 150-4 stroke nice bike but hate that it's doing this please help

Is the Air-Filter clean?
Has the bike sat for awhile? Old fuel? Ethanol added fuel?
Has it sat in the rain? Stored uncovered?

My guess, needs a new fuel pump or fuel lines
Might be a little clogged

Inspect the air filter
Clean or replace as necessary.