First, check the sight glass and see if there is fuel in bowl when you are experiencing this problem.  If you don't then perhaps your bowl is going dry, but it would be likely do to a crack or some other catastrophic failure.  If you do then, with a flashlight and your choke wide open, look down the throat while opening the throttle quickly and see if you see a squirt.  If not, the accelerator pump circuit of your carb is bad.  If the vehicle has sat for a long time it could be that it is just clogged with varnish and it might clear up with an additive like techron or Seafoam.  Hope this was helpful.