What JoG said. If there's old fuel left in there (or in my case, you used food grade vinyl hose for fuel hose in an emergency then forgot about it

) it gums up and the needle can become stuck in the main jet. The engine can't make enough vacuum to lift the slide against that stickiness. So, it will fire up and idle just fine, but you crack the throttle open and the slide and needle just go nowhere, engine stops.
The vinyl hose in my case dissolved over a few weeks and left a horrible green goo in the float bowl and main jet. Took ages to make it nice.