I have a 1987 Chevy Pickup with a crate engine. I start it and it runs for about 15 -20 seconds and then shuts off. I have replace the air intake hoses, fuel lines, fuel filter. When it was turned on then started to idle low i dumped gas into it and it shut off immediately. Could it be getting too much gas? Would that mean the carb is clogged possibly?

Did you try adjusting the choke maybe getting to much air or not enough at start up.

What size motor? Is it a TBI set-up?

Its a 350 5.7L - not fuel injected. Have not tried the choke adjustment but can. We have had it running for up to 2-3 minutes then hiccups and then chokes and shuts off.

Check to see if the carburetor is flooding out. Run while you can and when she dies look down the intake throat to notice any fuel, wet spots, and smoke down at the throttle plates.