My 5.7l vortec is having issues starting. It's in a 96 suburban. I just bought it and it didn't seem to do it when I bought it. I have 42 gallons of good gas in it.

When i go to crank the motor it will only start with throttle pushed down or pump it like a carb. And it will fire up in about 5 seconds or more.. it throws no codes or check engine light and runs very smooth and no hesitation. Any ideas what it might be im a little bit stumped? It doesn't matter if a prime the pump with key it still needs throttle to start hot or cold

 

Is there any black smoke that comes from the tail pipe when it finally starts?