Paxtoned 416 won t fire with lid on when engine is cool

Aftermarket hi flo filter but the rest is norms stuff. I have the floats that came with the kit. But th carb already has the newer style uncrushables in it.
As far as the way i start it i have tried to start no pumps. 1 pump. A couple. It has almost popped on a couple occasions but never saved it. I just got the pump in today and got it ready to fire but it wouldn t start with the box sealed. I didn t wanna pull the lid cause i had gas all over my hands and was running out of time for the day. I m gonna try tomorrow and see how it acts with this new pump and reg

Thanks to everyone with your suggestions. I m hoping its something stupid i have overlooked but i want to get this figured out before too long. I just got it back from the guy who built the engine and he wanted to address the pump first so i am. Just figured i d throw this out there before i dive head first into it