what causes a weak spark?

'66 mustang, 289

i've verified that my timing is correct at TDC (checked the harmonic balancer, and put my finger in the #1 cylinder and cranked it. blew my finger out at DTC and everyting was okay)

i have a mallory unilite electronic dizzy that i had running on my older motor (in the same car) with all the same wiring etc. and it worked fine. i put a new motor in this car and hooked the dizzy up again, and i have a really weak spark. same coil, wires, and new plugs. plugs are gapped to .035.

i had this motor running in my other coupe before i pulled it and it ran just fine. when i swapped it into my red coupe, using all the ignition components from my old motor (which all worked fine) and put on the new one. i have a very weak spark. i tried swapping the dizzy to the old one, same thing. tried swapping coils, still weak.

any ideas?

figured it out!

it was a fluke .. i checked again and it sparked fine

but part of the problem was it wasn't getting fuel. the hose clamp between the tank line and the fuel pump was loose . . so it was just sucking air, not fuel, to prime itself.