New Engine Won't Start

If you have spark but it's weak, you have unwanted resistance or a poor ground. You need to find the source of the voltage drop or high resistance in the wiring. Take your voltmeter and place it at various points along the suspected wire until you reach the point of the high voltage drop; i.e., should be 12V to the point that 12v needs to get to. If you have less voltage to that point, there's your problem and you need to repair or replace the wire or connector. Try measuring on the ground side too. If you have high voltage going through somewhere that should be 0 on the ground side, then you have a bad ground.

If you have a way to measure it, perhaps look up the spec for how much current is being drawn by the starter, should be around 150+ amps. If it's drawing less, you have unwanted resistance in the wiring.

Might try whacking the the starter too too see if that helps, might be a poor ground. That's the old school way.