Low voltage defiantly could be the problem. If you have a 6al or better make sure you dont have a 3000 chip installed. Some of my buddies use to think that was funny to put a 3000 chip in when someone just built a new engine.
If you really want to know what the true firing order is,
Start with number one. Make sure both valves are closed.
make the dampener t.d.c. mark align up with the pointer.
then
just rotate crank with a wrench one quarter of a turn,
whatever the cylinder is that both valves are closed,
it the next cylinder that is supposed to fire....
it is probably #5,
or #3 and so on........