Granddad and myself recently rebuilt the engine, now that it's all put back together we can't get it to start. 1 and 3 plugs are firing, 2 and 4 are not. It's not the coils or the plugs, if we move them to 1 or 3 they fire just fine. Timing is spot on, it's been double checked. We have no idea where to go from here. Any ideas? Any help is greatly appreciated, thanks in advance.