3&4 not firing

[j-no]

Hey everyone, I'm new to this forum and fixing up bikes.  I just picked up a 76 CB750F SS, that needed a little TLC.  She wouldn't fire up when i bought her, but the carbs were really gummed up and the fuel was contaminated.  I purged the tank, changed the oil and cleaned out the carbs so far.  Now she will fire up, but has a rough idle and it seems as though 1 and 2 fire like clockwork, but 3 and 4 aren't firing.  The timing and points seem to be properly set, but it seems like an odd combination of cylinders to be missing.  The only thing i can find that connect those two cylinders is the fuel line.  Is it possible that i missed something in line when i was flushing the system, or does this sound like a more serious mechanical issue?  Thanks

[CB750Winger]

   Take the spark plugs out and hold on the head to make sure you have fire. Could be fuel flow to those two carbs but check spark first. One ignition coil fires 1-4 and the other fires 2-3. Could be wires switched on those two but unlikely #3 would reach.

[Re-run]

Always replace old plugs. If you do not know the bikes history, just do it. D8EA is the plug you need and they are only about $2 each. Less than a set of good points.