If it is firing on enough cylinders to run - it should fire on all cylinders with some tweaking.
I just ran through a start-up of a rebuilt GMC - it backfired through the carb and only fired on a couple of cylinders. I realized that I had the correct firing order arranged on the cap in the wrong direction!
Fixed the firing order for Clockwise rotation and it fired right up and ran like a dream. 20 minutes at 2000 rpm later . . . and it was ready for road test.
