Honda CBX750F

I’ve had my CBX750F (1984) for about 7 years now. I work abroad and am only home in the summer for a couple of months. Every time I leave she’s mothballed: fuel tank emptied and removed, engine run to burn off fuel in the carbs, plank under the back wheel, small amount of oil squirted into each cylinder and engine turned over without plugs in. Plugs replaced and battery on an intelligent charger.

Every time I’m back I put in fresh fuel and she runs great. Last year I replaced the spark plugs, HT leads and caps as she was struggling in the wet.

This summer she is only running on the two left cylinders. The two right ones are stone cold.
I’ve checked the plugs on those cylinders and fuel is there as well as checking for sparks.
I’ve put new plugs in again (iridium ones), cleaned out the float bowls, put in new float needles and gaskets but this hasn’t solved the problem.

Any ideas would be greatly appreciated.
Thanks