I just recently bought a 1990 Zephyr 550 with only 5500 miles on it and cylinders 3 and 4 don't seem to run. Does anyone know what I can test. I installed new plugs and checked the spark (it's fine). When I remove the plugs from cyclinders 1 and 2 and try to start the bike, nothing happens. If I then check the plugs in 3 and 4 they appear to be dry while plugs 1 and 2 are wet. If I do the reverse, and remove plugs 3 and 4 the bike starts on only cyclinders 1 and 2 but runs real bad and the plugs get a white powder on them. I then checked to see if the carbs were getting fuel by removing the drain screws and each carb had fuel in the float. It also appears the cylinders 3 and 4 have compression, but I have not tested this with a compresion tester. When the bike is running on cyclinders 1 and 2, there is not any valve knocking or backfiring. Any advice would be greatly appreciated.