hi . neding help 503 rotax with mikuni 34 carbs only firing on mag side cyl . clutch side plug is wet and there is lots of spark . i have swapped the plugs and it still runs on mag side. i can get it to fire a little on the clutch side when i use the primer and pull the plug wire on the mag side but it wont run . there is 120 psi on both cyl and this thing is strarting to make me drink . i know the carbs need setting but i should be able to get it running on both cyl. there dosent seem to be any vacume or not very much on the clutch side. when i put my hand over the carb i cant feel very much suction. open to suggestions.
I believe your problem is in your pto side carb. First I would check the slide adjustment using a mirror check they are set the same. Check the idle adjusters are set the same. If this doesnt help I would pull carb apart checking pilot jet, main, all air circuits, needle and seat, float adjustment etc. I usually pull it apart and using carb cleaner with the extension, spray throughout to see that nothing is plugged. Also make sure fuel pump is supplying fuel to pto side carb. I have seen a kinked fuel line do the same thing.
Look in the exhaust port as well. I had a machine with similar issue and it turned out the previous owner had pulled the exhaust manifold off and when he put it back on, one of the gaskets was covering half the port.