hey it almost sounds like you have an air leak that is where i would start first make sure all your clamps are tight and the carbs are properly seated in the intake boot. then make sure your carb caps are on tight that is where i would start and i don't think you are that far off on jetting you should probably be around a 320 or so but your best bet is to to a plug chop or check. Any other questions you can pm me if you are still having problems.