Backfiring usually means it's too lean. If the carbs have been dry for a long time, it's possible the leather cups of the accelerator pump have warped or split. Wearing safety glasses and using a good flashlight, look down the primary side of the carb and pull the throttle open. There should be a good solid squirt from the accelerator pump nozzle. If it's spotty or puny, you've likely found your problem and changing the pump rod/cup isn't a big deal. You'll probably need to buy rebuild kits to get the parts, but ask around and perhaps someone has a spare- or two. Good luck
