Has anyone run into this problem before. We have 2 units in the shop with the same problem. Both have the GM 2.2 engines on propane with distributorless ignition. The ignition systems check out fine. The LPG regulator tests good. There is some play in the throttle shafts of the carbs. Any other ideas? The units are both hard starting cold and will backfire through the carb. Once running they seem okay but will back fire when restarted. They will start right away with a little shot of starting fluid.