Two dry-gas V-twin engines operate on propane or natural gas to reduce emissions, increase engine durability and improve fuel economy. They have a two-barrel mixer for gaseous carburetion in place of a traditional carburetor. Both engines have an oil-cooler adapter on the oil-filter base and oil cooler to optimize oil temperatures. The GX620K1 develops 14.5 horsepower on natural gas and 17 horsepower on propane. The GX670 develops 16.3 horsepower on natural gas and 19.4 horsepower on propane.