It's more than just the carb, I run the stock carb on mine and I'm getting 18-19(I also removed all the junk smog stuff) and there's room for more just don't have the time to play with it. Is there a lot of miles on it?, if the motor is tired and need refreshing mileage will suffer, what kind of condition is the ignition?, if that isn't up to spec that will contribute to bad mileage, how is the exhaust system? be surprised how a bad muffler can kill mileage, just to mention a few things..