Changing jets rarely increases economy more than slightly and often ruins performance. Make a thorough check on the condition of the motor with a compression gauge and make sure valves are in good condition. In a vehicle, check for dragging brakes or extra friction. Do not merely take a customer’s word on mileage; run a gas test with a scientific mileage tester. Check float lever in carburetor bowl and make sure the fuel pump pressure is not excessive. Then, if there is still cause for complaint, the various points as outlined above should be carefully checked over.