bad gas mileage on 1500cc intruder motorcycle?

im geting about 22 miles a gallon on a 1500cc intruder. I drive all city miles mostly in downtown riding I find this to be vary low mileage . I inflated my tires to correct psi, changed my air filter. ran carb cleaner through the bike. going to change spark plugs. I noticed it pops alot if im coasting down a hill in gear with the clutch out if I pull the clutch in it doesn't pop anymore. it has aftermarket pipes on it not sure if the previous owner changed the carbs around the bike runs great sounds good just poor gas mileage.