Lots of things to look at. What kind of boat is this on? Is it possible you have some wet floatation foam? If you are handling the boat the same, I would look for the extra weight somewhere.
gas milage has much more t do with the boat and the setup than it has to do with the motor.
check your boat....
is your prop slipping?
is your hull clean?
extra weight in the hull?
wet foam in hull?
weight distribution?
water conditions changing?
operation styles?
