I have a 2015 Mercury 150 with an older 9.9 Mercury kicker added within the last two years. (When the problem started) Both motors run great on a full tank. Once the fuel gauge reads 3/4 or less the 150 runs terrible at low speeds, seems fine full throttle. Kicker is good also. Dealer that did it said it was fuel related but have run many tanks(always premium) through it and motor serviced routinely. Any ideas?