I have a 2004 4 hp Mercury outboard on a small fishing boat that stays at the dock. When not in use, the motor is tilted up, so the lower unit is out of the water. The motor is difficult to start initially. Per the instructions, I choke it for two pulls. It will often start on the first or second pull, but then dies immediately. Per instructions, I push the choke in. It will then take anywhere from 6 to 15 pulls before it will start and continue to run. I have replaced the spark plug and set the slow and high speed carb adjustments as the owner's guide suggests. After running, it will restart easily. What should I check or replace?