Mercury 8hp 4 stroke trouble

I have a Mercury 8hp 4 stroke that is about 6 yrs old that doesn't like to keep running. Yesterday it first  ran 15 minutes no problem. But on the way in it will run about a 1/4 of a mile and shut off. The only way to get it started is take gas line off the motor, pump the bulb till it is hard, reinstall gas line. It will start 1st pull run another 1/4 mile and shut off. With line installed I can't get the bulb hard and I can pull for ever and it won't start. Seems like it is a fuel problem,  the fuel pump maybe? Anyone else have had this problem?  Thanks!!

The vent on the tank cap will relieve the pressure in the tank if it gets over 5lbs. There is a relief valve in the gas line between to keep high pressure fuel from entering the motor. All I know is if I leave the cap slightly open to relieve the pressure and don't get the bulb hard by pumping when vertical the motor doesn't run worth a darn. Yesterday it ran great for miles and never missed a beat, will try it again this weekend. Should have bought another Yamaha instead of an Mercury. I have 2 Yamaha's on my Lake Michigan boat that are nearly 20 yrs old and they will troll for hours or run for 10 miles in or out and have never missed a beat!!

You shouldn’t have to squeeze the bulb before every start.  Something else is wrong..  CARPIO 

After reading a few posts here, reading some articles online and rereading my owners manual I think my problem was mostly operator error. I did have a problem with a bad spark plug  a couple weeks ago but I didn't realize the bulb must be in a vertical  position when squeezed so that is why I couldn't get the bulb hard. Also this motor is designed to run with a pressurized tank with up to 5lbs of pressure and if you loosen the cap which I did, it does not run. So yesterday I did a test run, pumped the bulb in the vertical position till hard, kept the tank cap tight, started the motor and it ran for over an hour. Restarted it about 10 times and never missed a beat except the 1 time I didn't get the bulb hard before starting. I guess what got me was the fact that there was pressure in the tank which is EPA mandated now and the bulb in the vertical position both unlike my other Yamaha or Johnson motors I own. Guess a guy sometimes should pay more attention to the instructions!!

When it quits is the primer bulb sucked in at all ? If not I'd focus on the fuel pump. A sucked in primer bulb would indicate a restriction between the tank and the bulb. It could also be the vent in the tank not allowing fuel to be pumped (creates a vacuum in the tank). You could try running the engine with the cap lose on the gas tank to eliminate that concern but make sure you don't spill fuel in the process.

yamatroller,check water pump flow,check thermostadt for opening up,sounds like shes overheating

I have a 2020 Mercury 25hp four stroke, new this spring. It is a Tohatsu motor with a Mercury label on the cowl. When I picked up the motor the lead mechanic walked me through the motor operation details. One thing he stressed was to hold the primer bulb pointed up in the direction of flow indicated by an arrow on it. Another item to note is the 6 gallon gas tank is non vented and will pressurize with the gas expansion from heat. There is a built in to the gas line valve that prevents the pressurized fuel from entering the engine. The gas line is also the useless (stiff as a broomstick) non permeable type line. The non permeable line and non vented tank are EPA mandated. I think those are just more things to go wrong. Maybe one of those features are present on your system and causing problems. 

Be sure you are holding the bulb vertical when pumping it up.

It's a 2014 and yes I believe it is a Tohatsu.