Trouble with motor loosing it's prime??

[walleyemaniac]

I have a '87 90hp Yamaha 2-stroke. While idling my motor started loosing it's prime. Replaced the primer bulb, connector on the end of fuel line to the motor, and new fuel filter. Gas line looks good, lines on the motor look good, and everything seems tight. Only thing I can't get a look at is the gas line from the side of the boat down to the tank. Boat is an '87 Crestliner with a 20 gal inboard tank. Can't smell gas anywhere in the boat so guessing everything is alright that way.

Have a question about the fuel pump on the motor. How do you test these to see if they're bad? Could it be this? The pump did get replaced (by mistake) about 4 years ago. So it's not the original pump on the motor. The motor runs fine at WOT or any operating speed above ilde. Just will die when idling. Idle is smooth and runs at idle fine till it looses it's prime. If I can't get it going I go back and prime the bulb and it will be ok until it runs out of gas again.

Any ideas????

[boatfixer]

You can use a fuel pump pressure/vacuum tester. Sometimes the diaphram gets worn or "stretched" enough that it won't pump at idle. Because it operates by using pulses from crankcase, the lower rpms may also mean weaker, or less effective pulses. A tester will easily determine e this.

[jentz]

If you have no pressure tester,Remove the fuel line from the pump to the carb. Have a empty small fuel container on site,put a fuel hose on the pump outlet and to the empty container.Start the motor (it will run off the fuel in the carb bowl) watch the fuel line and see if its pumping effencently into the empty container at idle of course.

[walleyemaniac]

Thanks guys, check it out in the morning and post back with results or further questions.