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It may be that when your Yamaha outboard motor begins to sputter and cough, particularly at high speeds, you think something's "obviously" wrong with the fuel pump. You can test that theory without taking the fuel pump off your motor and without returning to shore. If you think you have a problem with your outboard motor's fuel pump, a quick test completed while you are boating can answer all but one question.
Squeeze the fuel primer bulb while the motor is running. If the motor runs rough when operating a high speeds, and if the motor smooths out when you squeeze the bulb, replace the fuel filters, clean out the fuel lines and repeat the test. If the results are the same, replace the fuel pump; if not, continue your boating activity.
Operate your motor across the full range of speeds without shutting it down. If the motor operates properly for several hours at high speeds and low speeds without faltering, the fuel lines or the fuel filters were the problem. If, however, the motor begins to stumble, particularly at high speeds, take note of the tachometer and any fluctuations in the motor speed.
Squeeze the primer bulb several times quickly, force-feeding the motor. If the problem disappears as you pump, but returns after you stop, replace the pump. If the problem doesn't disappear, or if the motor stalls, the problem is somewhere other than in the fuel pump.
- If the problem isn't in the fuel pump, that leaves the question unanswered, but one possible answer lies at the fuel tank; check the fuel tank vent first. A clogged vent will cause fuel shortages to your carburetor at high speeds, as fuel is pulled from an unvented tank, a vacuum develops that your pump can't overcome. It's a condition called vacuum lock.
- "Yamaha/Mercury/Mariner Outboard Repair Manual 2 - 250 HP 1995-2004"; Seloc Marine; 2007