Re: 1979 Evinrude 25 HP Fuel Pickup Problems.
welcome to iboats!
The fuel line has a primer bulb, which is just a chamber with two one-way valves. Squeezing it pushes the contents (air, on the first squeeze) through the fuel pump into the carbs, releasing the bulb causes it to draw fuel in from the tank. When the carb bowls are full, float valves close off and prevent more fuel from entering. At that point the line is pressurized; the bulb should be firm, and it should pretty much stay that way all day. When the engine's running, fuel flows freely through the bulb.
Two quick-disconnect fittings are at either end of the fuel line, one connected to a fitting on the tank, at the other end of which is the pick-up tube, and the other is connected to a fitting on the engine, which leads to the fuel pump.
The fuel pump has I guess two chambers separated by a check valve, and a rubber diaphram. a vacuum line from the crankcase flexes the diaphram creating suction that draws fuel in through the inlet and through the check valve, and a spring pushes it out toward the carbs.
As fuel is drawn from the tank, it is replaced by air entering through the vent. If the vent is closed or obstructed, the fuel pump will be drawing against a vacuum, which it's not designed to do.
If you can't fill your carbs by squeezing the primer bulb, first make sure there is gasoline in the tank (just kiddin). First try holding the bulb vertical while you squeeze it - some (not all) of those bulbs have ball-type valves that rely on gravity to close them off. Make sure the arrow on the bulb is pointing toward the engine. Listen closely at the tank fitting while pumping the bulb, for the sound of sucking air. Those fittings do wear out after awhile and aren't very expensive. A new fuel line/bulb & fittings every few years aren't a bad idea.
If the bowls are full and the line is holding pressure when you start the engine, and the bowls empty out while the engine's running, first check for leaks, either air leaking in or fuel leaking out.
Try squeezing the bulb just while the engine's running. If it won't open back up when you relax, then you have an obstruction between the pickup tube in the tank and the primer bulb. Or else a faulty bulb. Or a blocked vent.
If you can keep the engine running by constantly squeezing the bulb, then if you can't find a leak, most likely you have a faulty fuel pump. You can get a repair kit from iboats for about $12, piece of cake.
