1996 Johnson/Evinrude Outboards 25, 35 3-Cylinder Service Manual, Page 60Get this manual
Description of Fuel System
1The two fuel pumps are of the diaphragm-displacement type and are operated by changes in crankcase pressureAlternately, suction and pressure in the crankcase are transmitted to the pump diaphragmsThe pumps are designed to operate togetherOne pump draws fuel from the fuel tankThe other pumps the fuel through the oil mixing unit and on to the three carburetors
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fuel pumps are different and react differently to crankcase suction and pressure pulsesThey must not be interchanged as inadequate fuel and oil will reach the engine causing poor performance and possible powerhead failure