Mercury Optimax Models 135, 150, Direct Fuel Injection., Page 142Get this manual
FUEL PUMP
CHECKING for RESTRICTED FUEL FLOW CAUSED by ANTI-SIPHON VALVES
While anti-siphon valves may be helpful from safety stand-point, they clog with debris, they may be too small, or they may have too heavy springSummarizing, the pressure drop across these valves can, and often does, create operational problems andor power-head damage by restricting fuel to the fuel pump and VSTSome symptoms of restricted (lean) fuel flow, which could be caused by use of an anti-siphon valve, are:Loss of fuel pump pressureLoss of powerHigh speed surgingPreignitiondetonation (piston dome erosion)Outboard cuts out or hesitates upon accelerationOutboard runs roughOutboard quits and cannot be restartedOutboard will not startVapor lock Since any type of anti-siphon device must be located between the outboard fuel inlet and fuel tank outlet, simple method of checking [if such device (or bad fuel) is problem source] is to operate the outboard with separate fuel supply which is known to be good, such as remote fuel tankIf, after using separate fuel supply, it is found that the anti-siphon valve is the cause of the problem, there are solutions to the problem; either 1) remove the anti- siphon valve or 2) replace it with solenoid-operated fuel shut off valve