1986 Merc 75hp 4 cyl. will not run right

[Paul t 608]

My 86 75hp 4-cyl. ran great for 7 years. It then began to "run out of fuel" at WOT. If I slowed to idle speed for a few seconds, she'd take off and run wide open - no problem. The problem became more frequent until WOT was only achieved for 10 seconds, then she'd die. Priming the bulb made no difference. Hitting the choke did no good. I took the carbs off and had a Merc dealer rebuild them. This made no difference. I rebuilt the fuel pump including the check valves, and no difference. So I took it to a reputable Mercury dealer, and they gave up after contacting Mercury Marine. I replaced the stator and the switch box. No difference - actually it runs worse now. Does not idle and dies at WOT. I replaced the fuel lines under the hood, new fuel lines to the tanks - as well as new tanks. No difference. If you think you know of anything I could possibly do - please contact me. P.S. compression is good on all 4. New spark plugs, too. My e-mail is: ptofte608@gmail.com. Thanks....

 

[CharlieB]

Re: 1986 Merc 75hp 4 cyl. will not run right

When you replaced the fuel hoses did you replace the quick connector fittings?

O-rings in the connector can fail to seal air out yet still seal the fuel in. Pumping the primer pressurizing the fitting seals yet the fuel pump sucking will pull air in and the motor runs out of fuel.


Prove it by temporarily substituting a clear hose between the fuel pump and carbs, run the motor and watch for air bubbles passing thru the hose, proving air sucking in somewhere.

Be sure to remove the clear hose once finished testing and repairing the system as clear hose is NOT SAFE for continued use.

If your model fuel pump uses a hose to the crankcase for the pressure/vacuum pulse, that hose may have softtened and cannot transfer the full pulse pressure/vacuum required for the pump to operate at full capacity.