94 Mariner intermittingly bogs down and speeds; choke helps

[lclarke]

My 94 Mariner 75 HP will run fine, then bog down, then speed up, back and forth, or sometimes cut off. It's intermittent. Sometimes it runs fine. Doesn't seem to be a problem when it first starts, but I'm usually running at slower speeds then. Giving it more throttle may make it cut off or speed up. Slow down and run at slower speeds and it usually runs OK. I can choke it, and it will pick up. Sometimes pumping the bulb helps. And it may run fine later when I've done nothing different. It's worse at higher speeds. I've rebuild the fuel pump, replace the fuel line and bulb, installed an inline fuel filter. Looks like the fuel is flowing OK, looking at the fuel in the clear filter. Anybody else seen this?

 

[hkeiner]

Re: 94 Mariner intermittingly bogs down and speeds; choke helps

If the choke and squeezing the bulb really does help (and is not coincidental to the random bogs) then you apparently have a fuel problem and not an ignition problem. That narrows down things considerably but there are still a lot of possibilities. If it were me, I would follow a systematic troubleshooting approach to find the problem rather than replacing random parts on a hunch (educated guess) as the latter approach has often been ineffective and frustrating to me.

The first thing I would do is check the vacuum in the fuel line between the fuel tank and the fuel pump when the motor bogs to make sure that the vacuum level does not exceed specs during the "bog". The following guide explains how to do this.

http://www.marinepartsman.com/Mercur...2001/EN_17.PDF


If the vacuum is OK, then the next thing I would check is the pressure in the fuel line between the fuel pump and the carbs when the motor bogs to make sure that the pressure does not fall below the specs during "bog." Your shop manual will provide the specs for this test. The connection method is similar to the vacuum test and you use a pressure gauge.

If both of these tests show no variance from spec during bog, then perhaps your carbs are the problem or you really do have an ignition related problem caused by heat related failure of component or loose/corroded wiring. Intermittant and random failures are the hardest to troubleshoot.

These are my suggestions. Others on the forum may have additional or better suggestions.