2003 Mercury 150 hp saltwater misfire.

I currently own two 2003 mercury 150hp saltwater series (carb'd) outboards. Both with about 190 hours on them. One of the engines has been giving me problems idling on the initial start-up, once its been run for awhile it behaves, but this weekend i decided to turn it up and i got on em pretty good for awhile just to see how much rpm they can turn, and it ran fine. stopped the boat, then when i went to take off again that same engine was shaking and surging like crazy! i took it easy for awhile cruising at 2000rpm then all of a sudden it picked up speed and rpm's by itself and was running fine. i took her back out and noticed everytime i hit 2800-3200rpm it does the same thing kinda like a misfire/surge and the boat gets to vibrating alittle. get past that 3200rpm mark and she runs good again, i did that several times and it still did it. boats 2 hours away so im taking any and all suggestions before i go back to work on it! thanks

Re: 2003 Mercury 150 saltwater misfire.

It definately sounds like a fuel issue.

If both motors are "sucking" from the same fuel tank, you can rule that out as the source - so up to the point where the gas lines diverge must be ok.

The crappy idle and surging can often be attributed to a bad fuel pump or cooties in the carbs. Clogged bleed lines could also cause the issues you note.

Since you have a distance to travel you might want to pick up a fuel pump "diaphram" kit to take with you, it's about 20 bucks (full kit is over 100) and even if it's not your problem it definately wouldn't hurt.

But test your carbs for flooding. Just pump up the primer bulb and look for gas leaking out in the throat.

Unlike the diaphram kit for the fuel pump I wouldn't pick up carb kits (3 required at 100 bucks a pop) until you know for sure they need a rebuild - sometimes "take a chance" swapping isn't too costly, other times it's just not worth it...

Re: 2003 Mercury 150 saltwater misfire.

Thanks Galamb, definitely a good suggestion. and i agree it does sound like a fuel issue. it just strikes me odd that it only does it at that certain rpm.

Re: 2003 Mercury 150 saltwater misfire.

That's why I'm kinda leaning to the fuel pump. A small pinhole in the diaphram might react differently at slightly different crankcase pulse rates (related to the rpms) - it can cause some weird results...