1998 Merc 90 runs rough low rpm

[TPyles]

I just got a 1998 Mercury 90 HP on a pontoon. It didnt run last year at all and I dont think the previous owner winterized the motor. It ran very rough and wouldnt idle at low rpm, it was only running on 2 of 3 until I changed the plugs and then it wouldnt idle for beans. I took it in and had the carbs cleaned and new gaskets on them to the tune of 175.00 and its a ton better, but it still runs a little rough at low rpms. It runs perfect above 2200 rpms. It is firing on all cylinders now and runs OK, but I think it could be better from 1000 to 2000 rpms. Should I just keep running fresh gas and hope it improves or what? (I already ran a can of sea foam through it). Any help would be appreciated.

 

[Texasmark]

Re: 1998 Merc 90 runs rough low rpm

Get yourself a couple of cans of Sea Foam and decarb it. I have a 90 and had similar problems. Have been running Sea Foam in my gas since January (just after I bought the boat and noticed the problem) and it has made a marked difference in smoothness and in ease of starting.<br /><br />Additionally, I opened my low speed jets (the ones on the side of the carbs that you can see with the spring on them) 1/4 turn over the factory recommended (starter setting) 1 1/4 open from closed. Your mechanic probably set them to the factory starter setting. <br /><br />I had a tad of hesitation upon punching out of the hole which the carb tweak eliminated.<br /><br />HTH.<br /><br />Mark