Merc 90 Rough Idle

[laser51]

I have a 1999 Mercury 90 Big Foot that only has about 40 hours on it. However when i'm at a slow speed or in idle it runs real rough and smokes. When i'm at a cruising speed it runs smooth.

 

[JB]

Re: Merc 90 Rough Idle

That engine has a reputation for idling rough, Laser. My son had one and hated it.<br /><br />I don't know if the 90 is one of the engines that disables one or two cylinders at idle, but some of the bigger Mercs do.<br /><br />Good luck.

 

[phatmanmike]

Re: Merc 90 Rough Idle

i know the merc 115 4 cylinder disable cylinder 2 and 4( i think) , it kills spark to 2 of them, not 100% sure wich ones, but at slower speeds, under 1500 rpm(ish) it does this so as to save fuel during lower rpm ranges and allow you to troll at slower speeds.<br /><br />im sure there are ways around that !

 

[Clams Canino]

Re: Merc 90 Rough Idle

The 90 is a triple and idles on all three. It's the 115 / 125 that use the 2+2 scheme. And all indications are that the 2+2 cannot be modified to run on all four all the time.<br /><br />Sounds to me like it needs to go in for your basic tune up with the possible addition of carb cleaning.<br /><br />-W

 

[steam_mill]

Re: Merc 90 Rough Idle

Guys:<br /><br />I've got the 03 90 Merc. It runs a little rough up to about 1800 RPM. I have been told by other member of the board that this is normal.<br /><br />It is hardly noticeable to anyone though. <br /><br />I have read on this board the making cyl 1 richer sometime alleviates a very rough motor.<br /><br />Do a search on 90 Merc....<br /><br /><br />Joe