90 Merc with intermitent low high rpm

[Newt255]

HI<br /> I've got a Merc 90 s/n 968693 that will sometimes rev to 4000 and other times only go to 3000 sometimes within 15 minutes of each other. It's got new plugs, fuel, filter, filter screen and lines. Idle seems to be a bit rough at times then its OK and smoothes out, no backfire, seems to pull up smooth but sometimes won't go beyond 3000 no matter what I do with the throttle, next time gets up and hits 4000.<br />Thanks for any help

 

[Bob H.]

Re: 90 Merc with intermitent low high rpm

Check tank vent and try adding some carb cleaner to the fuel.

 

[Laotzu]

Re: 90 Merc with intermitent low high rpm

Sounds like fuel supply. Has the boat sat for any length of time. Maybe dirty carb(s).<br /><br />Jerry

 

[Newt255]

Re: 90 Merc with intermitent low high rpm

Thanks I just got back from the lake, tank vent is OK tried a little cleaner, all seemed fine until I brought the throttle down a little rapid and took some water splash around the cowling, now it developed a miss as well as low RPM. the RPM problen wasn't there before that and I thought maybe it was just a little crud somewhere. So I took cowling off and tried to find spark jumping somewhere but couldn't, guess its time to go grab the book and go through the ignition and electrical now. I don't know if the two are related or not because the miss wasn't there before at least not like it is now. I've got an idea I've missed somthing in the spark end of it. Thanks again, I'll try to let you know

 

[Clams Canino]

Re: 90 Merc with intermitent low high rpm

If it's in inline 6 , serch on the "timing and synch" post I did on this forum. <br /><br />-W