Merc 90 rough just above idle

[Aviator04]

Hey guys, my ‘06 90 elpto 2stroke idles well, but gets rough with vibration just over idle rpms and then clears up over 2k. At 3k+rpms this thing runs superb(as well as my 4 stroke). Any ideas what could be causing this low speed roughness? Plugs, fuel pump new & carbs cleaned. My carb adjustment screws are 1.5 turns out. My elevation is 950’. Should I attempt leaning out the carbs any?

 

[Texasmark]

I had an '02 triple 90 and have an '02 115 quad. Service manual says that to properly set the low speed adjustment, get on the water, engine warmed up, and adjust for best hole shot. Since you have 3 carbs, this will take some time and you are starting from the right opening. Sounds to me like you need to open up a bit...running a bit lean. One at a time, tweak for best response then go to the next. When finished repeat the process. If that doesn't work go to plan B. On plugs, there are 2 listed in the manual, a surface gap for lots of high speed running (designed to stay cool) and a conventional for slow running mostly. For less than the cost of a Hamilton, you could try a different plug in your plan B.

 

[Aviator04]

I will try that. Don’t these adjustment screws need to be relatively close to the same position? For instance if one is set 2 turns out, the other carbs should be set similarly? Did you have any vibrations at all throughout the rpm range on your 90? Once mine is at 3k + rpms it’s rock solid. I’m impressed how good these run up at speed! How does your 115 compare to the 90? Is it night & day?

 

[Faztbullet]

This on a aluminum boat???

 

[Texasmark]

I will try that. Don’t these adjustment screws need to be relatively close to the same position? For instance if one is set 2 turns out, the other carbs should be set similarly? Did you have any vibrations at all throughout the rpm range on your 90? Once mine is at 3k + rpms it’s rock solid. I’m impressed how good these run up at speed! How does your 115 compare to the 90? Is it night & day?

The boat was a 17 ½' tin boat with a stepped hull....may see if I can post a picture of it after I finish this posting......The 90 with me alone (my usual configuration) was GPS 48-49...my Avatar is a picture I snapped with my flip phone one day. The boat was BIA certified for 130 hp......I can't imagine having that kind of hp on that boat....the 90 was plenty. The 115 is a lot more engine and obviously at 25 more hp, roughly 20-25% more power you would get better more Whoopie.

The current 115 is on a 2002 Crestliner 1750 Fish Hawk (you can pull up the catalog on the www if so interested) and with an early edition 20P Laser II prop on a 55* day pushes that boat at 46-47.

I tried to upload a picture and it exceeded the 2 MB limit on posting so it wouldn't take. Since I have nothing to do today, I will see if I can reduce the pixel count somehow and get it posted.

 

[Faztbullet]

Avaitor..is this on a aluminum boat??

 

[Aviator04]

Faztbullet: no, it’s on an 18’ fiberglass bowrider..

 

[Texasmark]

Faztbullet: no, it’s on an 18’ fiberglass bowrider..

Faz, would you share your reason for your question as others that do have aluminum boats (as I do) would appreciate a heads up on a potential problem.....thanks.

 

[QBhoy]

Hi. Is this happening when in gear and travelling ? If so, it could be the prop condition

 

[Aviator04]

Qboy: that’s what I was originally thinking too. But The prop looks fine. I’m going to try adjusting the low speed screws like mark said and see what happens. Just waiting for a nice day which are few & far between here. I might also invest in one of those carb tuners/ synchronizers