Thread: Mercury 75 hp 2+1 running rough

I bought this boat about four months ago and it had bad gas in it. Since that time I've run 4 tanks of gas and a can of seafoam through it. It ran 100% better after that, but still idles rough. I just had the carbs rebuilt and I can't tell the difference in how it runs. The mechanic says it's a 2+1 engine meaning at less than 1800 rpms it runs on 2 cylinders and the third one kicks in at 1800 rpms. He says the rough idle is normal and he checked everything, timing settings etc. It's a 2002 Mercury 75 hp. Has anyone had this experience and if so is this normal?

I have a 2003 Mercury 75hp I have never noticed a rough idle before.

Is it 2 or 4 stroke?

I have 25hp 4 stroke that idles rough too.

It's a two stroke oil injected.

I've got a 2005 two-stroke that idles fine, maybe you have a clogged filter somewhere!

Idles and runs on all 3 all the time.

Normally its carbs that cause the poor idle on those.
If not that, timing or too low of idle speed.

TMD