Rough idle in neutral and while in gear

[bighermHK45]

1999 Mercury 115 HP 2 stroke 2+2 outboard. Engine starts well. While in idle it shakes pretty hard and vibrates the 24' pontoon it is mounted to. While in gear the rough running continues until I hit the "magic" 5 mph (GPS) mark and it runs smooth from there to WOT.

This engine is designed to operate at low speeds for trolling. I cannot possibly troll with how it is running now and trolling at 5 mph is not an option. So, I need to know where I should start looking for adjustments or repairs.

Thank you in advance for any practical experience and advice.

 

[ondarvr]

It's not designed to troll, they just drop two cylinders in an attempt to make run better at low RPMs. When everything is correct it works well, but it can be irritating if the carbs are dirty or need adjusting. People frequently complain about these motors running odd or rough before the other two cylinders kick in.

You need to start with all the normal things, spark, compression and fuel. I'd look at the plugs on the two low RPM cylinders and see if they look the same, could just be a dirty carb.

 

[bighermHK45]

Something is definitely wrong. The top three plugs were fouled from my 4th of July trip (went through 2 1/2 tanks of gas). I fired it up in the driveway and it idled rough and would surge at idle. I then inspected the spark plugs and discovered they were fouled. I replaced the spark plugs with new ones and it idled better, however the motor is running so rough the motor moves a couple of inches left to right at idle and is idling higher (around 1200 rpm in neutral). It seems like it is getting plenty of fuel, so I am assuming it is not getting enough spark to ignite the fuel mixture...