MERCURY IDLE ISSUES AND WOT RPMS

[Stephen2902]

New to forums, having issues with my 90HP 2 stroke 3cyl. Have spark and good compression 120 in each. Starts right up no hesitation. But it sounds like I’m getting a misfire or something. It kinda shakes and sputters at idle but will never turn off or die out. Revs up in neutral 5000-6000 even though your not supposed to do that I guess I had to see. I cannot get it over 4500 rpm’s. If I trim up I may be lucky and I can get it to 4800. Dropped my prop to a 13 pitch and everything. It’s a pontoon boat but I had a 75 mercury 2 stroke on it last year and she would go a lot better. But this one just doesn’t sound like it’s opening up when it’s underway. Anything will help. Thanks. Here’s a vid of it idling. Not sure if it could be a bad stator or a bad coil on a cylinder.

 

[Thirsty Endgrain]

Have the carbs been rebuilt?

Do you have a timing light?

 

[Stephen2902]

Have the carbs been rebuilt?

Do you have a timing light?

The carbs have not been rebuilt. The reason I don’t think it would be the carbs is that in neutral her rev is very clean and and she opens up with no hesitation. And I don’t have a timing light.

 

[Thirsty Endgrain]

Was wondering if maybe the flywheel key sheared, 'cause it looks like the timing is way off, easy to check with a timing light. As far as the carbs, since it sounds like this is a new to you engine, I would personally want to rebuild them so that I knew that they were good, you're just guessing otherwise. Since it's running so rough and you say it has spark and compression, fuel (carbs) is where I'd be going next. Did you do a spark gap test, or just confirm that there is spark?

 

[ThomW]

Revving well in neutral doesn't really mean much. It takes much more fuel to "rev" under way and moving the boat. Definitely worth checking flywheel key...but either way, new to you motor I would pull the carbs and clean them up well.