115 Merc 2+2 intermittent miss in middle rpms

[mistermoon]

I've got a 1997 Mercury 115 4-cyl 2-stroke outboard on my 27' pontoon. It is a 2+2 motor. <br /><br />The motor runs great below 2000 rpm (in 2 cyl mode) and also above 4000 rpm up to wide open throttle at ~5000 rpm. Between where the bottom two cylinders kick in at ~2200 rpm to 3800 rpm, the motor has an intermittent miss that causes rough running. This is a big problem because the miss is right in the rev range that I prefer running.<br /><br />My mechanic has been through all four carbs twice(!), checked the CD modules, checked the stator, checked the rest of the electrical system, checked the fuel, and verified that it has good compression. He's also spent hours on the phone with Mercury trying to diagnose this problem.<br /><br />The only thing he ever found was an extra part, an o-ring, lying in the bottom of the #3 carb bowl, probably by some previous mechanic. <br /><br />He's at his wits end and honestly, so am I. What could we be missing? Has anyone ever had a similar problem with thier motor? And most importantly, how did you resolve it? <br /><br />Thanks in advance!

 

[SlowBoat5]

Re: 115 Merc 2+2 intermittent miss in middle rpms

See post by "timmytes" concerning a 125HP problem.<br />last post on 7/15. I would do flips if this problem could be resolved.

 

[mistermoon]

Re: 115 Merc 2+2 intermittent miss in middle rpms

Update: The Mercury Regional factory service rep took a look at my boat. He thinks the problem is that the mixture is too rich on one of the carbs. He has recommended rejetting that carb two-thousandnths smaller. We'll see come Tuesday...

 

[quantumleap]

Re: 115 Merc 2+2 intermittent miss in middle rpms

Let us know what they find.

 

[mistermoon]

Re: 115 Merc 2+2 intermittent miss in middle rpms

Well, I think we've finally got it. It boiled down to three problems:<br /><br />1) Previously mentioned junk in the carb bowl. <br /><br />2) On Mercury's advice, replaced the 12 psi crankcase poppet valve springs with 28 psi springs.<br /><br />3) Smaller carb jets to correct the over-rich condition that was ultimately causing the miss. <br /><br />As for it being fixed, well this weekend will ultimately tell the story. But if it is, I've got the best Merc mechanic around.

 

[SlowBoat5]

Re: 115 Merc 2+2 intermittent miss in middle rpms

mistermoon, did you get a chance to run your boat? If so, is the problem resovled?

 

[mistermoon]

Re: 115 Merc 2+2 intermittent miss in middle rpms

Yep, she runs great. I can't say what a pleasure it is to take the boat out and have the engine run so smoothly. And yes, I've got a pretty good mechanic, "Satisfied Customer" is his middle name.