Merc 115 Hesitation/Rough Idle

[Matchrocket]

When my 95 Merc 115 is cold, it will idle fine and it starts will out any issues assuming you remember to pump the fuel bulb <br />Anyway, once the motor warms up, it will idle very rough, and the idle speed drops from ~2000 RPM to ~1300 RPM. The motor shakes and acts like it is missing. Also, when its up to running temp and I shut it off for a couple of minutes when I start back up and stick it in gear, it will stall. I have to put it nuetral and rev the motor for a sec, then stick it in gear. My family ski's behind it a lot so when I am staged, in gear, waiting to go. the motor shakes really bad and again sounds like it is missing. When I hammer it to WOT, it will hesitate for a sec then go. When running 3000 - 6500 RPM, it runs just fine. I have already replaced the plugs. Sorry for the long post. Any suggestions?

 

[mellowyellow]

Re: Merc 115 Hesitation/Rough Idle

a stuck float in the bowl is one possibility<br />(one of several)have you ever had the carbs removed<br />and rebuilt? try tapping on the bowl to un-stick the float.<br />check and see if gas is running out of the carbs<br />when it happens.<br />good luck,<br />M.Y.

 

[ad4par1]

Re: Merc 115 Hesitation/Rough Idle

I agree with M.Y. That needle might be sticking.

 

[Matchrocket]

Re: Merc 115 Hesitation/Rough Idle

I have never had the carbs of. Thanks for the advise.

 

[Spidybot]

Re: Merc 115 Hesitation/Rough Idle

The 4-cyl 115's are 2+2. They run on the two upper cyls only when below 18-2000 rpm (which is also why you find idle mix screws on these only).<br /><br />Try adjusting the idle mix on those two carbs. Turn them all the way in (count the turns) and be careful not to tighten them as they are delicate. Then turn them out so that they both match the average of what they were when you started. This should be a good starting point for tuning (1/8'th turn at the time). It's common that the motors run too rich (too much fuel in the fuel/air mix) and you lean it by turning the screws in (clockwise). <br /><br />It is normal that there is a noticeable change when the motor shifts to running on all 4 cyls.<br /><br />The 2+2 provides for better long-term low rpm running (when correctly adjusted) and works better than the average 2-stroke when trolling.

 

[MIKE F]

Re: Merc 115 Hesitation/Rough Idle

You're not really running that engine, at 6500 RPM, are you? Max RPM is supposed to be 5,250 RPM. Oops just saw your other post - you DO realize 6,500 RPM isn't right.

 

[Matchrocket]

Re: Merc 115 Hesitation/Rough Idle

Yes actually I do realize that isn't right. I kinda wondered if that was a little to high, checked my manuals and crapped my pants. <br /><br />I just picked the up the boat from being winterized and I had them compair the tach in the boat with their shop tach. Mine was off by just over 1500 RPM. They could not get it adjusted right so they replaced it. MUCH MUCH better now.