1990 115HP 2+2 high idle

RWD9146

Recruit
Joined
Aug 12, 2012
Messages
2
Hi all, trying to help a friend out with a 1990 115 Merc 2+2 motor with a high idle. Here is the problem as described to me.
The idle runs much too fast, around 1200-1500rpms, making it difficult to shift. However, only on a couple of occasions, it settled down to around 500rpms, and still idled smoothly. This is random, and I see no pattern to it. He found an adjustment in the linkage which he turned as far as he could which made a tiny improvement.

Did some research for him and found out that this a 2+2 motor. Motor has good compression and runs like a champ up the rev range. No mid RPM stumble when the accelerator pump kicks in. Was wondering if the accelerator pump is giving cylinders 3+4 too much gas during idle? Read that's usually these pumps don't give enough gas, resulting in low idle, not the other way around? Other options found for high idle were, possible air, leak, causing the high RPM, or should I just see if I can adjust the fast idle by turning in the two carbs with the low speed jets?
Am I in the right ballpark ? Any suggestions from the Merc gurus gladfully accepted.
Thanks,
Bob D
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
Twiddling with an adjustment is not a good way to solve any problem. Using a decent service manual, first perform a proper link & sync which gets carbs and ignition timing linkages in sync. Don't ask us how to do that because it is a lengthy procedure. And don't start in the middle of that procedure unless you want to do it over again. Then follow that with proper throttle cable adjustment. That alone will probably fix the issue.
 

RWD9146

Recruit
Joined
Aug 12, 2012
Messages
2
Thanks for the information. I just passed it along to the owner and he did inform me that [FONT=&quot]when he accelerates quickly there is a delay or hesitancy for a second and then the engine surges. So its good to know that is all link & sync related, and not the accelerator pump.
Bob D
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