93 merc 115 outboard running rough at idle???

I have done some basic test to this motor compression is 125-130psi but at 1/4 throttle or less under load it is missing. Just had my muffs on it and running it i can unplug the bottom 2 cylinder plug wires and hear it has good spark but doesnt affect the way the motor is running... Had the carbs apart and cleaned them and didnt seething wrong in side of them other then the 2 top carbs had idle mixture screws in carbs and the bottom 2 just had plugs in them. Just bought boat so not sure how long problem has been going on but has been getting harder to start up the last couple of times out on the lake. doesnt want to idle unles you give it a little gas or hit the choke putton on igntion. Anybody have any ideas on where a guy should start?? Been a mechanic for about 15yrs on cars but these outboard are a little differant but can work my way around them if i am pointed in the right direction. Thanks in adive to any advice

Re: 93 merc 115 outboard running rough at idle???

Your 4 cyl 115 is of the 2 + 2 design. The bottom two carbs do not have idle circuits and only allow enough fuel mix for lube but now enough for combustion until throttle opening and engine speed flow enough air to start pulling fuel thru the main.

Idling on only the top two cyls until the throttle opening activates an accelerator pump to squirt the two injectors and assist the lower pair to begin firing.

Carb cleanliness, fat idle mixture on the top carbs, accel pump setting and injector cleanliness are essential to this system.

Check this thread for a factory service manual.

http://forums.iboats.com/showthread.php?t=309079&p=2072211#post2072211

Re: 93 merc 115 outboard running rough at idle???

So basically your telling me that is normal when you pull the bottom two plug wires off while it is running not to changet the rpms. so then i should be looking into the top cylinders and tune those better so it will idle and start better? What rpm range will the other 2 cylinders start to work then? Have a service manual ordered but will not be in until next week. so kind of going at it blind folded now. Thought i was onto something earlier when i was pulling spark plug wires and nothing happen to bottom two cylinders but that is normal you say?

Re: 93 merc 115 outboard running rough at idle???

That is true, I have same motor but little newer. not sure exactly when it kicks in, but the throttle arm pushes down the accelorator pump and the bottom two cylinders kick in. Guessing around 1200 rpms??. Justa guess. if you have it idling you can take a screw driver and depress the button on top of the accel. pump and it will rev and you'll hear the other two kick in. Your problem seems to be different. Let me know if you have other questions. I have been troubleshooting a lot of this motor.

Re: 93 merc 115 outboard running rough at idle???

I had sometime to look at this yesterday and think i got the rough idle out of the motor with the muff on it runs fine but after about 30mins on the lake yesterday afternoon it was back to it spitting and sputtering self. I did notice at just above an idle it cleaned up and then at about 1900-2000 rpm it would start to almost miss fire it sounded like. i give it a little bit more gas and then it cleans up and runs perfect at higher rpm range. Somebody has to of had the same or simliar problem, trying to keep this boat out of the shop and fix myself if i can get any suggestions i am willing to listen???

Re: 93 merc 115 outboard running rough at idle???

You may want to try cleaning the carbs. I did that on mine and it made a big difference. Keep us updated.