120 Force Outboard by Mercury

by Wesley
(Houston, Texas)

So I just bought this boat from a buddy and it has a 1996 120 HP Force. The problem is to get it to idle while it is in the lake... It will idle all day long on the water hose but when you put it in the lake it will not stay idling.. I checked the reeds and the reeds are fine I took the carbs off and cleaned them they didn't look bad at all. I put in new plugs but I haven't had a chance to take it back to the lake since I have done those things, so any ideas if I get out there and it is still doing the same thing?? Any help or advice would be awesome..

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Jul 11, 2021
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Adding to prior comment
by: Mark

Just adding to my prior post. I believe your motor is being smothered by exhaust fumes. Mine would idle fine on muffs but would not idle at the lake. It took me years, a few parts and tinkering to figure out the problem. Try pulling the cowl off while you are in the lake and see if it idles or stalls. If it idles try putting the cowl back on and see how the fumes are building up inside the cowl.

The mid leg seals purpose is to stop fumes from rising up and smothering the motor. The steering linkage is on the front of the mid leg and recessed in a channel. That channel runs the entire length of the mid leg and is a direct path for exhaust fumes to rise and get into the cowling. The seal isn’t the best design and will shrink over time. If the motor idles with the cowl off try taking the mid leg back cover off and pump the front full of grease just above the seal.

Jul 07, 2021
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Motor dying at idle
by: Mark

Just went through this and finally figured out the same exact issue with my Force 120. I know what your problem is. On the front side of your mid led there is a 3-4 x3-4 " vertical channel that your shifting linkage runs down from the engine all the way to the top of the lower unit. If your mid leg seal is shrunk even a small amount the exhaust gases rise up through lowers shifting rod hole and follow the shifting rod linkage right up to the cowling. I made a new mid leg seal out of 1" rubber mat material. One thing you can do is pull the back plate off the mid leg and pump grease into the front area just ABOVE the mid leg seal and try to really get about at least 1/2 a tube of it in the front center so it blocks the shifting rod channel from allowing fumes to rise up through that channel. I think I could have just used the grease method and saved myself the hassle of making and replacing that mid leg seal. The goal is to get enough heavy grease that is fills that void around the linkage and stops the fumes from rising. Worked like a champ on mine

Jun 12, 2021
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Similar problem
by: Anonymous

My 120 HP Force is doing something similar. If I pull off the cowling it will idle all day long on the lake . If I leave the cowling on it will idle for a minute or so then shut off. I figured out the the exhaust fumes are backing up into the cowl and not exiting the prop at idle. It runs fine with any extra throttle and I can cruise without any issues. You might try to pull the cowling and see if yours does the same thing. I replaced the exhaust gasket but it still does it. I have NOT found out why it is doing this or what needs repaired.

Jun 30, 2014
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idle in water vs. on hose
by: Anonymous

From my experience and what most manufactures say, idle adjustments should be done with the boat in the water under load ( in gear ). This will make it idle a bit higher on the hose in the yard where there's no back pressure. I did this on my 1990 Johnson 40 hp and it does help with stalling at idle. Also remember the ignition timing is the major contributing factor for idle speed. Don't just crank out the low idle screw alone ( if your model has one). Adjust the spark advance linkage as well. I would even adjust the spark advance linkage before I turn the idle screw at all.

Mar 16, 2014
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120 horse force won't idle in the water
by: Bruce

I had about the same problem with the 115 Evinrude. It would start fine and run all day on a flush in the driveway. But put it in the water it would not even start but if it did it would run.
When I bought the boat somebody had taken it apart and put some replace a piston and when they put it back together they did not use new gaskets. Initially I looked over the motor to see if I could see if anything was wrong by giving it the old Eagle eye. I found some silicone that was smeared over a gasket I figured sormebody had someone your finger and just wiped it off later the marina said there was a vacuum leak right there where I feel that silicone off. When I replaced that gasket motor ran perfect.
sounds like you have a vacuum leak.
It is easier for the motor to start out of the water because it only has to suck air but if you put the motor in the water it will have to suck water up out of the lake and air so it won't be able to suck enough gas to start the motor.
Yes my motor did start in the water when I was having trouble I would have to nearly wear the starter out to get it going but once it got going it would run but I guess that just depends on how big the vacuum leak is.