Mixture screw adjustment and explanation on 2003 mercury 125 2+2

[dog22]

Hi everyone I have a few questions regarding the mixture screws on a mercury 125 2+2 the current Setting that I have in the motor right now is 1.5 turns out on both carburetors the engine seems to run pretty well for most parts in all bands of power and without bogging down during acceleration on takeoff but I get the occasional misfire or cough when is been trolling for an hour or so seems to me that is loading up with fuel when trolling for a long time the question that I have is do these screws control the air or fuel that enters the motor

 

[silverbul]

do these screws control the air or fuel that enters the motor, yes. look up your motor to get the factory spec starting point. see how far off you are or not. i would say mess with it, maybe a 1/4 turn in and see. i was having the same problem and found out i got the wrong factory starting spec, readjusted and now my outboard runs great. i was at 2 turns out, ran okay, now i am at 1 1/4 and runs like a bat out of, well you know.

 

[Dave1027]

The mixture screws are for fuel idle mixture and are only present on the top two carbs. 1.5 turns is the starting point but should be tuned individually to provide a smooth idle and also a smooth transition when you throttle up.

The bottom two carbs have a fixed idle mixture that only provides enough fuel for lubrication but not enough for the bottom cylinders to fire. There is a throttle linkage actuated accelerator pump that injects fuel into the bottom cylinders to get them going when you throttle up.

A cough could actually be a sign of a lean condition. It might also be too much fuel getting to the bottom cylinders causing them to attempt to fire when they shouldn't. Read the plugs. Have any black ones?

 

[dog22]

The mixture screws are for fuel idle mixture and are only present on the top two carbs. 1.5 turns is the starting point but should be tuned individually to provide a smooth idle and also a smooth transition when you throttle up.

The bottom two carbs have a fixed idle mixture that only provides enough fuel for lubrication but not enough for the bottom cylinders to fire. There is a throttle linkage actuated accelerator pump that injects fuel into the bottom cylinders to get them going when you throttle up.

A cough could actually be a sign of a lean condition. It might also be too much fuel getting to the bottom cylinders causing them to attempt to fire when they shouldn't. Read the plugs. Have any black ones?

Thanks for getting back to me this makes sense to me now I understand that those mixture screws only control the fuel that goes into the motor while on idle I think that is what is happening the engine is loading up with fuel when running on idle for an hour or so and 1 of those bottom cylinders is traying to fire occasionally the plugs look black in appearance the question I have is that turning the screw in will lean the mixture and turning the screw out will richen the mixture please correct me if I m wrong

 

[dog22]

do these screws control the air or fuel that enters the motor, yes. look up your motor to get the factory spec starting point. see how far off you are or not. i would say mess with it, maybe a 1/4 turn in and see. i was having the same problem and found out i got the wrong factory starting spec, readjusted and now my outboard runs great. i was at 2 turns out, ran okay, now i am at 1 1/4 and runs like a bat out of, well you know.

That is what I was thinking turning those screws 1/4 turn in on both carburetors and give that a try

 

[Dave1027]

The procedure is to adjust both idle mixture screws for best idle. Then try throttle up acceleration tests in the water (not on a hose). If there is any hesitation, turn both screws counterclockwise 1/4 turn. Then repeat acceleration test.

 

[dog22]

The procedure is to adjust both idle mixture screws for best idle. Then try throttle up acceleration tests in the water (not on a hose). If there is any hesitation, turn both screws counterclockwise 1/4 turn. Then repeat acceleration test.

Thanks for the replay that is what I m going to do when I get the boat in the water again the mixture is set pretty close right now the acceleration from stand still is very good with no hesitation and the motor is running good on idle for the most part what I m thinking is that the motor is a tad on the rich side I will try to turn in both screws 1/8 and see if the problem goes away or gets worst because right now as the motor has been trolling for an hour or so it seems like is loading up with fuel and 1 of those bottom cylinders is randomly firing due to to much fuel or it could be the other way around it could be a random lean cough due to slightly lean condition I can't tell the difference