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Larry B.
Larry B., Marine Mechanic
Category: Boat
Satisfied Customers: 574
Experience:  Mercury/Mercruiser Certified/Verado/DTS / Yamaha Two and Fourstroke Certified 27 years experience
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I have a 2001 Mercury 125 hp EXLPTO outboard. It has a rough

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I have a 2001 Mercury 125 hp EXLPTO outboard. It has a rough idle at slow speed below 5 mph. I understand that it cuts out some cylinders at slow speeds, but do not know which ones. I pulled individual plug wires while at idle: when plug wire 1, 2, or 3 was pulled I saw no real difference in the idle (still rough), but when I pulled plug wire 4 the engine idle smoothed out?

Can you tell me which cylinders it runs on at slow speeds? or do they change?
If cylinder 4 is one of the 2 used at idle, why would the idle smooth out when disconnected?

Any help would be greatly appreciated,
That motor idles on the top two cylinders, If you look you will see that the bottom two do not even have idle mixture screws, just enough fuel gets by to lube the bottom two cylinders at idle. The bottom two cylinders get extra fuel upon acceleration from a accelerator pump, this pump has fuel pump pressure on it when the motor is running, when the throttle is raised, the plunger is pushed down against a diaphram, this causes greater than fuel pump pressure to be exerted on two check valves mounted on the starboard side in the intake ports of number 3 and four. The fuel squirts into the intake port to give the motor enough fuel to accelerate.

I have seen this problem before, you have a check valve that is leaking,remember it has fuel pump pressure on it and it takes more than that to open the valve when it is working correctly so fuel is being injected into that cylinder.It will cause a rough idle and a miss or spit sometimes.

Luckily it is number 4 you can get to it without removing the plate the electrical components are mounted on. You should be able to see it just under the plate, a small brass fitting with a hose running to it, take the hose off and remove the checkvalve, it can be taken apart, just pay attention to how it comes apart, there is a spring and a needle valve like you would find in a carb inside it. It may have a piece of debris in it or it is stuck open.
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