1995 Mercury 40hp 4cyl - sputters and eventually dies at idle under load

[pharris1992]

Hey all,

I need some help with my 1995 Mercury 40hp 4 cylinder. I don't think they made a lot of the 4 cylinder model as I cannot find much information on them online. I cannot seem to figure out what is causing the stumbling and dying at idle speed under load. I have tried everything I know to do, which I will list out below. Thank you in advance, I know this is a long post!

This engine starts and runs decently well. At medium throttle and up, I believe it runs normally. At idle speed under load (once warmed up properly), the engine will start to run rough, stumble, and eventually die. When that happens, it is often very difficult to restart.

Here are all the things I have done so far:

• Disassembled, cleaned, and rebuilt carburetors with rebuild kit. I could probably use some help with fine tuning here, but I did this per the Mercury service manual.
• New spark plugs
• Checked compression; 120 on 1-3, 110 on 4
• Tested spark gap. Good strong spark that jumps a ~1/2" gap
• Tested coils with multimeter
• Replaced fuel filter
• Tested stator, got inconsistent readings, so I replaced. I hindsight, I'm not sure I tested it properly or that replacement was necessary.
• Tested switch: good readings
• Tested switch box: bad readings. Replaced with cheap aftermarket unit. Readings are good on new unit.
• Fuel pump diaphragm had a small leak; replaced

I have tried adjusting the mixture screws with the boat tied to the dock and in forward at neutral speed. It is hard to do this because it dies, but I have them adjusted as well as I can. If you look in photo 2, I have the idle advance screwed in all the way; this is the best chance I have to keep it running at idle under load.

All of the above was done to try to fix the idle issue, with the exception of the switch box. The engine died one night on the water, and would never restart over the next few days. When I realized that was bad, I thought I had found my issue, but it continued to run the same as before after replacement. None of the other things I tried have made a change to the issue.

The last initial piece of info I have is the picture of the spark plugs. The top two cylinders have plugs that look like the left one. The bottom two plugs are like the one on the right, with looks of streaks all over them; not sure what that means.

Thanks for reading the long post! I am happy to take any more photos or videos, or answer any questions that would help pinpoint the issue.

 

[racerone]

They made THOUSANDS of that style 4 cylinder over the years.----I have a 1962 model in the back of my truck.---A 1954 model apart on the shelf.------Did you replace carburetor floats ?------Did you replace water pump impeller every 2 / 3 years ?-----Made sure exhaust housing is not plugged ?-----Crankshaft seals could be an issue.----Checked the reed valves for any debris holding one open ?

 

[pharris1992]

They made THOUSANDS of that style 4 cylinder over the years.----I have a 1962 model in the back of my truck.---A 1954 model apart on the shelf.------Did you replace carburetor floats ?------Did you replace water pump impeller every 2 / 3 years ?-----Made sure exhaust housing is not plugged ?-----Crankshaft seals could be an issue.----Checked the reed valves for any debris holding one open ?

I did not replace the carb floats. I did replace the impeller, but I have only had this for about a year; not sure of the total history.

How would I check the exhaust housing?

I was also reading about the reed valves, but that is not something I have ever looked at before. Do I need to pull the carbs back off to see those? How would I check for proper operation? Thank you!

 

[pharris1992]

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It looks like this is the diagram for the reed valve. Is there a way to test or inspect these before buying a new one? One thing that seems a little odd after looking at this diagram is that the spark plugs on both 3 and 4 look identical, but much different than those on 1 and 2. I would have to think it is an issue somehow affecting both cylinders, but maybe that is not correct.

 

[merc850]

Hookup a timing light to the Nr. 1 cylinder, start motor watch timing light if motor stops the light will go out then try to restart; the light should flash unless the problem is ignition. Try it with the other plugs.
If the light doesn't flash it's ignition if it does it's fuel supply, an engine running out gas dies out slowly if the spark fails it quits instantly.

 

[pharris1992]

Hookup a timing light to the Nr. 1 cylinder, start motor watch timing light if motor stops the light will go out then try to restart; the light should flash unless the problem is ignition. Try it with the other plugs.
If the light doesn't flash it's ignition if it does it's fuel supply, an engine running out gas dies out slowly if the spark fails it quits instantly.

Hey guys, I finally had some time to take a look at this. I don't have a timing light, but I have an inline spark tester that I used to perform the test you described. I went down to the boat ramp to do it, as it won't seem to die as easily just running on the hose.

All cylinders had spark until the engine quit, and start sparking again as soon as I hit the key to restart. Seems like that is not the problem. I also used my multimeter to check all the resistance readings for the coils, stator, and switch, and then a DVA to check running voltage per the manual. All checks out.

What next? I am not sure how to go about checking the reed valves at @racerone mentioned. What about timing; could that be the issue?

I still have the oil injection system operating on this. Is it possible that too much oil could be going into the engine? Not sure if that is possible or would cause the issue, but just trying to think through everything.

 

[racerone]

check flywheel key ?

 

[pharris1992]

check flywheel key ?

I had the flywheel off when I replaced the stator and all was well. Could something have happened to it since then? No change in running condition before/after stator replacement.

 

[pharris1992]

Is there a guide or video for reed valves that anyone can point me to? I have searched around here and on youtube and don't know that I have a great understanding of how they can be "checked". Do I have to tear the whole thing down just to visually inspect them?

 

[merc850]

Time to check the fuel system; sometimes I tilt the motor all the way up then pump the line bulb fuel should run out of the carbs then tilt it down and see if will start.