Troubleshoot Mercury 90 bogs over 4200 rpm

danport

Cadet
Joined
Jul 19, 2011
Messages
6
2009 Mercury 90 Hp 2 stroke ELPTO
Will not get above 4200 RPM and not getting expected high end speed/power, and has a rough idle.

I did the "rag test", covering each carburetor in turn. The engine dies when I cover the inlet of the top and middle carburetor, but continues to run when I cover the lower carburetor, suggesting a problem with the lower cylinder.

Now for the weird part. Trying to check for spark, I used an in-line spark tester. I thought I might see spark on the top and middle cylinders and not the problem lower cylinder, but I can not see spark on any of the 3 spark plug wires. I thought the outdoor light was too bright but I tried it on my lawnmower and can see the spark easily in the same light.

It must be getting some spark as it starts and runs, although poorly.

The plugs are NGK BUZHW-2 as recommended by Mercury but now discontinued. Specs say they have a 35 Ohm resistor, but I am getting an open circuit on all 3 plugs when I check for continuity / resistance.
 

danport

Cadet
Joined
Jul 19, 2011
Messages
6
2009 Mercury 90 Hp 2 stroke ELPTO
Will not get above 4200 RPM and not getting expected high end speed/power, and has a rough idle.

I did the "rag test", covering each carburetor in turn. The engine dies when I cover the inlet of the top and middle carburetor, but continues to run when I cover the lower carburetor, suggesting a problem with the lower cylinder.

Now for the weird part. Trying to check for spark, I used an in-line spark tester. I thought I might see spark on the top and middle cylinders and not the problem lower cylinder, but I can not see spark on any of the 3 spark plug wires. I thought the outdoor light was too bright but I tried it on my lawnmower and can see the spark easily in the same light.

It must be getting some spark as it starts and runs, although poorly.

The plugs are NGK BUZHW-2 as recommended by Mercury but now discontinued. Specs say they have a 35 Ohm resistor, but I am getting an open circuit on all 3 plugs when I check for continuity / resistance.
Compression test shows 120, 118, 120
 

wn6ngp

Petty Officer 1st Class
Joined
Aug 12, 2012
Messages
211
Sounds more like a fuel delivery problem. If you are doing that rag test not on the lake just earmuffs you can get lots of unusual things since you don't the have back pressure on the exhaust. Can you have a buddy squeeze the bulb while you are on the water at your attempted speed.?
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
38,026
Do not run it anymore.-----Clean bottom carburetor first.----Check fuel pump diaphragm.
 

danport

Cadet
Joined
Jul 19, 2011
Messages
6
Thank you. I cleaned the carburetor, it looked perfect with no crud or blocked jet. I will have a buddy pump the primer bulb when at full throttle. The rag test was done on muffs. I will look in to checking the fuel pump.
 
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