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1999 Mercury Force 120 - Loss of Top end speed

Posted By: HappyNest

1999 Mercury Force 120 - Loss of Top end speed - 06/11/22 09:09 PM

Hi everyone, I hope your all having a nice weekend.

I have a 1999 Mercury Force 120 that has dropped from a top end speed about 45 MPH to about 40 MPH. The prop is in good shape and the boat hull is clean. The Motor runs and idles good but stops at about 40 MPH and 3200 RPM. I have gone through the fuel system and it is clean including taking samples of the fuel hose. I replaced the reeds, Compression is about 108 on all cylinders. I found a visually weak spark on #2 cylinder so I swapped modules around but it remained weak on #2 cylinder.

I took ohm readings on the Stator Green/White to White/Green is 724k Ohms. I have ordered a DVA to check the output voltage.

I checked the trigger Black/White to
C1 - Purple 796k ohms,
C2 - White 4.15m ohms,
C3 - Brown 850k ohms and
C4 - Blue 628k ohms.

I found a paper at www.outboardignitions.com that indicates the Stator should be 500-700 ohms and 180v. The trigger is indicated that it should be 800-1100 ohms.

Any thoughts on things to try or what might be wrong before I go purchase some parts?
Posted By: AggieDave

Re: 1999 Mercury Force 120 - Loss of Top end speed - 06/15/22 01:13 PM

Fuel injected or no?
Posted By: boatman025

Re: 1999 Mercury Force 120 - Loss of Top end speed - 06/15/22 05:09 PM

no injectors on that motor possible stator
Posted By: AggieDave

Re: 1999 Mercury Force 120 - Loss of Top end speed - 06/16/22 02:04 PM

High speed carb ports clogged?
Posted By: HappyNest

Re: 1999 Mercury Force 120 - Loss of Top end speed - 06/18/22 08:23 PM

Sorry I have taken so long to get back with you. Thanks for the help.​​​​​ I cleaned the carbs real good but that was a good place to check. The procedure helped me identify my errors in assumption.

Update: having a procedure and specs is the ticket.

Stator 725 ohms and 192 volts so it seems to be good,

Trigger low side all 4 are open and and low side output is 1.9-3 volts on each one this is well above the 0.5 volt minimum

back to the CDI modules high tension side. One is a newer CDI and it 2275 ohm so it is good
Three are OEM, two out of three 1100 ohms
one of the OEM reads open sometime and other times it fluctuates 25 - 200 ohms

We are ordering a new module from CDI.
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