1989 ish Mercury Force 120 ignition issues

sblanco

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Sep 3, 2020
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I am in need of help on my 1989 ish Mercury Force 120. I have ignition issues that I can not seem to properly diagnose. I will go back and give an explanation as to how I got to this point… the point of wanting to smash this thing into many little pieces…

The engine ran great for many years and then being a Force, it lost compression due to piston and ring failure. I did a complete engine rebuild, new pistons, new crank, all new bearings, all new seals and gaskets. After the rebuild, the engine ran for me but it had a dead misfire on cyl#2 and an erratic misfire on cyl#4. After going through the CDI diagnostic sheets and schematics I found that swapping the switchboxes around had no effect on the cyl’s misfire and when I tested the stator and trigger, they both tested normal… I figured it was a faulty trigger. I replaced the trigger with a used part and the misfires continued, I then changed the switchboxes with used parts. The dead miss on #2 was corrected and the erratic miss on #4 now became erratic on #3. So, I know that used parts can be defective and I know that my knowledge of this engine is limited…. I just need it to run

I’ve also done further testing on the trigger for this engine using a timing light. With plugs out and grounded and cranking to set timing I used the timing light to show the trigger point of the cyls. I marked the flywheel for each cyl and while cranking, placed the timing light pickup on #1 and it flashed on my #1 mark, #2 flashed only on #2, #3 flashed all 4 cylinders, #4 flashed only on #4. So I thought that can’t be right and changed the trigger back to my original trigger and re-tested. I got the same results. Not sure why… Maybe I’m missing something or maybe there’s more to it…

I took the boat out onto the water to test as I thought maybe I just need to run it for a while and clear out some of the old fuel. On the water the engine ran rough, misfiring and not responding to throttle as it should have. It did run good though at WOT.

I should also mention that this is a Prestolite system.

I pulled the spark plugs to try and read them and I also thought maybe there was moisture on them but wasn’t sure. It looked like tiny water droplets but otherwise the new plugs where dark brown and black on the face… not washed looking.

The method I use to determine if I have a misfire is to use my timing light and place the pickup on the plug wire while running or cranking and if the light flashes its good and if it doesn’t flash it’s a misfire. So, my understanding is if it was a fuel related misfire it would still spark and read on my timing light as a spark.

So, my questions are

1, Am I testing the trigger properly and do I still have a faulty trigger?

2, Is there an ignition conversion to a newer style system that’s more reasonably priced for parts?

3, Is there something I’m missing in my diagnoses?

4, Are there alternative switchboxes to use… cheaper ones

5, What else can I test?

Any help would be greatly appreciated!
 

Nordin

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Jun 12, 2010
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sblanco I think you trigger is good.
I think you have one or two defective CD boxes.
There is a blocking diod in the box which purpose is just to block the other box from affect the other one.
There are no test to check this other then put new unused CD boxes in and check.
You can change over to Mercury style Thunderbolt system but parts for that system are not cheap either.
 

jerryjerry05

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May 7, 2008
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Little drops of water on the plugs??? NO WAY should there be ANY water in the combustion chamber, any at all. Water on the plugs show the head gaskets probably bad or not sealing???
Had a 88/85 that started missing, did a comp test and 145 all 3 cylinders.
Then I noticed 1 tiny drop of water on the plug of the missing cyl. Yup bad gasket. Removed gasket and the compression ring had a very tiny hole next to it.
Running bad at low speed and good at high speed: that sounds like your sucking air, probably from the port cover gaskets, one tiny backfire, cough or stutter can blow the port cover gaskets.
Start the motor and spray starting fluid around the intake side, under the carbs and under the coils/packs. If that changes how the motor runs?? then it's sucking air.
 

sblanco

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Sep 3, 2020
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Thanks for the quick reply’s. A couple of thoughts from the comments… I will definitely check for intake air leaks and see if I’m sucking air… I definitely don’t want to be running lean and pop something
Water on plugs is a mystery as it’s all new gaskets in there already. I will check the head gasket and exhaust gaskets. Is the a way to pressurize and test for leaks with a leak down tester?
The cdi switchbox diode… there’s no way to test them? Can I install new diodes to confirm? Which circuits are diode protected?
New cdi modules will run me somewhere in the neighbourhood of 800 Canadian by the time they ship to me so I need to be sure before I order lol
Thanks
 

Nordin

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There are no way to check the CD boxes, they can dump voltage to the coil as your are doing but if the blocking diod is bad it interfere the other CD box and you got issues like you have.
The blocking diod is in the CD box, almost impossible to change.
The purpose of the diod is as I said before to protect the CD boxes to interfere with each other.
I have the OEM troubleshoot instruction and if you PM me I can email them to you.
All parts in this ignition system can be tested but not the CD box.
With a DVA meter (or a VOM with a DVA adapter) you can check if the boxes put out 180 VDC or more to the coils, but as yours are putting out voltage to the coils in inconsistent order you never know if they are okey.
 

sblanco

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Sep 3, 2020
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Thanks for all the assistance on this. It’s looking like I need 2 new modules… I called standard magneto and they are not making anymore of the blue modules…
I am seriously considering and looking into a change over to the cdm system. Has anyone done this and can anyone provide info for me?
 

jerryjerry05

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May 7, 2008
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Just buy the black and us solder to connect the cut off ends.
I've seen NO difference in the Blue and Black
When they started making the black they changed the connectors so you'd have to buy black coils.
 

sblanco

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Sep 3, 2020
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Thanks for all the help on this. I found a couple of used blue CDI boxes that have corrected my misfire problem.
I’m now trying to determine why I still have water droplets on all 4 plugs. Engine seems to still run a bit rough so I’m guessing it’s caused by the water intrusion. This is a rebuilt engine with all new gaskets and seals on the power head.
When I installed the exhaust plate gaskets I sprayed some gasket sealer on them. Should I have used something else. RTV sealant?
What is everyone else using to form a proper seal? The gaskets came from a rebuild kit so I’m sure they are not the best quality?
Any thoughts would be greatly appreciated!

I also have found when running the engine for several hours my battery voltage reads over 16 volts and it’s causing issues with my fish finder equipment. It’s a new voltage regulator so maybe I have the wrong one? Thoughts
Thanks
 

Nordin

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Jun 12, 2010
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When I rebuild or do anything with an OB that has to seal or reseal something I use Permatex Aviation no 3 form a gasket.
My experience is that it is useful at the engine block or at the LU.
It is easy to use and apply cause there is a brush built in the cap.
 

jerryjerry05

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The gaskets should be thick and stiff with a sealer already on them. The factory used no sealer just good gaskets.
The mounting surfaces might need a mill job. Over time the heat causes them to round off and makes resealing tough.
Usually when I do a rebuild and if there is no problem with the exhaust, I don't take it apart because it can be a b%$#@& to reseal.
 
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