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89 Merc 70hp spark returns after compression test?

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  • 89 Merc 70hp spark returns after compression test?

    Just starting to diagnose intermittent spark(hard starting/bogging on water) on cylinders 1 and/or 3.

    Day 1 diagnosing-- motor starts hard (on muffs).Check for spark on plug wires with timing light. #1 and #3 dead somehow started on just cylinder 2. Do quick compression test. 90-70-90 (not at WOT). Figures it starts on the worst compression cylinder. Put plugs back in and motor started right up. Spark on all 3 cylinders now motor working great. Did 1 can Seafoam to 1gal gas decarb to try to help compression in #2.

    Day 2-- motor starts hard again.No spark in #1 this time. Check compression to see if Seafoam treatment helped.Still 90-70-90 darn! Put plugs back in, starts right up with spark on all cylinders. Put boat in water,starts,idles and runs perfect. That lasted for about a week and now the motor is hard to start again.

    Any idea why the spark is coming back to all 3 cylinders after a compression test?

    BTW I am going to run the motor until it blows up with the bad compression. The boat can still do 36 mph. DVA adapter just arrived today and trying to not replace any unnecessary parts on a dying motor

  • #2
    Welcome aboard Watermn,

    Sounds like an intermittent electrical problem, my guess is the spark plug wires/cap.
    Check inside the plug for rust/corrosion also check the ignition wire-connectors that maybe loose/disturbed during the compression test.

    BTW Seafoam is not a cure for all ill ( lol )
    "Success is walking from failure to failure with no loss of enthusiasm." - Winston Churchill

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    • #3
      Ok thanks, I'll give the plug wires a good look. I have more time now to be more thorough in testing things.

      Yeah,the Seafoam I gave about a 1% chance to fix the compression. I don't believe you can fix things from a can generally, but the motor sure did smoke nice. Lots of black slime(?) did get removed which I guess was the carbon from the cylinders.

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      • #4
        it's mostly from the chemicals in the Seafoam then anything, but what the heck
        "Success is walking from failure to failure with no loss of enthusiasm." - Winston Churchill

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        • #5
          Remove the plugs from block hook up cables & ground-wire all 3 plugs to block push lever to full forward in gear.
          Jump start the starter (watch out for moving prop) get the compression readings & check for spark.
          Note; a good battery is required for this! a weak battery will not produce enough RPM's to generate power to cause the plugs to spark

          Another thing..... when a (faulty) Stator warms up the can stop working properly
          Last edited by may the force be with you; October 4th, 2019, 01:54 PM. Reason: just remembered
          "Success is walking from failure to failure with no loss of enthusiasm." - Winston Churchill

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          • #6
            Speaking of stators lol

            Here's an update to maybe help someone reading this post.

            I had checked resistance on the stator: Low speed 2230 (service manual 3600-4200) and High speed 102.4 (service manual 90-140). The service manual says to replace the stator assembly if " meter readings are other than specified". I had the flywheel off to check the stator, the flywheel magnets and the trigger so I said screw it and installed the new stator I had ordered.However,I should have waited until the DVA adapter came to check the voltage of the stator as the new stator I ordered did not fix the intermittent spark. Maybe the old stator was perfectly fine? Does a low resistance on the Low speed mean the stator is going to fail soon?
            Anyway, on with the story, started checking voltages with the DVA adapter: New stator and trigger voltages were fine as well as the stop circuit, but the voltage going into the coil from the switch box on cylinder #3 was "Zero". I put everything back together and started the motor. Checked the 3rd cylinder with a timing light and it was getting maybe one spark every 20 seconds.Swapped in a new switch box today and this is by far the easiest the motor has ever started. 2 short cranks max will start 'er up. I can't believe how well the motor idles and runs with crappy compression.

            The good-- learned a ton of things about outboards (thanks to this forum)
            The bad-- Probably bought a stator I didn't need.

            may the force be with you --I'll try the compression test you describe above when I get the boat out of the water for the winter ! I was wondering how you get the motor to turn in-gear when it's not running....

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            • #7
              push lever to full forward in gear.
              Don't do this! Use the throttle only feature to advance the throttle while the motor remains in neutral. Actually you can check for compression with the remote in idle. There may be a slight difference between carbs wide open and carbs in idle.

              Jump the solenoid to spin the motor for a compression test. Do the spark check separately.

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              • #8
                Originally posted by ga_boater View Post

                don't do this! Use the throttle only feature to advance the throttle while the motor remains in neutral. Actually you can check for compression with the remote in idle. There may be a slight difference between carbs wide open and carbs in idle.

                Jump the solenoid to spin the motor for a compression test. Do the spark check separately.
                well said!
                "Success is walking from failure to failure with no loss of enthusiasm." - Winston Churchill

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                • #9
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                  Service manual states 200 rpms with throttle fully open. Manual also states that any number below 120 psig you can expect to have problems. I'd say that you have 2 problems, spark and compression, and I use Sea Foam in every tank and I really doubt it is capable of curing your compression problem.
                  If you are new to boating or have a new boat, a knowledgeable friend could show you how to operate your boat and save you a lot of grief, maybe some money, and maybe your life.

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