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  1. R

    Mercury 500 Strange Ignition Problem?

    Update, better late than never! Long story short I took all of your advice and went out troubleshooting with a spark checker, timing light, and CDI manual. Spark on and 1 and 2 could not jump 1/2" gap, it was faint. 3 and 4 could jump gap. Interestingly, timing light showed some flashes on all 4...
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    Mercury 500 Strange Ignition Problem?

    Update. Haven't had much time to mess with engine so I swapped out plug wires and went fishing. It appears cylinders 1 and 2 have problems as engine will idle but won't rev up past about 2500rpm. They are wet with fuel though so they definitely are getting gas but not firing right. Time to...
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    Mercury 500 Strange Ignition Problem?

    Racerone, before retesting ignition, I wanted to look inside engine, I felt like I was going to snap a bolt on side cover so I looked through plug holes with camera and what I found looks like bad news. 2 horizontal scratches on right side (port) on cylinder #1, cylinder #4 had 2 horizontal...
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    Mercury 500 Strange Ignition Problem?

    Engine sounded a little better after changing wires but kind of hard to tell, the idle is definitely higher now and needs to be reset. Going to go through carbs again to make sure I didn't miss anything before next test run. Does anyone know how to remove the red gasket sealant stuff - it is...
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    Mercury 500 Strange Ignition Problem?

    Racerone, I think I may have screwed up the test. I did test in daylight and just saw white flash on all 4 and called it good at the time. I am redoing this test in dark to verify, I don't want to be too confident but think this may have been issue. Have you seen bad plug wires cause problems...
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    Mercury 500 Strange Ignition Problem?

    Update. I bought the manual and started testing EVERYTHING on the motor from stator, trigger timer, coils everything. Stator and trigger are correct ohms. But All four coils tested out of spec, 1270 ohms from plug wire to ground. This didn't make sense to me so I pulled off plug wires and noted...
  7. R

    Mercury 500 Strange Ignition Problem?

    No damage, its just deep below the cowling so hard to get access to. I can't use a socket to tighten on the plug I have to use an open wrench.
  8. R

    Mercury 500 Strange Ignition Problem?

    Nevermind, Guage should hold peak pressure, the valve must have got bumped. Do you think compression is good enough at 115?
  9. R

    Mercury 500 Strange Ignition Problem?

    Okay I think engine might be blown. I rechecked compression (warm this time) after running in a barrel for fifteen minutes in neutral 1 - 128 psi 2 - 130 psi 3 - 128 psi 4 - 115 psi Some may say its okay but here's the concerning part, after checking compression I noticed the pressure falling...
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    Mercury 500 Strange Ignition Problem?

    Wow all this advice is great. To me I think there is electrical problem because motor seems to "die" down at times and fire back up if trigger is slightly moved up or down slightly, then it surges back up like it got a cylinder or two back. If I check voltages of trigger do I do this at idle or...
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    Mercury 500 Strange Ignition Problem?

    I checked and all wire colors match. I tried adjusting carb screws on water but did not help top end, and didn't get rid of bizarre low rpm misfire. I can try to screw out more though. I will get Guage to compression check. Thank you for advice on this motor
  12. R

    Mercury 500 Strange Ignition Problem?

    Racerone, yes, I did spark gap test at 1/2 inch after pulling plugs. It was broad daylight, I should have done at night but saw a white flash on all four cylinders
  13. R

    Mercury 500 Strange Ignition Problem?

    Need help diagnosing a motor that was given to me. I believe its a 1980 mercury 500 thunderbolt ignition. My buddy didn't have time to use it so gave it to me. All I know is wiring is new. Compression is good. Just did carbs and water pump. Motor starts easily and idles okay, but when you move...
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