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  1. #1

    Default One cylinder won't fire

    My ex-friend gave me a boat that has a 1979 Mercury 70 hp (blue) motor. I have been trying to get it to run properly for over a month now. I'll run down what has been done to date. Replaced...Stator, rectifier, plugs, rebuilt fuel pump, rebuilt water pump, cleaned both carbs and replaced floats and needle/seats, replaced fuel filter and installed a glass type in fuel line between pump and inlet, replaced all fuel hoses, new gas tank/primer bulb, repaired all corroded and damaged wires.

    Motor runs well for 5-10 minutes then loses power. Goes from 30 mph to 8. Take home, clean carbs...take out and runs for 5-10 then bogs down. Found #2 plug isn't firing. Compression good...130 psi. Swapped #2 coil with #1 and won't fire. Have gone through troubleshooting guide for CDI switchbox and resistances are good. Yesterday, I purchased an in-line spark tester. Looked at each cylinder and they all looked the same...getting volts from coils. As a last ditch effort, I installed a new plug...still not firing. Don't know what to do next. If the #2 cylinder isn't getting fuel/lubricated, it would over heat and blow it up. If it's getting fuel and spark, it should fire...even if the timing is off, it should fire. Take the #2 plug out and it looks brand new with a little evidence of fuel on it.

    I'm completely stumped!
    Last edited by JEH63; 09-24-2018 at 02:07 PM.

  2. #2
    Join Date
    Sep 2010
    Location
    ontario
    Posts
    15,362

    Default Re: One cylinder won't fire

    The # 2 cylinder gets fuel from both carburetors.----Perhaps it is a reed valve problem.

  3. #3

    Default Re: One cylinder won't fire

    Quote Originally Posted by racerone View Post
    The # 2 cylinder gets fuel from both carburetors.----Perhaps it is a reed valve problem.
    Thank you, I believe you have it! After looking into it further, that's the only thing that makes sense. Learning more about this motor than I ever thought would happen!

  4. #4

    Default Re: One cylinder won't fire

    Quote Originally Posted by JEH63 View Post
    Thank you, I believe you have it! After looking into it further, that's the only thing that makes sense. Learning more about this motor than I ever thought would happen!
    Thanks again Racerone...broke it down. Certainly are differences in them. Some seem to stick a little. Others seem to open freely. My guess is the "sticky" ones should be replaced. Manual says to always replace complete sets. What constitutes a set? My motor has a total of 8 boomerang looking flaps. Would a set constitute one complete side (2 flaps). I looked to purchase and they all call a set (3) of the boomerangs...still confused but not near as much as I was. Not sure if I should replace them all but one set is $65-$85!

    ...I know...Break out another thousand!

  5. #5
    Join Date
    Sep 2010
    Location
    ontario
    Posts
    15,362

    Default Re: One cylinder won't fire

    Are any reeds broken ?------If not look for other issues.

  6. #6
    Join Date
    Sep 2003
    Location
    New Tripoli, PA, USA!
    Posts
    15,692

    Default Re: One cylinder won't fire

    It's probably NOT the reeds, and here's why: These are weird motors seeing the center cylinder breathes through both carbs. If a reed was stuck open it would mess up both the top and bottom cylinder's operation, and that sucker would barely run (if at all). There IS a possibility you have a water leak from a leaky gasket in the exhaust plate (left side); had that happen to me once.

    If it's not that it, then it's most likely ignition related. To find out, you need to buy or borrow a DVA meter (or a DVA attachment to your present meter. Got mine on Amazon for about 4o bucks and it works great.) Even at cranking speeds you can accurately diagnose what's gong on.

    Jeff

  7. #7

    Default Re: One cylinder won't fire

    Thanks to both of you for your input/advise. Reeds appear to be undamaged. The ones in this motor seem to be made of metal. I read somewhere that older ones were metal but materials have changed over the years and now they're some sort of composite. FastJeff, I agree if one/two were stuck open but what if a couple were sticking closed and not opening enough? Seems that would cause not enough fuel to get in the cylinder and not adversely affect cylinders 1 & 3.

    Prior to getting inside this motor, I went through and measured resistances from the stator, coils, and control box. All seemed within the specs. One thing I failed to mention is when I installed the new stator, the wires were shorter than the ones on the original unit Therefore, I installed it 60 degrees off (read on another blog this was acceptable) in order for the wires to reach the rectifier and control box. I have since pulled the stator, lengthened the wires, and reinstalled in the original configuration...no difference.

    Not sure if this was a valid test but I disconnected the triggers and tried wiring only cylinder 2 to the #1 trigger/coil. Still wouldn't fire. That leads me to believe the #2 cylinder just isn't getting enough fuel.

    Figure I have it broken down this far, may as well replace gaskets, seals, and give it a general cleaning. Got my fingers crossed! Thx again!

    Now, different problem...do either of you have trouble typing on this site? Seems like it misses 1/4 of the things I type and have to go back and edit. Things that make you go, hmmmmmm...

  8. #8

    Default Re: One cylinder won't fire

    Quote Originally Posted by racerone View Post
    Are any reeds broken ?------If not look for other issues.
    I believe it was reeds. I tore the motor down and cleaned the mating surface between the reeds and the reed blocks. Also replaced internal gaskets and seals. All cylinders are firing now. Last thing to do is check timing. Thanks for the advise!

  9. #9
    Join Date
    Sep 2003
    Location
    New Tripoli, PA, USA!
    Posts
    15,692

    Default Re: One cylinder won't fire

    Update, please!

    Jeff

  10. #10

    Default Re: One cylinder won't fire

    Quote Originally Posted by fastjeff View Post
    Update, please!

    Jeff
    Hi Jeff,

    I believe it was the reeds. I tore the motor down and replaced all the gaskets and seals. More than half of the reeds that serve the #2 cylinder were sticking. I simply cleaned the mating surfaces between the reed and the reed block. This cleared up the sticking issue. I put it all back together and fired it up. Just need to go through the timing and it should be good. So, it was either sticking reeds or a compromised internal gasket...

    Thanks for your input!

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