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  1. #1

    Default Yamaha 6HP 2-cyl. 4-stroke doesn't fire

    It is an F6MLH serial 60N L 1005268 J.
    It seems to have plenty of compression...judging from the pull start.
    We used starting spray and the plugs are wet...so I guess there is fuel.
    There doesn't appear to be any spark at all...on removing the spark plug and grounding it.

    The engine has less than 5 hours on it...looks like new.

    So, what is the most likely cause? I don't want to start throwing parts at it.
    Any particular part that is prone to failure?
    Last edited by whitegreg56; 09-29-2021 at 07:59 PM.

  2. #2

    Default Re: Yamaha 6HP 2-cyl. 4-stroke doesn't fire

    First easy one to try is kill cord and if that is the correct one and fitted, is the cut off switch working correctly ?

  3. #3

    Default Re: Yamaha 6HP 2-cyl. 4-stroke doesn't fire

    I disconnected the kill switch so the ignition is NOT shorted to ground.
    Engine still does not have spark.

    Found out the electronics module is NLA.
    There is one on eBay for ~$500!

    I'm going to reseat all the connectors and see if anything changes.......

  4. #4

    Default Re: Yamaha 6HP 2-cyl. 4-stroke doesn't fire

    And try a different ( new or known working) plug, I have had quite a few fail on customers engines.

  5. #5

    Default Re: Yamaha 6HP 2-cyl. 4-stroke doesn't fire

    I noticed that the ignition coil has two high-voltage leads to the sparkplugs
    and only two other wires.

    Does this engine have "wasted spark"?

  6. #6

    Default Re: Yamaha 6HP 2-cyl. 4-stroke doesn't fire

    I should think so but donít have the manual to hand. Let me know what you have checked so far, Iíll be back home in a few days, Iíll look up the troubleshooting resistance values and post them up.

  7. #7
    Join Date
    Sep 2010
    Posts
    22,832

    Default Re: Yamaha 6HP 2-cyl. 4-stroke doesn't fire

    Most 2 cylinder 4 strokes fires both sparkplugs each time pistons come to TDC.-----So yes it is a wasted spark on one of them.

  8. #8

    Default Re: Yamaha 6HP 2-cyl. 4-stroke doesn't fire

    Definitely a wasted spark system.

    Good idea. Not throwing parts at the symptom of some problem. Which might not exist.

    Do you have a multi meter with volt and ohm settings?

    1. Have you tested the ignition coil, the high tension leads and the spark plug caps?

    2. Have you disconnected the white wire bullet connector on the lead from the CDI assembly to eliminate the kill system?

    3. Have you tested the charge coil for voltage to the CDI assembly?

    4. Have you tested the pulser coil for voltage to the CDI assembly?

    5. Have you tested the CDI assembly for voltage to the ignition coil?

  9. #9

    Default Re: Yamaha 6HP 2-cyl. 4-stroke doesn't fire

    Well, I got a few pages of the ignition section from the service manual at the local Yamaha parts place.

    Did some resistance checks on the ignition coil and the sparkplug caps (internal resistor).
    Sure enough, the ignition coil primary came back low...specification is 0.6 ohms.

    I used a digital ohm meter. They aren't very accurate at resistances of less than 1 ohm.
    But it came in at 0.2~0.3 ohms cold. And, when heated up with a hair dryer, 0.3~0.4 ohms.
    So, it looks like a temperature dependent "short".

    The motor was sold (along with a sailboat) before replacing the ignition coil.
    So, the end may never be known!

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