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Thread: Zenoah 45hp ultralight engine has lost power, one cylinder smokes and runs cooler

  1. #1
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    Zenoah 45hp ultralight engine has lost power, one cylinder smokes and runs cooler

    I have a two cycle 45 hp ultralight engine on an airboat. I did not change my prop since last year and I have dropped 300 rpm's that I can get full throttle. I was getting 5900 rpm which is where it is supposed to be, now I can only get 5600, sometimes 5650 rpms. I cleaned carbs, synced them, etc. Exhaust pipe runs cooler also. We swapped carbs, swapped spark wires from the other side, same problem. Compression is good, did a leakdown test and that's good. New spark plug also. When I accelerate the CHT on the bad cylinder will momentarily drop, and the muffler runs cooler than the other side, but CHT gauge says they are the same when idle. This engine was just rebuilt by someone who has been repairing small engines for quite some time, but he is baffled by this also. The engine only has 20 hours on it since overhaul. If anyone can help please do! Thank you so much.

  2. #2
    Registered User Hot Wings's Avatar
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    Re: Zenoah 45hp ultralight engine has lost power, one cylinder smokes and runs cooler

    How is the crankcase compression/seal?

    When I start my diagnostic routine if the problem is in any way, even remotely, related to the most recent repair I start there - even if I was the one that did the repair. ~80% of the time that is where the mistake/bad part can be found.
    Topaz likes this.
    Conventional wisdom and practices yield conventional results. If that is good enough for you:
    Problem solved.

  3. #3
    Registered User clanon's Avatar
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    Re: Zenoah 45hp ultralight engine has lost power, one cylinder smokes and runs cooler

    If you ruled out carbs ; fuel pump (clogged output); switch CDI sparks wires ; i will say piston rings ...

    PS:ZENOAH GZ50N OWNER'S MANUAL Pdf Download.
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    Registered User Doggzilla's Avatar
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    Re: Zenoah 45hp ultralight engine has lost power, one cylinder smokes and runs cooler

    If it has good compression, spark, and fuel, than I can only assume that its a timing or valve issue. It can have proper compression in a test, but perhaps a valve is floating at high speed. Just imagine a partially compressed charge being blown out into the exhaust. The cylinder would run cooler, the exhaust would be cooled by the relatively rich mixture flowing into it, and it would put out smoke from incomplete combustion. It would also provide normal operating temps at low speed, when the valve is not sticking, but would have reduced top end performance as the valve gradually begins to float.

    Perhaps one valve spring is weaker, or if it has hydraulic lifters, it will clatter if one of them doesnt have proper oil. A bad hydraulic lifter will not open the valve all the way.

    I dont see any detailed specs in the owners manual about the cams, valves, or timing, but Im sure someone else can elaborate.

  5. #5
    Registered User clanon's Avatar
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    Re: Zenoah 45hp ultralight engine has lost power, one cylinder smokes and runs cooler

    there is no valves!
    Not even reeds on this design.Just piston travel doing all the intake-exhaust
    Zenoah 45hp ultralight engine has lost power, one cylinder smokes and runs cooler-g50c-.jpg

    Timming is picked up from the only point on the magnet-flywheel
    PSid you try another CDI?
    "Do what you can, with what you have, where you are"

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    Registered User Doggzilla's Avatar
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    Re: Zenoah 45hp ultralight engine has lost power, one cylinder smokes and runs cooler

    Quote Originally Posted by clanon View Post
    there is no valves!
    Not even reeds on this design.Just piston travel doing all the intake-exhaust
    Attachment 25184

    Timming is picked up from the only point on the magnet-flywheel
    PSid you try another CDI?

    Well, that simplifies any answer to my hypothesis.

    Looks like dual carbs. What kind of mixture is used?

  7. #7
    Registered User Dan Thomas's Avatar
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    Re: Zenoah 45hp ultralight engine has lost power, one cylinder smokes and runs cooler

    Three possibilities:

    1. Weak spark.

    2. Weak spark.

    3. Weak spark.

    A CHT drop on acceleration indicates that the plug is not firing when the cylinder pressures get high. Start looking at the ignition system.

    90% of all engine hassles are electrical, yet most folks chase fuel system possibilities instead.

    Dan

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