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When I first start the engine it runs as smooth as a baby,s bottom for about one minute then sounds like it is missing on one cylinder and vibration increases significantly. Engine has 360 hrs and is run on auto gas 5-10 % ethanol

No EGT or CHT sensors

What has been done:
Carbs just overhauled.
Differential compression test. Done hot, all cylinders 80/80
Regular compression test..automotive style. Done hot. All cylinders within Rotaxj specs.
Carbs synced.
New plugs.

After the engine is hot, this problem is greatly reduced or
goes away.
Could this be a valve issue?
Thanks
  • Re: Rough idle when engine is cold

    by » 10 years ago


    Hi Marlin,

    What rpm are you warming up at?
    Mag drop numbers?

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell


  • Re: Rough idle when engine is cold

    by » 10 years ago


    On engine startup idle is 1700-1800. Runs smooth. After brief period engine settles into idle speed of 2000 and starts to run rougher. Warm up is 22-2300.
    Mag drop is - 50 each.

    Idle mixture screw was set 1.5 out on each carb. I lenriched mixture on cyl 1-3 approx 1/8 turn and saw slight improvement. Additional enrichment had minimal effect.

    Removal of plugs after brief idle shows all to be sooty which I understand is normal.

    At 28-2900 rpm engine smooths out and runs great to max rpm

    Carb overhaul and Syncing the carbs were done per the videos on this site.

    Thanks, Marlin

  • Re: Rough idle when engine is cold

    by » 10 years ago


    Hi Marlin,

    I see the carbs were just rebuilt. Did it have this same vibration before the rebuild?
    You should be warming up at 2000 or above until the engine reaches operating temp. Even then you should try and avoid longer periods of low rpm idle.(<1800)

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell


  • Re: Rough idle when engine is cold

    by » 10 years ago


    Hi Roger,
    It ran much worse prior to the carb rebuild. It was impossible to even come close to synchronizing them.
    Thanks
    Marlin

  • Re: Rough idle when engine is cold

    by » 10 years ago


    Hi Marlin,

    You still may have a fuel delivery issue that wasn't discovered yet especially if the rebuild made it better, but didn't totally cure it. You may need to at least pull the carbs, flush some canned carb cleaner through every orifice followed by some high pressure air. Don't leave an orifice out. Make sure the idle jet has the center hole open which may need to be cleaned out with a tiny piece of wire. Make sure the mixture jet in the carb bowl is cleaned out. Make sure you have no air leaks. Make sure you take the Banjo bolts off and make sure they are clean and no crud is lodged in them.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell


    Thank you said by: Marlin Kirkendall

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