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Hi folks,
My 503 DCDI just surprised me. 300 TT. 0 since top OVHL. Earlier today, it ran for an hour like a sewing machine. Smooth as could be. Five hours later, when performing a pre-flight run up, it is breaking up at high RPM's. Not much changed, except the ambient air temp cooled a few degrees, I added more 91 octane fuel & oil pre-mix (at 40 to 1), from the same Shell station to the tank. Post mortem: Pulled the plugs .... a little black but not more than expected.
What changed!?! :evil:
Where to begin?
Thanks for ideas and suggestions.
john
  • Re: High RPM break-up after overhaul ... gremlins

    by » 8 years ago


    Petrol mix should be 50/1...note sur if that would cause you a prob though,

    Are the plugs just black around the base of the threads...what colour is the centre electrode.

    Nick.

  • Re: High RPM break-up after overhaul ... gremlins

    by » 8 years ago


    Nick,
    Only running 40 to 1 for a short while for initial break-in. I'm thinking I will add gas to dilute to 50:1, but I'm not sure that the extra oil would cause misfire / breakup.
    Plugs are black around the base, as I would expect, given the oil ratio. Not fowled, though.
    Center electrode is clean / silver.
    Any thoughts appreciated.
    Thanks for the follow-up.
    john

  • Re: High RPM break-up after overhaul ... gremlins

    by » 8 years ago


    Did you re-touque the heads after you run in??

    To much oil makes the mix weak so less cooling for the pistons...Rotax do say not to baby the engine on run ins...dont think it does it any favours,

    Was it ok before you had the heads off.

    Nick.

  • Re: High RPM break-up after overhaul ... gremlins

    by » 8 years ago


    The 40:1 would not have caused this. Sounds like you may have developed an air leak or fuel delivery issue. Go back an check everything. Consider all nuts and bolts hand tight and wrench loose until you put a wrench on them to check them for torque. I would get back to 50:1 too. The more oil added the more fouled plugs and less cooling. Fuel is the number one cooling agent in your engine.

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


  • Re: High RPM break-up after overhaul ... gremlins

    by » 8 years ago


    Gentlemen,
    Thanks for your thoughts. Much appreciated.
    I'm in the process of looking every thing over in painful detail.
    I'm going to reduce ratio to 50:1, retorque heads (thanks! I was going to do it at 5 hours, but now is a good idea) and run again paying close attention to EGT's when it breaks up. They might shed some light ....
    I'll get back with results.
    john

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