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Thread: Rotax 503 EGTs

  1. #1
    New Member trevor9's Avatar
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    Rotax 503 EGTs

    Fan cooled Rotax 503, single carb, dual ignition in an SSDR flexwing.
    EGTs at tickover are the same, EGTs at the climb(6000 or so rpm) are the same, but in the cruise (5000 rpm) there is a difference of 50 to 75 degrees.
    The ignition timing has been checked , the engine runs fine and all the carb bits and pieces are standard.
    A plug check shows the PTO cylinder running richer than the mag end cylinder.
    Any 2 stroke experts left out there that can advise please?
    thanks


  2. #2
    Trainee Pilot tomshep's Avatar
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    Yes, the mag end runs hotter so leaner.
    Swap the gauges over and check the temperature imbalance is the same to rule out a dodgy gauge.
    Which is the hot pot?


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    New Member trevor9's Avatar
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    Thanks for the quick reply
    Hot end is indeed the mag end cylinder, any idea why it would run hotter Is it not better served by the fan?


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    Trainee Pilot tomshep's Avatar
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    Not as well served as the other one is by the prop. You sit with your body shrouding the back of the engine and the front pot (PTO) gets the airflow.


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    Co-Pilot BobH's Avatar
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    trevor9,

    If yours is fan cooled then take off the top of the engine shroud (half a dozen screws to loosen that's all) and see if there's a deflector inside it. There should be one to redirect some of the breeze onto the mag end cylinder. I had two fan cooled 447's in years gone by, one of which had a deflector in the shroud and one that didn't. The one that did have the deflector ran well, but the one that didn't have the deflector seized on the mag end cylinder because the air goes rushing past that one and on to the PTO cylinder instead.

    I believe that with the 503, two different sizes of deflector have been used. The later deflector was bigger than the original deflector, as it was found that even with a deflector, not enough cooling air was being directed to the mag end cylinder. So they made it bigger. The person to ask about this is Simon at Eccleston Aviation (their website is here - https://ecclestonaviation.co.uk/). He should be able to tell you which is which, and if you've got the smaller deflector he could probably either sell you a shroud with a bigger one, or give you the dimensions so that you could pop rivet an extra piece of metal to the existing deflector yourself.


  6. #6
    New Member trevor9's Avatar
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    Checked the shroud and it seems I have the smaller deflector, it measures 60mm at its tallest and the curved edge is 75mm.
    Simon at Eccleston was a good help but does not have a larger deflector to hand to give me the measurements.
    Anyone any idea and where are my tin snips?


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    Captain kawasakiinit's Avatar
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    i need one as well so will be interested to see the measurements thanks..
    The more people I meet the more I love my cat..


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    Captain Roger Mole's Avatar
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    Does anyone else think that this forum is becoming a bloody good reference for technical information? Personally, I think better than the BMAA forum was and also better then the Facebook group which is too trivial for my taste. Then again, I'm not a Facebook fan at the best of times - nice for the ladies who want to 'share' their latest shopping trips and their kids' latest teeth but that's about it.

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    Co-Pilot Wexfordair's Avatar
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    I agree. The information on here is kept and searchable in the future if needed. However there is no denying that the biggest numbers are on Facebook and Twitter etc.. despite the fact that good information eventually gets pushed aside.
    Back on topic, at the next decoke or sooner if you like, remove the heads and exhaust and see if there are any signs of melting, I'd doubt it. You could also use a borescope to look into the plug hole for signs of over-heating. Another great tool is the choke, it works at all rpm's so you can pull it on during cruise and it will enrich the mixture if your egt gets too hot. The cooling fan doesn't really have as much bearing on egt's as does fuel mixture, carbs and rubber sockets are the place to look and give your jet needle a look at. You could possibly raise the hot one by one notch to enrich the mixture for that cylinder at cruise rpm. But 50-70 degrees is not usually a problem, unless of course they are both very high to start with. Egt related failures are usually melted pistons as far as i know. The probes themselves can read differently at various temps as well so swap the probes and try that.


  11. #10
    New Member trevor9's Avatar
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    Thanks for the replies. I will try the suggestions, neither of the EGTs are anywhere near the max it just irritates me they are not similar throughout the range


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