fbpx

 

Looking for plug read.  Reference attached image.  Both plugs are from the same 912ULS #4 top.  One is from ~60 hours ago, prior to a plug change and needle clip adjustment.  The other is current.  To my eyes, one is too lean, and the other is too rich (especially considering this is the lightest plug/location of the bunch).  Home airport is 200 MSL, and most of my flying is below 6000.

 

9151_1_IMG_2706_cropped.jpg (You do not have access to download this file.)
  • Re: Looking for plug read / mixture assessment

    by » 4 months ago


    What did you do to the needle clip and why?

    Air filters can play a part in this too. Air filters mounted right on the carbs, separate airbox from an MFG or a Rotax air box. K&N air filter oil application.

    The one on the left tells me you were idling too long or too rich. Run the engine up to 3500-400 for a few minutes and then immediately turn it off. This will keep the dry black soot from accumulating on the plugs and give you a more accurate assessment of the plugs. The one on the right is fine. Is the paste on the plugs to normal thermal paste or an anti seize? It looks like the normal thermal paste?


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


  • Re: Looking for plug read / mixture assessment

    by » 4 months ago


    The one on the right looks normal. I would recommend using the silicone thermal paste over the antiseize.


  • Re: Looking for plug read / mixture assessment

    by » 4 months ago


    Roger & Sam, thanks for replying.  Here are the quick answers:  Filters are K&N, lightly oiled, clean, one per carb (no airbox).  Thermal paste is Super-Lube brand, suggested by a couple Rotax mechanics.

    Now for the "why" on the needle.  To keep this readable, I have to abbreviate some unpleasant history with a certain Rotax mechanic.  I had a long battle with fouled plugs, rough warmups, and very dark oil at 25 hour intervals (maybe 2% 100LL).  New plugs, fresh oil, and a harsh lecture detailing new procedures made little difference.  The "lightest" colored plug (above left) was still black; and the others....oooof.  At the next inspection I went to a different Rotax mechanic who--at my request--helped me inspect and lean the needles by 1 notch (clip moved from #3 to #2).  As I understand it this procedure should not have been necessary in my home climate, but wow what an improvement.  Or so I thought....

    Since then I've had 2 more oil changes.  The latest oil analysis shows a significant uptick in aluminum and chrome over previous samples.  They say it's attention-getting, but only recommended a wait-and-see approach for now.  This got me to thinking about the needle change 6 months ago, so I pulled the plugs, and this thread is me thinking out loud.  

     


  • Re: Looking for plug read / mixture assessment

    by » 4 months ago


    Hi Chris,

    Most of the time I'm a fan of leaving the needle position in the #3 slot. There are times that a needle position change is warranted, but usually for special circumstances. The carbs do a pretty good job running for the normal operator in their original setup. Dark oil means nothing as far as lubricating. Dark oil is just from the combustion process and carbon. That's its job.

    Idling will cause some plugs to have a dry black soot on them. Each engine is its own personality so it may be different plugs. Don't just idle and check plugs. Run it up for a while and shut it off right away. Running it up like when you fly burns that dry soot off. Your carbs may need to be internally checked and or overhauled depending on TTSN hours. Maybe you have a heavy float? Clogged idle jet? Crab floats not set properly for their height in the bowl. Carb sync can be in play here. Oil samples are helpful, but each on its own may not be fully accurate or a tell all. Most oil samples are comparisons from against other Rotax engines with the comparable times on them. Oil samples can be affected by when you catch the sample. i.e. at the begining of the flow in the middle or towards the end of the oil drain. Then time on the engine (older vs newer), type of fuel and oil can play a part in sample readings. I do a lot of oil samples as engines accumulate time and I see many with elevations in certain areas, but are usually gone the next oil sample. Unless the sample indicator is off the chart then most are a watch and see at the next sample.

    Overall sounds like you may need someone with experience that can go through the carbs and then do a good sync. Personally I would put the needle clip back in #3 position. You don't want to ever get in a bad situation that may cause excessive leanness. 


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


  • Re: Looking for plug read / mixture assessment

    by » 4 months ago


    Roger.  You've given me a few new bits to consider.  

    I'm probably due to weigh the floats again, but they've been spot on every time so far, and this problem dates back to when the engine was brand new; about 280 hours ago.  I've personally dialed in carb balance at almost all rpms; checked a couple times a year.  I don't see how a clogged jet could result in over-rich, but I'm listening.  In any case, seems unlikely to affect left and right side equally.  I've done the extended length run-up and throttle chop approach (it helps), but mostly I just fly the thing for 30 minutes and check after landing.  My hangar is close to an uncontrolled runway.  It's not like I'm driving around the airport, or waiting on ATC.  Oil is sampled hot, and midstream, as best I can.  That is fairly inexact.  I always use Aeroshell Sport +4.  Never had to add any between changes.  I run almost exclusively 94 octane, zero-ethanol, unleaded fuel from the same pump.  Actually maybe that in itself is something to change up.

    "Float height" is not something I've considered, or had checked, unless this is just another expression of buoyancy.  Weight has been checked multiple times.  Is there more?  What/how do I check?

    "Different plugs" is an interesting suggestion, especially given the long running debate that considers only 2 (both of which I've tried, by the way).  What other choices would make sense to combat this buildup? 

    "Special circumstances".....as in unsolved mysteries?  ;) 


You do not have permissions to reply to this topic.