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Hi All,

First condition inspection one year of ownership and 60 flight hours after carb rebuild, new sprag clutch, rubber replacement, and gearbox inspection by a Rotax-certified A+P on a 1998 912ULS in a Rans S7 with 700 hours. I'm trying to educate myself and my A+P/IA on carb sync.  Before I set the mixture screws to 1.5 turns out, I found the current positions at 3/4 turn on one carb and one turn on the other.  After enriching to 1.5 turns, syncing carbs, and replacing plugs, it starts on first try where it had begun occasionally taking two tries.  It also recently has a high pitched, but not very loud whine at idle that comes and goes abruptly with no change in input or output/gauges.  The only significant change during inspection was compression on #4 cylinder went from 78 to 62/80 (still at 62 after a two hour flight). 

--the manual still recommends setting mixtures at peak RPM vs video says 1.5 turns out.  Since a dual manifold pressure gauge is used to sync throttle cable positions, any reason not to use that to set mixture at peak power, or should I stop overthinking it and just use 1.5 turns?

--possible that previous A+P found peak RPM at that lean setting?

--risk of damage from lean setting that would affect compression or engine noise?

--when it wasn't starting on first try, the prop came to an abrupt stop but not a reverse motion.  It caused a significant shudder of the 700# airframe, about the same as shutdown one ignition after the other.  Is some vibration normal or is there risk of damage at start or shutdown even if prop is not "kicking back"?   (Soft start modules are on order). 

Thank you!

 

 

  • Re: Carb Sync Education

    by » 5 months ago


    The screws set at 1 1/2 turns affect only the idle, not the mixture. Mixture is changed by the needle clip settling.


    Thank you said by: Craig Chaffee

  • Re: Carb Sync Education

    by » 5 months ago


    Hello Craig

    The serial number of the engine might help.  Early ULS engines did not have an overload clutch.  That did not become common until about 2003.  If this indeed has 700 hours you should look at a gearbox reshim.  The high pitch whine you have is most likely something in the gearbox and would have nothing to do with ignition. 

    As Sam pointed out the mixture screw is for low speed and not high speed of the engine.  1.5 is the factory setting and I have never seen it necessary to change it.  

    As for compression if you have a low cylinder after say 10 hours, it has not balanced back to match the others, then you perhaps have stuck rings or valves not seating.  This could be from be from many things but can only be fixed by removing the piston and head  then cleaning or replacing the rings in that cylinder.  Before you do that be sure that you do your compression test by a leak down method, not peak pressure.  In a leakdown you can take some time and listen to your intake at the carb side (would be intake valve not seating) or at the exhaust side (most common problem exhaust valve not seating).  Your A&P should know this one.  Before you take off the cylinder try to "spike" the valve to see if you have just some carbon holding the valve open slightly.  This is a standard aviation practice that might save you from taking it all apart to correct the compression. In the type 915 and 916 engines they explain how to do this in the MML.  

    Cheers

    37719_2_valve spike.jpg (You do not have access to download this file.)

    Thank you said by: Craig Chaffee

  • Re: Carb Sync Education

    by » 5 months ago


    Great, thanks for the education!

    Sorry I wasn't clear--gearbox was sent to Lockwood 60 flight hours ago,  and their logbook sticker includes reshimming.   

    Compression test was differential/leakdown and no noise was heard from intake or exhaust, but it may have been missed over gurgling oil tank, other shop noise, and senior ear drums!  We will recheck in a few flight hours.

    Serial # 4425367.   Strictly 93E10 or 91 non-ethanol fuel.  No metal in filter or magnetic plug.  Hoping to avoid engine or sprag clutch repair!

    Regards.


  • Re: Carb Sync Education

    by » 5 months ago


    Is it possible that after the hose change, the oil system was not properly purged? (not to be confused with burping). That can leave air trapped in the lifters which will end up destroying the valve train.

     


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