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Hey All,

So today I have had an issue raise its ugly head, and I am hoping maybe someone else can shed some light on this.

Firstly, 912 ULS always seems to run a little rough around the 2500RPM mark, and I put it down to needing a Carb Balance.  Other than that, it has always run great, and all is well.  This is what happened today...

I did my usual warm up (oil above 50) and then went to do the run up.  Had it up to 4000 RPM, and then dropped a Mag like a usually do and noticed an instant and definite miss.  Only missed a few times and then went back to normal.  It dropped the usual 100rpm, but it got my attention.

I dropped the other mag, same RPM drop, but smooth.  No issues.

I then went back to dropping the first mag, and again, it missed a few times.  I increased the rpm a little to see if it had an affect and the miss went away.  At this point I shut everything down and took the hood off.  Checked all the plugs leads were on securely etc etc. Could not see anything obvious.

Jumped back in, and it didn't misbehave.  Gave it a good few runups and all was well.  

Flew to my destination, and leaving the destination - no issues.  Cruised nicely at 9500ft. :)

Returned to my hangar, and I again did a mag drop test, and no issues at all. So I figured at that point it may have been a lead.

 

UNTIL....


I remembered someone telling me once about rough running at say 2500RPM may not be carb related.  So I did an experiment.

At 2500rpm, it's running rough.  I drop a mag, and now it's REAL rough (the same one that missed this morning).  Drop to the other Mag, and it smooths out a bit! Not great - but smoother.  Back to the first Mag and it's rough again.  Both Mags, rough, but not that rough.

Above 3000 -4000RPM, all smooth.  Both mags drop 100rpm, and I could not replicate the issue from this morning.

2500, they drop RPM, but one mag runs pretty rough, compared to the other.  It sort of feels like a vibration rather than a random miss that I had this morning.

 

Any ideas?

I am thinking to start working methodically - Swapping one plug at a time to see if I can observe it following a plug - If it is a plug?  Might not be.

 

Anyways...

Just FYI - On the cruise, it's smooth as!  5200RPM and no issues that I can sense.


Cheers

John

 

  • Re: Rough Running and strange things with Mags

    by » 2 years ago


    Try doing mag check in the air at cruise throttle setting.  Should be very smooth with only slight drop.  I do this almost every flight.

    Most likely your problem is carb related.  Sounds odd, but most likely the case.  You might start with floats.  Weigh floats < 7 grams for pair.  Maybe switch floats from L- R carbs.  Check throttle arm return springs to see if cable is being pulled taught and arm follows cable extension.  

    Rainbow Aviation has a three-part series on the Bing 64.  See.... https://electricmotorglider.com/wp-content/uploads/2017/02/CV-Carb-Web.pdf

     

     


  • Re: Rough Running and strange things with Mags

    by » 2 years ago


    Thanks man. 

    Floats were replaced with the SB and the annual about 5 or so hours ago. 

    I checked all linkages and movements etc etc. 


  • Re: Rough Running and strange things with Mags

    by » 2 years ago


    Not quite enough info.

    Try a carb sync just to rule it in or out of your problem. Sync it at around 3300 - 3500 for the high rpm and then at idle. Don't use 2500. Then let us know if your issue still exist. We need to determine if it's fuel related or electrical. So just check it to make sure. It just takes a few minutes and it doesn't cost anything. Let's rule in or out the most common cheap causes first. It will make you problem solving a little more narrow in its scope of things to consider.


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


  • Re: Rough Running and strange things with Mags

    by » 2 years ago


    John Fleming wrote:

    Thanks man. 

    Floats were replaced with the SB and the annual about 5 or so hours ago. 

    I checked all linkages and movements etc etc. 

    Hum? Coincidence? Maybe not. Were the float levels set properly? Are the plug colors normal? I had similar symptoms after a "professional" carb overhaul last year. Mine was running so rich that it wouldn't start with choke.


  • Re: Rough Running and strange things with Mags

    by » 2 years ago


    Thanks lads. I’m waiting for my buddies carb sync tool and I will get that done. 

    while I’m waiting I pulled the plugs. And piston 1 are black (pax front) while all the others are grey. 

    How does one piston run rich?

    34758_2_9BA887C8-60D2-406D-9D8E-DA6BC5371D25.jpeg (You do not have access to download this file.)

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