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Thread: How to prevent cracked muffler on 912uls

  1. #41

    Default Re: How to prevent cracked muffler on 912uls

    this is awesome to hear. I STRONGLY suggest avoiding the innovate motor sports AFR gauge. I’ve killed 3 in just this tuning event - even my wife was looking at me asking WTH I was doing because there is no way 3 die in the same fashion only in my hands. Well… I returned it and the gentleman behind the counter told me he was getting a lot of these returned. Take that for what you will. I went with the AEM unit and it was actually 6 bucks cheaper and didn’t have a single issue during yesterdays almost 4 hours of flying / engine time.

  2. #42
    Senior Member Delta Whisky's Avatar
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    Default Re: How to prevent cracked muffler on 912uls

    That's great feedback - the AEM unit has been on my buy list for a few days now.

  3. #43
    Senior Member Delta Whisky's Avatar
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    Default Re: How to prevent cracked muffler on 912uls

    Since losing my exhaust stinger I’ve been hunting the culprit hiding behind the scenes. Some felt that it was most likely engine vibration while others leaned towards the hole in the cowling was too small and startups and/or shutdown shakes caused the stinger to hit the cowl and caused the breakup of a fine marriage. I leaned towards the vibration theory as there was plenty of evidence supporting the same but outside the scope of this report.

    While waiting for delivery of a new muffler, Nickolas’ postings on managing air fuel mixture conditions caught my attention as mixtures on the extreme edges of acceptability will often cause vibration.

    Once the muffler arrived an AFR gage was installed and I found the only adjustment needed was at idle and a new and improved idle was noticeable.

    I next attempted to see if there was any way to get a feel for the “exhaust pipe hitting the cowl” theory and the best I could come up with was the trusted blue tape approach seen here:


    Exhaust pipe not touching.jpg


    After no fewer than 50 start-and-stops (from a fairly wide range of rpm’s) the pipe has yet to make a mark on the tape and the clearance is about ½ of the original clearance.

    On to the vibration theory - and here’s where the report gets interesting.


    Background: after about 10 (original) hours on the plane, I attempted to balance the prop/engine but couldn’t do better than its original set up. Meaning: no added weight except for the dynamic balancer. BUT – my attempt to balance was performed at 4,000 RPM only and the results were for the most part unrepeatable, run-to-run.


    Now, with the loss of the stinger my attention was more focused and I started over again. A base line run was made with the original set up – with the still installed dynamic balancer and spinner and runs were made at 2,000, 2,500, 3,000, 3,500 and 4,000 rpm. Then the same runs were made with the spinner removed (remember, zero additional weight). Next, following the DynaVibe instructions, I balanced the engine/prop to a fairly respectable level. All in all, the results looked like:


    First prop balance run.png



    Clearly, my label of “respectable” is for the higher rpm’s in the runs following the baseline runs. Nothing I did would eliminate the weird waviness and I started thinking the dynamic balancer may not be up to the task. So off it came.

    Tests and set ups were rerun at the same data points and it became clear that something is still happening at 3000 rpm (about 1200 at the prop) but all of the traces show a respectable amount of similarity. (I stopped after run 8 when noticing how it was starting to flatten out across the tested rpm range.)

    Finally, a flight was made and the results are presented here on the same graph.


    Second prop balance run.png




    Bottom line, I’m more convinced 1. that it was vibration that caused my issue, 2. the dynamic balancer influenced my vibration survey (favorably) quite a bit but not enough, 3. the dynamic balancer has its own set of issues, and, 4. I’m going to check propeller balance at each condition inspection. BTW, the very minor vibration I used to feel in the pedals is now gone.


    Oh yeah, any and all reasonable offers for a slightly used dynamic balancer will be kindly received; the highest bidder will be notified of acceptance NLT 1200 noon this coming Monday.

  4. #44

    Default Re: How to prevent cracked muffler on 912uls

    this is outstanding news to hear that yet another person has seen drastic improvements as a result of the AFR tuning process. Thank you for reporting your results.

  5. #45

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    Default Re: How to prevent cracked muffler on 912uls

    Quote Originally Posted by Eric Page View Post
    I have a new, unused exhaust system for a 912UL/ULS, including down-pipes with cylinder flanges and muffler with welded-on stinger. It dates from 1995. No springs or other hardware. I'd take $775 plus shipping. Not interesting in parting it out.
    Do you still have this? I’m looking for an exhaust for my 4-1200 round cowl w 912uls

  6. #46
    Senior Member Eric Page's Avatar
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    Default Re: How to prevent cracked muffler on 912uls

    PM sent...
    Eric Page
    Building: Kitfox 5 Safari | Rotax 912iS | Dynon HDX
    Member: EAA Lifetime, AOPA, ALPA
    ATP: AMEL | Comm: ASEL, Glider | ATCS: CTO
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  7. #47
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    Default Re: How to prevent cracked muffler on 912uls

    The other day I pulled my cowl off on the Kitfox 4 and look what I found. A little bit of fabing up some support pieces, a trip to the welder and $50 and it was all good again. Probably for good this time with the bracing. JImChuk
    Attached Images Attached Images

  8. #48

    Default Re: How to prevent cracked muffler on 912uls

    Quote Originally Posted by avidflyer View Post
    The other day I pulled my cowl off on the Kitfox 4 and look what I found. A little bit of fabing up some support pieces, a trip to the welder and $50 and it was all good again. Probably for good this time with the bracing. JImChuk
    good thing you found it before the stinger decided to detach in flight. Keep your eyes on the corners of those welds for a while. Typically welds round a corner in an effort to reduce the stress concentration at the weld interface.

  9. #49
    Senior Member Slyfox's Avatar
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    Default Re: How to prevent cracked muffler on 912uls

    check your gearbox. static should not fall under 300. just rebuilt my box it was about 250. now it is smooth on shut down and start up. that will crack your exhaust right now.
    steve
    slyfox
    model IV 1200-flying
    912uls
    IVO medium in-flight
    RV7A-flying
    IO-360
    constant speed prop

  10. #50

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    Default Re: How to prevent cracked muffler on 912uls

    Just wanted to give an update since I started this thread. My muffler cracked at 200hrs and per the advice of John McBean, I enlarged the cowl hole for the stinger until I had about 5/8" clearance all the way around the stinger. I am now at over 700 hours and my muffler is good as new. This is certainly not proof that the the cracked muffler was caused by the muffler hitting the cowl on rough starts. Still, there really are not any disadvantages to giving the stinger more wiggle room, and heck, it might save you a $1000 muffler replacement



    Quote Originally Posted by 3kdscf View Post
    200 hours on my engine and my muffler got a huge crack, clearly this is not ordinary wear. Talked to John McBean and his first guess was that the stinger was banging against the engine cowl on startup. I looked and sure enough, saw evidence on the stinger that it had been hitting the cowl. The distance from my stinger to the cowl is 3/8" which would seem like plenty but in a rough cold morning start, clearly that is not enough. Rather than buy another $995 muffler every 200 hours, I think I will increase the distance to 5/8" clearance And while I am at it, going to replace my engine mount vibration isolaters too in case those are shot and causing excess movement.

    Attachment 29987Attachment 29988Attachment 29989

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