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Thread: Muffler baffle

  1. #1

    Join Date
    Apr 2012
    Location
    Lexington,NC
    Posts
    9

    Default Muffler baffle

    I have reason to believe my 1993 Kitfox IV 912UL 998 hrs. muffler may have been repaired incorrectly leading to a rough running engine (straw grasping).

    The previous owner welded a plate on the end of the muffler canister where the exhaust outlet is - I can only guess that it was to repair a cracking issue. When I was reinstalling the engine and muffler, I looked in the inlet holes and noticed that the baffle (tube with holes in it inside the canister) did not go to the end and was offset toward one side of the canister.

    Has anyone had their muffler open to see how the baffle should be? Could the baffle location be causing this grief?

    The carbs are synched an the engine will idle at about 1800 for about 30 secs before getting too rich and overflowing. It is rough through 2000-2800rpm even though the carbs stay synced and smooths out a bit at 3000. At 3000 the carbs do not overflow but you can hear the engine stumbling a bit. At any point, the engine will run on either ignition with about 100-150 rpm drop. Every time I check plugs, the rear cylinder plugs are light tan and the front cylinder plugs are totally covered in dry black soot. All the compressions are 78/80.

    Previously I posted about overflowing carbs and thought I had resolved by installing a fuel pressure regulator (which I should not have to use) and running the fuel pressure at 3.5psi. I have since realized that the engine can vibrate but it cannot shake or it will surely overcome the floats and overflow.

    I have checked and rebuilt carbs twice, checked ignition coil wire and grounds, changed plugs, sprinkled holy water, etc, until I am one step away from making someone a really good deal... - on a plane with one of the most tedious engines devised.

    Any advice will be appreciated and considered.

    Thanks,
    Charles

  2. #2
    Birdseyeview's Avatar
    Join Date
    Sep 2017
    Location
    Oregon, Ill
    Posts
    198

    Default Re: Muffler baffle

    Charles, I've been chasing fuel overflowing from the carb vents for some time now and I think I have discovered the cause as a bad case of the Brown Sticky Stuff (BSS) fouling the carbs. There was much discussion about this issue in past forum posts so I won't repeat that here. You didn't mention seeing this BSS in your carbs so I assume it must be another cause for you but I highly doubt that your issue is caused by the exhaust pipe repair that you mentioned. I mention my findings about BSS just in case you haven't looked for it in your carbs during cleaning of them. During my investigation of my fuel overflow issue I discovered a few interesting tidbits from a highly respected Rotax technician that may be of interest to you as well:

    1) Engine vibration - 912 vibration can be aggravated on the UL and ULS engines by the orientation of the intake manifolds. From the factory the 912 intake manifolds curve inboard and this is done intensionally by Rotax to reduce the transmission of vibration energy from the engine to the carbs. The closer the carbs are to the engine center line the less they vibrate and the less the float needles have to strain to seal fuel inside the bowls. On the earlier Kitfox models (prior to the series 7) the intake manifolds are reoriented to both curve outboard to avoid interference between the air filters and the older style engine mounts. This manifold orientation increases the vibration moment arm and thus more vibration of the carbs. As a result of increased vibration at the carbs the float needle valves are more easily overpowered. The engine mount system was changed on the series 7 to a ring mount and there is some thought that this change was done to avoid the interference mentioned earlier and to reorient the intake manifolds back to their Rotax preferred original design orientation and thus reduce vibration at the carbs. Reduced vibration at the carbs usually equates to less likely to vent fuel out the overflow tubes.
    2) Float Bowl fuel level - According to the Rotax owners forum the float valve position should be set to cause the fuel level in the bowls to operate at 13mm below the mating edge of the float bowl. This is roughly equivalent to about half full of fuel in the carb bowl. I bent the float pivot armatures in my carbs to more closely accomplish this and it reduced the fuel venting of my carbs quite a bit. However, it didn't eliminate it because the BSS was continuing to build up in my case and was fouling the carbs making them run rougher over time in between cleanings.
    3) Float bowl valve seat - The valve seat in the Bing 64 is not replaceable if it is damaged or corroded, but it can be reconditioned to seat better. If one or both seats aren't sealing properly fuel overflow can happen. The Rotax overhaul facilities can do a pressure test on the carbs to determine if the float valve is strong enough to seal off the bowl sufficiently at the test limit pressure of 6 psi. If not, a seat reconditioning may be called for.
    4) Fuel pump pressure - The older German made Pierburg fuel pumps are supposedly "bullet-proof" and they are usually producing the correct operating fuel pressure of 4.0 to 4.5 psi. However, many of the newer fuel pumps are known to produce higher pressures and when this is combined with any, or all of the items listed above, it can lead to or aggravate, fuel venting.

    I hope this info helps you isolate and overcome your issue - it was very enlightening to me as I struggle through my own issue with fuel venting in the overflow tubes.
    Larry Olson
    Kitfox Series 6 - 1st Flight Oct 2021
    Tri-gear, smooth cowl
    912 ULS

  3. #3
    Senior Member
    Join Date
    Apr 2009
    Location
    Chisholm Mn
    Posts
    1,562

    Default Re: Muffler baffle

    Just a comment on your post Larry, I have a Jabiru engine in my Avid Flyer, and it uses a type 94 bing carb. The type 94 is an enlarged version of the type 64 used on the 912. I have changed the float bowl valve seat on that carb when I went to gravity feed for fuel delivery. You run a 8MM fine thread tap into the seat, screw a matching bolt in with full length threads on it. Make a U shaped bracket with hole in center that the bolt goes through. A nut is screwed onto the bolt, and tightened to pull on the valve seat. Carb is warmed somewhat to make it expand a bit. It worked fine. I think I read how to do it on the Bing carb web site, but couldn't find it just now. They do sell new valve seats there though for the type 64 carb as well. JImChuk

  4. #4

    Join Date
    Apr 2012
    Location
    Lexington,NC
    Posts
    9

    Default Re: Muffler baffle

    Larry,

    Thanks for the detailed response. I have checked off each item listed as double/triple checked.

    I went back into the CDI wiring again today, more aggressively, mainly due to having exhausted (no pun intended) other alternatives and not putting much stock in the exhaust baffle theory.

    After disconnecting the CDI boxes from the coils, I checked continuity through each coil to ground and jiggled the wires. Eureka - I found an intermittent ground that was the typical good insulation holding together a broken wire at the terminal.

    I am going to enjoy my "win" for the day and give the engine another run tomorrow.

    Charles

  5. #5
    Birdseyeview's Avatar
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    Sep 2017
    Location
    Oregon, Ill
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    198

    Default Re: Muffler baffle

    Charles,

    It's always a great feeling of relief when you find some defect that is easily fixed, especially after chasing the unknown solution for a long time. I hope that what you found fixes your issue once and for all. I hope to hear all about your outcome after you run it again. I'm sure there's more builders and flyers than just me that's learning about this engine as we all have struggled through various issues along the build path.

    Jim,

    Its good to know there are repairs out there like you described. Thanks for the info.
    Larry Olson
    Kitfox Series 6 - 1st Flight Oct 2021
    Tri-gear, smooth cowl
    912 ULS

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