FYI,  UL mufflers (at least for the 350iS) now come from the factory with a reinforcing gusset to the exhaust pipe flange.  I repaired my original (which had cracked), but ordered a new one in the event of future cracking/needed replacement.  Likely was a identified issue...

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While I was cleaning up from my oil cooler leak,  I decided to look at my muffler again.  I have the newer style as noted by the web as in David's lower picture.  That web cracked and was coming loose in all the places where it was attached to the muffler.    Also,  there was another crack forming outside of the weld from the tailpipe holder.   It was about a cm away.  I'm having it re welded and will weld the reinforcing gusset completely down to the muffler.  Not just tacked in three places.   The fact that the metal was fatigued and failing away from the crack makes it more likely in my mind that this problem is not completely fixed.  Look closely at your muffler. 

Has anyone done prop balancing yet?  It seems like there is a pulse/vibration  off this engine (350 iS)  that may be greater than some of the aircraft design and parts are designed to hold.  ( Eg horizontal tail that also had to be reinforced).  

Any thoughts? 


Just talked to Gary Motley last week, he is working with friend on prop balancer. You might contact him to see what they come up with. Maybe you can talk him into doing a You Tube video on it.



I posted on the ULP owners forum about prop balance. It was within reason at the start but was able to almost eliminate the vibration after the balance. I don’t think harmonics from the engine is causing problems but just an opinion. 

Fred, that is disappointing to read!   I ask the guy who fixed my muffler, who is a certified aircraft welder, what he would do to improve the muffler.  He said "make it with thicker material."  If it cracks again, I'll have him fabricate one for me but with thicker s/s material.  Meanwhile, "fly it and watch it!" 

Regarding dynamic prop balancing, I have no backing plate on my spinner and at a loss as to where I would attach balance weights if needed.  I think I have to make a ring (like the prop crush plate) with a slightly larger outer diameter than the prop flange for attaching weights.  Open for ideas.

My muffler that cracked had the piece already to stiffen the flange. I had my welder go ahead and completely weld the extra support to the muffler body. It was just attached at three small areas. The main fracture was outside the weld on the flange. 



Update:   5 hrs down the road,  I did my 100 hr tappet adjustment.   Decided to take a close look at the muffler as I had it all re welded couple flying hours ago.  Well,  the other side cracked.  Again, the was crack was not in the weld.  I called Wicks and ordered a new muffler.  On a good note,  New oil cooler is working fine.  Rest of the 100 hr inspection was ok. 

Fred,  I am just curious if Wicks is now stocking the new design muffler for the UL engine?  I welded mine, but am sure it’s on borrowed time.

Robert,  I called and talked to Wicks.  They have one of their guys who specializes on the UL engines.  Unless you know what you want, just ask the staff to direct you to him. I ordered the new muffler and got it yesterday. It appears to be a new design. The attachment tube runs all the way thru and they welded it to the other side. I think it is heavier. I have not installed it yet.  

Thanks Fred, I will call and order one along with new exhaust pipe gaskets in case I have trouble getting the old muffler off.

Pulled the cowl to change the oil on my 750 (350is) a couple of days ago and while inspecting everything, found my muffler cracked similar to David’s picture.  I was able to weld it without pulling the muffler, but I have the newer muffler with the gusset and the gusset had broken off the muffler also.  My plane has about 120 hours on it.  I know they try to keep everything light as possible, maybe too light on the muffle material. I’m sure vibration causes the problem, but I have been impressed on how smooth the UL engine runs compared to some planes I’ve flown.  So it goes!

I was just at Zenith open hanger days and talked to Robert Helms about the muffler issue.  He informed me that they now have a new design with the flange for the exhaust pipe going all the way through the muffler and holes drilled in it inside the muffler.  They believe this will solve the cracking problem.  He didn’t indicate what it cost, but said Wicks has it and to call them when ordering one and specify the new design. I’m hoping the welding will hold for a while, but my heater shroud is not all that far from my now welded crack so a new muffler may be in the works. I hope everyone with a UL engine is aware of the cracking problem and keeps an eye on the muffler.  My mechanic found a small crack in the muffler on my Piper a few years ago and didn’t  give me a choice - I had a new muffler.

Thanks for the heads-up! My muffler is off at the moment, but I will check it when I reinstall it this week.




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