YS 91 Exhaust Problem
 

I have a YS91 AC mounted in my Aeroworks Edge 540 60/90 and I can't seem to keep a muffler on it. After several flights every muffler I've put on it has come loose. The second muffler I tried ( a stock YS 120) actually "split" at a weld seam. On the third muffler ( a stock 91FZ muffler) that I tried the tail piece came off during flight. I even used high temp. red silicone sealant on the bolts that attach the tail piece to the rest of the muffler. "Performance Specialties" suggested that the exhaust is over-heating and that probably need to get more air flow to the motor. I have an area of nearly 3" x 4" already cut out of the bottom of the cowl with the stock holes in the front. Has anyone else experienced these sort of problems, an if so what was the cure?

MUFFLER
 

GET A AAP MUFFLER FROM CENTRAL HOBBIES. THAT WILL CURE YOUR PROBLEMS.............
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YS 91 Exhaust Problem
 

I heard the same things about the YS muffler problem, so I just omitted it....easier to cut the cowel, too....

http://home.internetcds.com/~barbados/exhust.jpg
http://home.internetcds.com/~barbados/exhust2.jpg

This is a YS 91FZ also...just used a flexible Hobbico exhaust deflector instead of the muffler.

Sounds GREAT, not that much louder and the cowel is easier to remove, too.

No worries about buying new mufflers anymore.....

YS 91 Exhaust Problem
 

Thanks for the comments and the pictures. I've ordered the tail piece of the stock muffler that vibrated off from YS Engines. I'm also going to fabricate a straight pipe in conjunction with threaded header that is still on the engine. Also, I believe that the prop that I was using (APC 15 x 6) might be the wrong prop for this engine, allowing it to rev too high (i.e. 9,500 rpm). I'm going to give a 14x10 APC prop a try.

YS 91 Exhaust Problem
 

For my situation, a 14-6 works well. Had lotsa problems with being unable to land properly with a 15-10 and a 15-8, 15-6 and a 14-8, went to the 14-6 and real slow on landing....Revs a little high 9,600 but I always run at 1/2 throttle......

.......your situation may differ.

Good luck with the muffler problem...you can always wire the parts on so you don't lose them.....

Join the club
 

Jones,
I have the same set up w an YS .91AC I had heard all the stories about the muffler. So I safety wired the top of the muffler to the plane. I blew the lower part off the muffler. I replaced just the lower part with anothe muffler from a NIB YS .91 I put a piece of safety wire on what I thought was the correct part of the lower part of the muffler were the threaded part was. I did it wrong and during the next session I blew the lower part off again.
I called performance RC and talked to them about my problem they have discontinued the muffler I have and want 49$ for the replacement muffler. He did have a used one they finally sent me a great price. I installed it correctly and during my last session the whole muffler came loose.
My solution will be to remove the muffler, clean all the threads with acetone then install with RED Loctie or what ever the auto shop recomends for the application then resafetywire the thing and a helo guy says that a tailboom clamp from a helo may fit and will help to hold the SOB in place.
I feel your pain!
Sparky

YS 91 Exhaust Problem
 

Well, bugger me......now why didn't *I* think of that?! I just spent ages agonising about how to get the cowl on and off my World Models Super Sports Senior and the muffler was the problem. Geez, I could have saved myself some mucking around by just putting the exhaust deflector on the end of the stub :rolleyes:

YS 91 Exhaust Problem
 

I'm curious about all these muffler problems. A buddy has a new 91FZ with no issues anywhere. I just got a new 91FZ. These engines do have the new, ugly muffler that has three bolts holding an end-piece on.

Is this the muffler you guys are talking about?

Steve

YS 91 Exhaust Problem
 

Elmshoot - I've tried high temp and red Loctite and neither held at all. In fact, with the high temp the muffler fell off right after I took off. I tried red hi temp RTV silicone and it held the longest but it still came off. Others have suggested using white teflon tape.

Steve - the last muffler that came apart was the the same as the one that you described. I bought it recently from Central Hobbies. However, my YS is an AC not the FZ. I'm not sure the FZ owners are having the same bad experience. Wiring it might be a good idea though, the replacement is $40.

How are you guys wiring the muffler so that it isn't lost? Do you have to drill holes in it? Is the wire attache to the motor mount are firewall? What type of wire are you using?

Last, will rubber hose ( used as an exhaust extension) take the heat put out by the engine?

YS 91 muffler
 

The same thing happened to me.

To prevent in the future do the following:

Install your new tail piece on the existing muffler and tighten. Drill a small hole through both pieces approximately 1/8" from the joint. The hole should pass through the threads. Tap the hole for an appropriate sized allen head screw. Apply loctite to your screw and tighten it to the muffler.

No matter how much vibration the screw will not let the tail piece come off.

Sean

Mufflers
 

Interesting...

I have a new YS .91 (FZ) and simply tightened it normally. (no locktite, wiring, etc.) That's with 15% fuel and an APC 15/6 prop.

I've test run the engine, removed and re-attached the muffler several times, and flew it all day last Sunday on my "Extase". ( http://www.nextcraft.com/rcpp_extase03.html ) No hint of it coming loose.

One thing I noticed while doing some propeller experiments...
The Master Airscrew props would bend, at high RPM, causing a LOT of vibration. The APC props are very smooth running, in comparison. Are any of you guys with the problems using props that "warp" at high speed? If so, a more rigid prop will help a lot.

Also, I've gotten really superior service from YS in the past. Have you contacted them?

YS 91 Exhaust Problem
 

Mike - I have only used a 15 X 6 APC prop on this engine in this airplane. Previously, the engine was mounted in s Super Sportster 60. In that plane, due to height restrictions, I ran a 13 x10 APC prop and didn't experience any exhaust problems.

I did contact Performance Specialties (YS Engines) and they thought that either the engine exhust was getting too hot ( not enough air flow) or that at high RPM ( i.e. > 9,000) some sort of harmonic vibration was occuring. I believe the engine is getting plenty of air flow, so I'm going to experiment with different size APC props starting with a 14 X 10.

YS 91 Exhaust Problem
 

I have a 91FZ mounted in a Dave Brown soft mount. Using an APC 14 x 10. Probably have 25 or so flights on the engine. No problems with the muffler, but I'll check it before I fly again.

YS 91 Exhaust Problem
 

For those of you not able to keep the muffler on and tight, you may like to try plumbers tape....you know, that teflon stuff they use on water pipe joints? I did this on both of mine and they're still exactly where I put them when I tightened them up. Just wrap 2 layers of the tape around the threaded ends of the stub (making sure you wrap it in the right direction of course, so it doesn't snag up when you try to screw it on) and tighten the whole thing up like normal. Works a treat :D No loctite required.

YS Muffler
 

I will try the teflon tape on mine before I fly this weekend. I am running a APC 15x6 which seems to be the best prop from the other thread on the Aeroworks 540T.
Yes I drilled holes in my muffler. I used a small numbered drill bit and was able to run a hole in the upper flange so that no hole was inside the muffler. I actually used safety wire but any small wire should work for a while before it fatigue cracks. I anchore the wire to a portion of the fire wall that is near the muffler.
Sparky

YS 91 Exhaust Problem
 

I think that who ever mentioned an rpm related problem may be right. I have two 91's and run an apc 14-10 on one and an apc 15-8 on the other. No muffler problems with the stock mufflers. RPM is 8600 to 8700. Using a lower pitch prop may be getting the rpm up to a level where the engine vibrates. These motors do not need to be spun up higher than 8600 to 8800.

YS 91 Exhaust Problem
 

Hey Crash-N-Burn, I removed the muffler from mine this arvo and just ran it with the Dubro deflector thingo instead. I agree.....it sounds excellent :D So much easier to get the cowl on and off this thing now.....will help, seeing as I'm currently playing around with regulator settings!

YS 91 Exhaust Problem
 

Good for you, MrBonk!

Initially, I was worried that running without a muffler would warp the valves, but the guys that have been doing it for years assured me that wasn't so.

Good flying, my friend! :)

YS 91 Exhaust Problem
 

I had a similar problem with my OS 70 Surpass, but in my case the muffler vibrated so much that it broke two different header pipes.

The solution isn't really to make the muffler stronger or tighter, but to stop the vibration. To solve my problem I braced the end of the muffler with a copper strip. One end of the brace was attached to the end of the muffler, the other to the plane. The brace isn't to strengthen the pipe, its only job is to stop the unrestrained wiggling of the muffler that creates stress on the header pipe and muffler, and in your case unscrewed the muffler.

I don't know if your cowel would allow this, but if so it should stop the muffler from vibrating loose.

Good luck!

YS 91 Exhaust Problem
 

What would cause the valves to warp?

YS 91 Exhaust Problem
 

Running without a muffler may not cause engine damage, but depending on where you fly it may lead to you loosing your field!

That certainly happened to us here in the Dallas, TX area. :(

Keith

YS 91 Exhaust Problem
 

After reading all of this thread and the talk of copper straps, safety wire, etc., all I can say is you guys must have some severe vibration problems.

I've been running OS four strokes (every size except the .40 and the multi-cylinder models) since 1988, box-stock, and the only exhaust problem I ever had was trying to keep an extension fitted to the 120 Surpass muffler on a CG Ultimate to route the exhaust out of the cowl.

I have several pals running YS 91s (an AC and a FZ) with absolutely no issues. My new 91FZ is scheduled for the break-in stand Wednesday. We'll see...

Steve

YS 91 Exhaust Problem
 

Steve,

I agree with you about the vibration issue. At first I thought maybe I hadn't balanced the prop and spinner very well. Like you I see people flying without braces all the time. However, even after re-balancing to the best of my ability I had the problem again.

An RC buddy of mine suggested that it may be a combination of the engine and my particular plane and the harmonic resonance created between the two (not sure if this is a bunch of hooey or not, but using the brace solved my problem). He told me that he had often encountered this type of issue when racing pylon racers. Also, a very knowledgeable LHS owner suggested the brace too.

Honestly, I wish I knew the answer. If it's something I'm doing wrong I would love for someone to let me know because I really don't like to have to brace the muffler like this.

Another possibility is that I could have had two faulty header pipes that had the threads cut too deep. Who knows?

BTW, the plane that had this problem was not the one picture, it was a H9 Ultra Stick 40. I've since switched the engine to the pictured Extra and used the brace on it as insurance since it was easy to do and out of sight on the bottom of the plane. The US40 now has a Super Tiger 51.

Keith

YS 91 Exhaust Problem
 

Well, at least I know that can't happen to us....there's no-one to complain :D We're in the middle of nowhere.....residentially speaking. The only person who lives even remotely close is a cane farmer, and he doesn't care.

YS 91 Exhaust Problem
 

I missed some posts, sorry!

Mrbronk, It is my understanding that hot valves exposed to cold air may warp. Dragsters with headers plug the exhaust after a run to keep the cooler air off the valves. With a muffler/exhaust pipe, everything cools at the same rate. Don't know why it doesn't affect 4-C R/C engines sans muffler...maybe it does, but their life-span is less than a warp...?

KeithB...I think your solution will fix the vibration/resonating problem. Good Job! I think that is exactly what it is, a vibration at the correct frequency that is Dependant on the airframe, that loosens the mufflers.

You won't have another problem, I'm sure.

As for the noise......

The YS91FZ sans muffler is maybe 15-20% louder than with. It is different, though....it has a lower frequency resonance that makes it almost sound the same.....

I can tell you that the gassers at our field are 50% louder than the YS without a muffler.

Our field is also in the sticks....no one for miles...if we had houses close, I wouldn't ever consider running without a muffler....noise is collective and additive, yes?

Happy flying, my friends... :)