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In Cowl muffler for OS.91FX

Old 06-13-2002, 08:47 PM
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Default In Cowl muffler for OS.91FX

At Toledo this year, I purchased at your booth this muffler so my OS 91FX to go into my 25% Aeroworks Edge. It's nicely done and fit very well. However, I cannot get the motor to run more that 3 minutes as it leans out on every flight. I have plenty of exiting air over the engine and muffler and always run my engines rich so they do not burn up.
I am not one of those "chat room" groupies but many comments are on the web sites regarding this exact problem..your muffler overheating the OS 91 while other mufflers do not do the same.
So, please get back to me either though email or this forum: an aerobatic plane with an engine always ready to die is a lousy combination. Plus, it has been a waste of my money.
Thanks for your time and we do appreciate your hosting of this site.
Old 06-14-2002, 09:17 PM
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Default In Cowl muffler for OS.91FX


It sound to me that you have too much prop for that engine/muffler. Remember, the in-cowl muffler is a Tuned Muffler so you might have to run a proper size or lesser prop. Either by diameter of pitch.

Over heating problem's are usually cause from over-prop or lean needle setting. In this case, I think you have a little too much prop for the engine/mufller set up.

Remember, muffler that are tuned must be run according to to it design and RPM range. Any deviation from that will cause some problems. Hope this help.

And Bob can answer the rest.

Sam S.
Old 06-15-2002, 01:30 PM
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Default In Cowl muffler for OS.91FX


Thanks for the feedback. Info on both good and bad experiences are always helpful in developing and refining products. I will make sure Dub sees the message. You may also wish to email him directly at [email protected] and he will get back to you quickly. You may also pass along this info to anyone in chat or message boards with concerns. Dub is always the first place to go for answers, and he is only a phone call or email away.

A few quick questions that can help me better diagnose the situation. First, what prop, fuel are you using. Second, how is your fuel system set up? Any header tank, filler valve, filter, etc. Also, have you been able to duplicate the situation on a test stand?? Knowing this can better help me/Dub/Dennis diagnose any problems.

The issue may be traceable to fuel foam or something similar. But since you mentioned that others have noted a similar problem it may be something else. We will have to find out. I do know that most of the feedback Dub has received about the Turbo-jett installation in the AeroWorks 540 has been very positive, but as stated above, we want to know about any potential issues or problems Jett customers experience.

Sam is correct about the tuning feature and the need for a correct prop size. But one of the advantages of the Turbo-Jett is that it runs over a very broad range of RPMs. Usually you can turn a bit more prop with the Turbo that you might with a regular tuned system. The OS91-Turbojett combo should run in the 10,000 rpm range (or higher) for best performance.

My experience with the OS91 and turbo combination has been pretty good. It turns a 15x8 apc at about 9500 rpm. ILaunch it at about 9300 rpm, and let it unload in the air. It takes a careful needle setting loaded like this, or the engine will lean and get hot. The combination really enjoys a 15x6 as it will unload well over 10,000 rpm on the top end, and is very happy in midrange. One other item of note, was I have had some difficulty with the stock OS needle valve. It seemed to "leak" or something, and my runs were not consistant. I replaced it with a Jett remote, and it holds the setting much better now.

Something that I did with my Aeroworks ARF was to block off the left side air inlet in the cowl. The only active air inlet is over the engine cylinder. From what I learned from guys flying the bigger planes with single cylinder engines, this helped cooling quite a bit. I figured it was worth trying on the smaller 540.

I hope this is helpful, and that we can resolve your issue shortly.


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