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Ruff Rotax 912

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Posted

Hey all! Here's a question about my Rotax UL912,  So flying along and slowing down, engine reaches 2800 rpm and starts running ruff, increase rpm and its smooth, and below 2800 its smooth again, but it did this several times! Any ideas on what could cause this, fresh fuel 91 octain, new filters, new carb boots, new battery, engine starts instantly and sounds fine at every other rpm.

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Posted

Not sure what is the cause, but did you see the thread on the Team Kitfox forum about never having to sync the 912 again.  According to those that have made this modification, the engine runs much better at low but above idle rpms.  JImChuk

http://www.teamkitfox.com/Forums/showthread.php?t=8116

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Posted

Thanks for that info 1avid flyer! Sounds interesting! I'm going to do a static run up this week end and see if it acures on the ground, but that does make sense in low rpm balance.

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Posted

This is not unusual I think, mine has a rough patch around the same area but it is easy to use slightly more or less throttle on the approach where I run into it.  One thing you can try is doing the pneumatic syncing at that RPM and see if that helps if you really feel like you need it to run at that RPM.  The bigger crossover tube sounds interesting, thanks for the link Jim.  I didn't know that Hal was marketing that mod.  The 912's intake manifold leaves a lot to be desired (at least the older ones) at low speeds the front cylinders run richer than the rear ones and I suspect that this may be true across the range of operation.

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Posted

(That's my thread on TKF)

I synced so many times...  There are subtle variances in the carbs that make them run different from one another.  The larger crossover tube helps mask this.  My engine runs incredibly smooth throughout the rev range now.  I used to have the common roughness between 3500 and 2500.  Now it's gone.

As that thread on TKF went on, it drifted into using a large tube between the standard small elbows.  This is totally useless.  The air needs to pass from one side to the other easily.  The small elbows won't allow this.  I don't care what is said on the thread.

The modification is fairly simple.  I used a 7/8" end mill to locate where I wanted the tube, then drilled it to size for a 3/4" NPT fitting to a 1" diameter tube.  Put a piece of pipe between with rubber hose to seal it and you're all done.

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Posted

We did somewhat the same thing when I was building gyrocopters for a company. It made a difference even after synchronizing the carbs with the right tools. I think every rotax has some kind of rough spot in it. My 582 has one around the 2200rpm mark . I've done everything I can think of to smooth it out but it's still there a little bit.

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Posted

chopndrag, my 582 is rough below 2000, and I think it's the C gearbox that has a resonant range.

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Posted

Your idle should be set at 2k that's where your roughness is coming from . I had a old tach that was reading off by almost 800rpms with a tiny tach . I replaced it and now they both match. I'm pretty certain mine running a little rough when I first got it was from the plane sitting and the faulty gauge showing a lower rpm . It was reading 2k on the dash and the tiny tach was around 1700rpms at cruise I thought I was around 5900 rpms when I was actually only around 5300. When I went through everything and changed the tach I noticed a big difference in performance. My plane has a little over 230 hours on it since being built so you can imagine how long it sat for.

chopndrag, my 582 is rough below 2000, and I think it's the C gearbox that has a resonant range.

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Posted

It sounds like the larger cross over tube is the way to go, at least it couldn't hurt. Ill be doing a static ground test this week end and see if it reacures.

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