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pjeffers(at)talktalk.net
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 Posted: Fri Aug 02, 2013 6:28 am Post subject: Rotax 912ULS Stumble below 4000 RPM on final ONLY** |
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Just a suggestion. Have you checked your carb vent pipes. Make sure they are both connected to the plennum chamber and that neither is blocked.
Pete Jeffers
From: SIDESLIP
Sent: 02/08/2013 02:35
To: rotaxengines-list(at)matronics.com
Subject: Rotax 912ULS Stumble below 4000 RPM on final ONLY**
--> RotaxEngines-List message posted by: "SIDESLIP" <Chad2007(at)rogers.com>
Hello all. I've got a Zodiac 601 XL-B with a Rotax 912ULS and a warp drive three blade.
It's set at 15 degrees pitch, giving me 5500 RPM CLIMB OUT, 5200-5300 static. This engine runs flawlessly on the ground. It has only 175 hours on the engine. New plugs, carbs synced, oil changed. Fresh fuel lines. All parameters are well within margins. Climbs and runs like a top, and is happy at 5500 cruise. The airplane does 100-110kts straight and level at 5500 rpm. Fuel burn is approximate at 3.5-4.1 gal/hr.
I fly my pattern at 90kts, with 70-80 on final.
When I pull the power back gradually to slow it for flaps and turning base, anything under 4k and it shakes and thuds. Maybe that speed unloads the prop? I don't know. Like I said, runs like a top on the ground. It's only running like a bag of hammers in the circuit pulling back under 4k. It is a slipper clutched plane.
Any help is appreciated!
Thanks in advance!
Chad
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C-GYXQ. 912ULS. 601XL-B
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BARRY CHECK 6
Joined: 15 Mar 2011
Posts: 738
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| Posted: Sat Aug 03, 2013 3:02 am Post subject: Rotax 912ULS Stumble below 4000 RPM on final ONLY** |
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It is difficult to play doctor over the internet. You have to be a bit more accurate in your description and what you have already checked. "Smoothly Smashed" is an oxymoron. Either you smoothly advance the throttle or you smashed the throttle. You say "I mean its only firing on half the cylinders. It's THAT sputtery." What jumps into mind is:
Condition of Spark Plugs
Condition of Spark Plug Wires
Mag output
Fuel Pump - Fuel Flow
Have you checked these issues? And YES, you can have a good spark and running condition at High RPM and lousy at low.
Barry
On Sat, Aug 3, 2013 at 6:48 AM, SIDESLIP <Chad2007(at)rogers.com (Chad2007(at)rogers.com)> wrote:
[quote] --> RotaxEngines-List message posted by: "SIDESLIP" <Chad2007(at)rogers.com (Chad2007(at)rogers.com)>
Brand new prop. Brand new hub. Brand new extension. Smooth as silk at full throttle. When I mean runs rough below 4000, I mean its only firing on half the cylinders. It's THAT sputtery. Although yesterday I flew it and brought the power back REALLY slowly, and it didn't seem to do it hardly all the way to idle. I let it idle, as I lost altitude, then smoothly smashed it to the firewall. ZERO hesitation and took me from 50kts DIRECTLY to 100kts! Stuffed me in the seat. I was pleasantly shocked. The slipper was checked by a Rotax tech only 20 hours ago. It was good.
The throttle cable thing sounds legit to me....
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C-GYXQ. 912ULS. 601XL-B
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Roger Lee
Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.
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| Posted: Sat Aug 03, 2013 3:36 pm Post subject: Re: Rotax 912ULS Stumble below 4000 RPM on final ONLY** |
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Carb sync, I hope mag drop is fairly equal and gearbox checked to see if it needed re-shimming? Even with only 175 hrs. if a plane has been around years things like the Bellville washers in the gearbox get crushed and need to be re-shimmed or replaced.
I could easily be throttle cables not moving perfectly in sync as you throttle up and down, especially down. Going up should be smooth.
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Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
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rampil
Joined: 04 May 2007
Posts: 870
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| Posted: Sun Aug 18, 2013 9:23 am Post subject: Re: Rotax 912ULS Stumble below 4000 RPM on final ONLY** |
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Here's another thought.
I have a similar issue in my 912s, at manifold pressures from 22 down to
18 inHg, the engine gets rough. If I advance the prop to more FINE, the rough range narrows. This would speak for overloading the engine or lugging, not under loading as initially suggested.
Over the years I've have several conversations with Phil Lockwood, Dean Vogel, Bud Yearly and others. The most likely, but still unproven in my case, is Bud's thought that in-flight one of the carb slides sticks. Bud has seen
a carb replacement cure the issue.
I may try it myself when I fly down to Sebring for my ten year maintenance.
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