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Stumbling 914 only at runup

  • Cluemeister
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3 months 2 weeks ago #21320 by Cluemeister
Stumbling 914 only at runup was created by Cluemeister
Here is what has happened the last 4 times I've used the plane. It's a 914 ULS, 2016 aircraft. The engine has been running fine for a year (carb issues a year ago, carbs replaced and has worked without issue since then).

The plane starts normally, and then warms up past 50C. I then start the runup. The rpms increase into the 3000's, and it's here where the engine stumbles roughly. As I push the throttle further up to 4000, the engine runs fine, and even at full 115% power application there are no issues.

This stumbling appears to stop after running the engine further. I have flown 3-4 times since this occurred, and no issues have occurred on take off, in flight or landing.

I have uploaded video of the stumbling on runup.

www.dropbox.com/s/ugbiv7zzzf3x7ux/914.MOV?dl=0

Any ideas? It's seems strange that it only occurs in the 3's, and not up at full throttle. It is a very pronounced stumble, so I'm concerned it is something that could get worse. Thanks!

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  • Bill Hertzel
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3 months 2 weeks ago #21321 by Bill Hertzel
Replied by Bill Hertzel on topic Stumbling 914 only at runup
It sounds like it is running lean in the midrange before the engine has fully warmed up.
Until the Intake Manifolds reach operating temperature some of the fuel sprayed from the carb jets remain liquid and wet the inside of the manifold instead of vaporizing.
Once the engine heats up another 20 degrees the issue resolves itself.

Did this start after the weather turned cold?
If so, you might want to set a richer mixture for the winter.
The cold dense air can burn more fuel than thinner summer air.

Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
whertzel1@yahoo.com
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3 months 2 weeks ago #21323 by Cluemeister
Replied by Cluemeister on topic Stumbling 914 only at runup
Thank you Bill. This did start about a month ago, when the weather turned cold. It's odd that it didn't happen last winter.

When you say set a richer mixture, are you talking adjusting the idle mixture screw on each carburetor ?

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3 months 2 weeks ago #21325 by Roger Lee
Replied by Roger Lee on topic Stumbling 914 only at runup
Try setting both needles in the #3 clip position.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell
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3 months 2 weeks ago #21374 by Cluemeister
Replied by Cluemeister on topic Stumbling 914 only at runup
Thanks Roger. So another dumb question. What's the purpose of the idle mixture screw vs a pin setting? Is that to make a minor adjustments between pin settings?

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3 months 2 weeks ago #21376 by Roger Lee
Replied by Roger Lee on topic Stumbling 914 only at runup
The idle mixture (percentages of air and fuel) are controlled by fixed idle jet, idle bypass, idle restrictor, and the idle air-bleed. The amount of mixture is controlled by the idle mixture screw.
The main jet needle controls the fuel flowing through the carb at higher rpms than idle.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell
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3 months 4 days ago #21498 by N874CB
Replied by N874CB on topic Stumbling 914 only at runup
Roger, if his jet needles are already on #3 would you suggest moving both to #4 ? Mine are actually staggered (1-3 on #3 and 2-4 on #4) which I understand is not uncommon on a 914.
Also, I appreciate all the input and advice you give here !

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2 months 4 weeks ago - 2 months 4 weeks ago #21555 by Cluemeister
Replied by Cluemeister on topic Stumbling 914 only at runup
So the shop opened it up today to move pins from 2 to 3. Opened it up and the diaphragm looks like a solvent has softened it up. They are a good shop, but new with Rotax (been through Rtoax training) but haven’t seen much to gain experience from. They have ordered new diaphragms from Lockwood. Thoughts?

Here’s a link to the image.

www.dropbox.com/s/qw3njc2573d1lv9/Photo%...1%2006%20AM.jpg?dl=0

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Last edit: 2 months 4 weeks ago by Cluemeister. Reason: Added file image to post

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2 months 4 weeks ago #21558 by RobSeaton
Replied by RobSeaton on topic Stumbling 914 only at runup
that is strange and alarming!

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2 months 4 weeks ago #21560 by Dennisr5
Replied by Dennisr5 on topic Stumbling 914 only at runup
Has there ever been an additive added to the fuel?

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2 months 4 weeks ago - 2 months 4 weeks ago #21561 by Cluemeister
Replied by Cluemeister on topic Stumbling 914 only at runup
I have never added anything to the fuel. I self fuel and get 91 E-free from the same station, a busy local convenience store chain that sells a lot of E free to boaters, motorcyclists, etc.

Edit: Since new, I've probably filled up 3 times with 100LL on long x country's, but haven't used 100LL for 6 months or so.
Last edit: 2 months 4 weeks ago by Cluemeister.

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2 months 4 weeks ago #21562 by RobSeaton
Replied by RobSeaton on topic Stumbling 914 only at runup
100LL will not cause that damage, some sort of solvent is more likely.
Methanol in your fuel? (it is used by the gas industry to flush the fuel delivery systems, you may have unintentionally gotten a gulp of it.

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2 months 4 weeks ago #21563 by Cluemeister
Replied by Cluemeister on topic Stumbling 914 only at runup
The station where I fill the 5 gallon cans, the fuel pump has only one hose, with two fuel choices, 87 with ethanol and 91 E free. So I guess the fuel remaining in the hose could have had it? Would it be overkill to put the first gallon of E free in my car tank, and then fill up the cans?

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2 months 4 weeks ago #21566 by Roger Lee
Replied by Roger Lee on topic Stumbling 914 only at runup
I've never seen a diaphragm like that one either.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell

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  • Bill Hertzel
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2 months 4 weeks ago #21567 by Bill Hertzel
Replied by Bill Hertzel on topic Stumbling 914 only at runup
I have seen rubber swell up like that when a fellow put Transmission fluid in his brake reservoir.
Any chance someone shot it up with a load of Carb Cleaner?

E0 or E10... This has absolutely nothing to do with whether the fuel had Ethanol or not!

Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
whertzel1@yahoo.com
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2 months 3 weeks ago #21577 by Cluemeister
Replied by Cluemeister on topic Stumbling 914 only at runup
Diaphragms replaced. Would you empty the fuel tanks as well?

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2 months 3 weeks ago #21578 by garrett
Replied by garrett on topic Stumbling 914 only at runup
I would and then flush the fuel lines.

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2 months 3 weeks ago #21579 by Roger Lee
Replied by Roger Lee on topic Stumbling 914 only at runup
It won't hurt to start fresh from the tanks and the fuel lines since you have no idea what caused it.
Fresh never hurts.

p.s.
Ethanol fuel won't cause this otherwise owners all over the world would have had thousands of problems by now..

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell

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2 months 3 weeks ago #21580 by Cluemeister
Replied by Cluemeister on topic Stumbling 914 only at runup
Thanks guys, they are flushing the tanks and lines and putting in 93 mogas with ethanol.

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2 months 3 weeks ago #21581 by garrett
Replied by garrett on topic Stumbling 914 only at runup
Joe the service manager at Lockwood told me years ago that the 0 E pump gas is just fuel with another set of chemicals. He had heard of damage from the 0 E pump fuel from the race track next door.

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