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03-24-2015, 09:50 PM
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Join Date: Mar 2005
Location: Queen Creek AZ
Posts: 541
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I could surely use some help - trying to figure out this engine problem
I have a Lycoming with about 50 hours on it that is behaving very badly.
It is cyclical roughness, happening every few minutes. It begins with roughness, followed by lower RPM's, higher EGT's (all) and a feeling as if it is self leaning! Fuel flow goes down by 2-4 GPH too (11+ down to 7 or 8 and EGT's rising to Peak). A few seconds (20-30) and it cures itself. Then in a few moments, it all begins again.
Isolating the mag does not improve or make it run worse or better. Same for the one Lightspeed II/Hall effect I am running. Feels like the engine is about to give up, then it takes a breath and goes on normally for a few minutes. It will run hard and even overspin the CATO prop (it's underpitched right now), then runs very poorly... like I need to find a place to land. Goes from 2750 down to 2450, and then, slowly, back up again. Scary!
Because if "feels" like a fuel issue, I traced everything in the (new) AFP 150 for loose fittings or fouled filters.... no joy. Cleaned everything and tightened fuel fittings and no change in the way it runs. Takes a few minutes on the way to altitude before it starts in, then never corrects. Same issues regardless of altitude or RPM. Could the mag be working intermittently? Hall Effect? If so, you would think isolating them would make it quit altogether, but it doesn't change anything except the EGT's go way up, and then I get nervous and switch back to both.
Any thoughts you may have are much appreciated.
Ron
RV-4
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03-24-2015, 10:13 PM
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Join Date: Sep 2007
Location: Mesa Arizona
Posts: 608
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I am neither a mechanic nor an expert, but it sounds to me like it could have something to do with a fuel tank venting problem
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Myron Nelson
Mesa, AZ
RV-10 N24EV
KITPLANES Contributing Editor
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03-24-2015, 10:30 PM
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Join Date: Jul 2009
Location: Lee's Summit, MO
Posts: 747
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fuel
How much fuel in the tanks when this is happening?
What engine? Injected or carb?
Will the engine do this consistently during a ground run?
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Robert Williams
Lee's Summit, MO
RV-8 - Empennage & Wings Done
Working on Fuse
O-360-A1A
1946 Cessna 120
Last edited by DaAV8R : 03-24-2015 at 10:35 PM.
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03-24-2015, 10:36 PM
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Join Date: Jan 2005
Location: Victoria, Canada
Posts: 2,251
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Do a fuel flow test on both tanks by disconnectiong the fuel line from the carb or injector servo and use the boost pump to run fuel into a gas can.
You should get about 25 gph.
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V e r n. ====
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RV-9A complete
Harmon Rocket complete
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Chandler, Az (Winter)
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03-24-2015, 10:38 PM
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Join Date: May 2008
Location: Sacramento
Posts: 377
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Fuel leak air not fuel
I had a similar issue with missing and chased electrical untill it just could not be electrical. Make a fuel hose clear tygon and run it from you red cube to the spider if you are fuel injected. Run the engine up to 1500 and check for air bubbles. It will bubble at idle but at 1500 rpm there should be none. If you see bubbles it is sucking air into the fuel. I traced my issue to the fuel valve pipe threads. Did not leak fuel but sucked air. Pulled the fittings off and sealed with permatex 2. Problem solved. Hope this helps.
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Dan "Nordo" West
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03-25-2015, 05:42 AM
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Join Date: May 2005
Location: X35 - Ocala, FL
Posts: 3,679
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I agree with the assessments that it sounds either like it is sucking air or is a clogged vent. Does it happen on either fuel tank? If so, then a clogged vent is less likely, but still possible.
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Jesse Saint
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03-25-2015, 06:18 AM
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Join Date: Jun 2011
Location: Indianapolis
Posts: 228
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Engine Problem
One more troubleshooting thing to do. Assuming that the engine is running on the mechanical fuel pump only, next time it happens do nothing but turn on the electric fuel pump. If the "self leaning" stops, then you would have confirmed that you have fuel line sucking air, or an obstruction/flow restriction causing the fuel in the line flashing to vapor/bubbles, thus reducing the flow to the engine.
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Larry Anderson
Indianapolis
RV-Super 8 N88XT SOLD
RV-10 QB N38LA Flying
Helicopter Instructor Pilot
Standardization I.P.
Instrument Examiner
U.S. Army Retired
EMS Pilot for LifeLine
At IU Health Retired :-)
Although Exempt,
Donated for 2020
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03-25-2015, 06:22 AM
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Join Date: Nov 2014
Location: Fayetteville nc
Posts: 23
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Clogged Vents
On wet tank (type of fuel tank design), a clogged fuel vent will most likely cause skin damage / collapse before the engine dies. Doesn't tank much negative pressure inside the fuel tank before the thin wing skins concave.
If this is a carburetor'd engine, I'd suspect issues with the float / needle sticking if there is adequate fuel pressure on the fuel pressure gauge.
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03-25-2015, 07:15 AM
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Join Date: Sep 2005
Location: Maple Grove, MN
Posts: 2,333
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Check for play in the mixture cable - verify that you cannot move the mixture arm manually with the knob locked.
Is the fuel system design per AFP's recommendations? I.e., pump and filter locations, etc.
Did the problem start abruptly?
Also, call Don at Airflow Performance. He may want to check out the servo.
__________________
Alex Peterson
RV6A N66AP 1700+ hours
KADC, Wadena, MN
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03-25-2015, 08:16 AM
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Join Date: Jan 2007
Location: Battleground
Posts: 4,348
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Quote:
Originally Posted by flynwest
I had a similar issue with missing and chased electrical untill it just could not be electrical. Make a fuel hose clear tygon and run it from you red cube to the spider if you are fuel injected. Run the engine up to 1500 and check for air bubbles. It will bubble at idle but at 1500 rpm there should be none. If you see bubbles it is sucking air into the fuel. I traced my issue to the fuel valve pipe threads. Did not leak fuel but sucked air. Pulled the fittings off and sealed with permatex 2. Problem solved. Hope this helps.
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I have done this. The only thing I can add is that it is no fun standing behind a prop spinning at 1500 rpm. Obviously, tie down and chock, and it wouldn't hurt to have a trusted buddy at the controls. Scary! 
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Bucker Jungmann
Fiat G.46 -(restoration in progress, if I have enough life left in me)
RV1 - Proud Pilot.
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