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06-05-2011, 10:39 PM
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Join Date: Sep 2006
Location: Townsend, Montana
Posts: 3,179
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rpm fluctuation, stumble..?????
I had a beautiful flight this morning, but my friend Neil thought since I have Emag/Pmag I had too much rpm drop. It drops about 100rpm...either side. It's always done that so I didn't think much about it.
Then when I reduced power to idle in the pattern I get a stumble...not the typical RV backfire, but a stumble. A touch of throttle seems to smoooth things out. But always runs a little rough taxiing when hot. It seems to be getting worse. So tonight I went out for a little practice, ran up the lake and back before entering the pattern. Once in the pattern, I pulled power and sure enough stumble. I get a rpm drop and I give it a little throttle to pick things up. 1800 and above seems ok, below that i wasn't certain the thing would even stay running. My stable approach went to **** and did a go around.....Runs great at full throttle. This time aeam the numbers I slowly retard the throttle, and it is stumbling and surging I thought it quit once(but maybe not). Makes for an exciting short final when it seems the engine will quit at any moment. I don't dare another go around, this will be a full stop!. I'm jockeying the throttle to keep it running....btw should I count all my bounces as landings
So I taxi back and several other guys notice she is running pretty rough. Once stopped I pull back to idle and the engine surges from 850-1030rpm. I try to lean the mixture, I try full rich, I try boost on & off. Started slowly a couple hrs ago and seems to be getting worse. Weather is warming up around here.
So I'm grounded until I figure this thing out....any ideas?
IO-360 parallel valve, 9to1 comp, AFP injection, Emag/Pmag w/auto plugs
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Retired Dam guy. Life is good.
Brian, N155BKsold but bought back.
Last edited by hydroguy2 : 06-05-2011 at 10:47 PM.
Reason: added engine info
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06-06-2011, 02:09 AM
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Join Date: Nov 2007
Location: NJ
Posts: 1,747
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Not sure what you are experiencing there but I suspect the first culprit is plugs.
However, I am relieved when I hear other injected engines run rough at idle when hot. Mine does too but lately I have found the more you lean it out the less rough it is. I think mine will just be a tad rough when hot but for me, learning to lean it out just the way she likes it will make a big difference.
Sorry I couldn't help with your issue. Good luck!
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My ATC opinion is NOT an official FAA recognized opinion, so any advice you get from me is ONLY my opinion.
Track my RV7A!!
Bought my flying -7A
Building an -8! (Fuse)
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06-06-2011, 03:34 AM
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Join Date: Mar 2007
Location: Sonoma County
Posts: 3,821
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If this was a carb engine, I would say check the carb mounting bolts.
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06-06-2011, 08:54 AM
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Join Date: Nov 2007
Location: Lee's Summit, MO
Posts: 48
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Which E Mag?
I just went through something similar. Always stumbled on the ground. Eventually the ElectroAir side failed a runup check.
Turned out to be a bad sensor. Replaced that and it runs smooth.
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06-06-2011, 05:47 PM
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Join Date: Jan 2005
Location: Las Vegas, NV
Posts: 488
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Sounds like an air leak to me
in that it gets worse under high manifold vacuum conditions. Look for a broken intake gasket at the cylinder head (found a small piece missing from one of mine), or other air leaks between the injection servo and the intake port.
Another possibility is foreign material partially plugging an injector or injector line, or even inside the spider. Do all cylinders drop out or just one or two?? Do a bottle flow test.
Doesn't sound like ignition to me. Ignition problems USUALLY show up during high power or high altitude operations in my limited experience.
Good Luck!
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Terry F.
RV 7A N457RV
250 hours and lovin it! 
Southern Nevada EAA Chapter 1300 - www.eaa1300.org
Paid VAF 03/17
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06-06-2011, 10:37 PM
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Join Date: Sep 2006
Location: Townsend, Montana
Posts: 3,179
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Busy today and tomorrow, but I'm going to download the Dynon info next. I think it is fuel also. this problem just recently started. Originally It was a minor hesitation when going to idle during a hot engine taxi. then I noticed it more in the pattern 2 flights ago. Last night was 78* and engine temps were up due to some slow flight. But CHT's and EGT's were tracking normally from what I recall. While all my hoses are firesleeved, my UNinsulated red cube is mounted under and between cyl 1 & 3....thinking that may be a problem.
__________________
Retired Dam guy. Life is good.
Brian, N155BKsold but bought back.
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06-06-2011, 10:50 PM
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Join Date: Jan 2009
Location: Salt Lake City, Utah
Posts: 554
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Red Cube
I think I recall the instructions state to insulate the cube if FWF. I did this but can't really remember if it's in the instructions or not. I used some giant firesleeve hose about 8 inches long, two slots for the bolts with hose clamps on both sides.
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A&P RV-10 Flying 1000+ hours
SuperSTOL Flying 170 hours
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06-06-2011, 11:01 PM
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Join Date: Sep 2006
Location: Townsend, Montana
Posts: 3,179
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I recall that some folks do, others did not. I did not insulate as i wasn't sure I even wanted to leave it there. It is a very tight fit and was already thinking of moving it aft of the FW. this gives me an excuse.
__________________
Retired Dam guy. Life is good.
Brian, N155BKsold but bought back.
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06-16-2011, 10:03 PM
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Join Date: Sep 2006
Location: Townsend, Montana
Posts: 3,179
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UPDATE:
Talked with Don at AirFlow Performance. re set the idle mixture and rpm setting. I was rich. Also insulated the FF redcube and an elbow on the servo. Pulled the plugs, checked the timing.
It ran better but still rumbled a bit during hot taxi. Ran good in the pattern though. I didn't like it still, so started tracing the fuel system for any issues. When I pulled the fuel filter, I found the inlet flare not loose, but not torqued properly.
Anyway got her all back together this morning, running great! no stumbling, bumbling or other scary schtuff. Good thing too as I had a photo shoot in HLN. We were shooting a video for the Big Sky Air Race along with one of the sponsors, RobertAllen Nissan. also did some low pass shots.
We got to close RY34 for about an hour. It was great fun

__________________
Retired Dam guy. Life is good.
Brian, N155BKsold but bought back.
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06-17-2011, 12:48 AM
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Join Date: Jan 2008
Location: Ashland, OR
Posts: 2,573
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So, what was it?
Was the not-torqued fuel line at the filter allowing air bubbles into the fuel? It is on the suction side of both pumps, but even then, low enough pressure to pull air in?
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Steve Smith
Aeronautical Engineer
RV-8 N825RV
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bought my old LS6-A back!! 
VAF donation Jan 2020
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