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Old 07-11-2021, 02:52 PM
walter's Avatar
walter walter is offline
Join Date: Mar 2005
Location: Mansfield TX
Posts: 356
Default Engine stumble

I have flown 17 uneventful hours behind my Barrett IO-390X. It has run perfectly until today. On climb out I noticed the engine loose power or stumble, slightly and briefly, three times and I immediately turned back to land. This has not happened before.

Yesterday the wx was a bit snotty so I de-cowled and looked for any anomalies - none were found. This mornings temperatures were downright awesome at 80. This engine never runs hot, cylinders always below 330 and it's hard to get oil temps above 190. DA was about 2000 ft this morning and field alt is 600.

I pulled the chip from the G3X and pasted today's flight on savvy. The attachment is a snapshot from run-up (prop cycled 3 times), short taxi to runway and the very short flight to landing. Notice EGT's on 1,2 are divergent from 3,4.

I pulled all the fuel injectors and checked for blockage - nothing found. I have dual PMags and each PMag powers one spark plug per cylinder - eg cylinder 1 has a plug powered by the Left PMag and a plug powered by the Right PMag, same for all cylinders. I have not checked the timing yet.

This takeoff was done with fuel pump on throughout the short flight. As I was climbing out I started reducing rpm from 2700 to 2500 when I started to feel the slight power loss.

Later today I did a PMag ground check again and both were functioning properly on the ground but I did notice that it would not idle unless the mixture was pulled way way back. Boost pump was not on. I de-cowled again and found that the mixture control is moving correctly stop-to-stop.

After every flight I post my flight to savvy. Up til today the EGT's on all cylinders have been in sync and look very normal.

Looking for suggestions on what to look for next.

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Walter Tondu
Flying RV-8A - Fastback, IO-390, G3X Touch, 74 HRT
Prior RV-7A Build Log
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Old 07-11-2021, 03:08 PM
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rv8ch rv8ch is offline
Join Date: Feb 2005
Location: LSGY
Posts: 4,133

I have gotten a stumble about 3-4 times, usually just once at about 1000ft during climbout. Seems to be related to warmer temps, but it feels like an air bubble in the fuel system or a little bit of water. I certainly don't think it's water. What fuel injection system do you have? I have an FM-200 and dual pmags. My stumble is just a very short cut that is gone before it can be picked up with my GRT even at 200 milliseconds resolution, but I sure hear and feel it, and I don't like it. I can't seem to "make" it happen when I try, either with another takeoff, or similar climb.

Any hints most appreciated!
Mickey Coggins
"Hello, world!"
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Old 07-11-2021, 03:39 PM
BillL BillL is offline
Join Date: Sep 2007
Location: Central IL
Posts: 6,215

Climb - Texas - was the boost pump on? Only thought, agree probably not water.

One check - look back at fuel pressure on savvy to see if it gave any corresponding change.

I was recently check pilot for my 10Friend and had the same thing happen but only once. We did 6 approaches.

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Old 07-11-2021, 03:48 PM
calpilot calpilot is offline
Join Date: Aug 2018
Location: Independence
Posts: 134

Not uncommon full power right after take off to have a valve stick momentarily. Causes a shot of cold urine directly to the heart! Issue goes away in seconds, hard to diagnose. Some engines have valve guides pressed into the cylinders without a final ream. ECI used "ball-izing" to final ream the cylinders and give a bit of a knurled inside to allow better oil adhesion. Not Kosher.... but I try 1/2 qt. Marvel Mystery Oil, see if the problem disappears. If it does, most likely a sticky valve.

DAR Gary
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Old 07-11-2021, 04:22 PM
blaplante blaplante is offline
Join Date: Jan 2010
Location: Southern California
Posts: 290
Default the plot ...

is that a mag check between 10:00 and 10:30? Am I reading it right that there was about 200 rpm drop on the mag check?

Looks like you pulled power once airborne and then the EGT on #1 and #2 went crazy?

Could you show us the CHT's too?
RV6A in phase 2 as of April 2016
Donation made Oct 2020
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Old 07-11-2021, 05:33 PM
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walter walter is offline
Join Date: Mar 2005
Location: Mansfield TX
Posts: 356

Not a mag check. EMag checks generally result in nearly zero rpm drop.

Electric Fuel (boost) pump on the entire flight.

Here's more information with Fuel Flow, Fuel Pressure, CHTs and Oil Pressure:

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Walter Tondu
Flying RV-8A - Fastback, IO-390, G3X Touch, 74 HRT
Prior RV-7A Build Log

Last edited by walter : 07-11-2021 at 05:36 PM. Reason: clarification.
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Old 07-11-2021, 05:42 PM
Latech15 Latech15 is offline
Join Date: Apr 2018
Location: louisiana
Posts: 181

Emags - -

I am running dual emags on a o-360 and generally see 30-40 rpm drop on one side and 50 or so on the other. Zero rpm drop would make me question grounding....
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Old 07-11-2021, 05:51 PM
walter's Avatar
walter walter is offline
Join Date: Mar 2005
Location: Mansfield TX
Posts: 356

I verified all the wire connections on both PMags after todays flight. Even pulled the retainers and made sure each set screw was tight.

As for the RPM drop I maybe get 10-20 rpm drop. Nothing substantial. Been the same on both planes I've built. (7A-8A)

I'm running a Precision Airmotive fuel injection system. On the ground I lean aggressively to get peak RPM and idle and taxi like that. Run-ups are full rich.

In the air for the first 10 engine hours I was mostly down low full power and full rich. Lately I've been flying LOP at reduced power settings.

The engine is broken in. Oil consumption is negligible. Since my last oil change at 5 hours I'm still at 7 quarts. (8 added during oil change).

This has been the smoothest running engine I've ever flown behind. Right up till the point where I commented this to my wife yesterday. I Jinxed myself.
Walter Tondu
Flying RV-8A - Fastback, IO-390, G3X Touch, 74 HRT
Prior RV-7A Build Log

Last edited by walter : 07-11-2021 at 06:03 PM. Reason: more clarification
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Old 07-11-2021, 06:40 PM
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pilotkms pilotkms is offline
Join Date: Mar 2007
Posts: 515

Another data point.
Similar setup. Lycoming/Aerosport IO360. Precision Airmotive FI. Dual Pmags - retarded 1 tooth from TDC. My fuel pressure, engine driven pump constant 25-27psi. With boost pump on 30psi. Your FP looks low.
Mag drop for runup 100 to 150 rpm on each. 700+ trouble free hours.
RV 7A RV #9700 May 2017
N325KS the Flying K
Built in SoCal KCCB, now in GA @ KPXE
750+ Hours & 7X cross the USA
OSH flyin 2018 & 2019 & Petit Jean 2019
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Old 07-11-2021, 09:48 PM
gfb gfb is offline
Join Date: Jul 2014
Location: Madison, WI
Posts: 784

This doesn't make sense but I had this once before exactly as you describe. It happened maybe 3 times total over maybe a dozen flights. No other symptoms. It was around the time I usually replace spark plugs so I did that. Has not returned (~250 hours since.)

I know, what are the chances of two plugs hiccuping on the same cylinder at the same time? Honestly it was probably unrelated but it has not come back.
RV-9A - started 01/2015, completed 07/19/2018, 500hrs 03/14/2020
RV-10 - started 04/2020
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