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Old 09-17-2017, 12:02 PM
Azjulian Azjulian is offline
Join Date: Dec 2015
Location: Scottsdale, Arizona
Posts: 146
Default Plagued by engine stumbling...

I have posted about this before and I thought that I was past the issues, but they seem to have started again. At full throttle I occasionally get an engine stumble, this happened again today twice just after take off and led me to abort the flight.

I have done the following

- taken both carbs completely apart, cleaned with air and carb cleaner
- checked all my fuel lines, at each connection I have run a volume of fuel over time test and I have run several gallons through the lines
- checked my fuel bowls for any contamination several times
- re-synchronized my carbs both mechanically and pneumatically.

The problem had all but disappeared until I experienced it at alititude (9500') on a trip to Payson recently. I confirmed my engine was on the float exchange service bulletin and I swapped all my floats out (one side was 7.7g). But after the float exchange problem seemed to get worse...

As you can see in the image below, when the issue happens its preceeded by the left EGT going 100F hotter than the right (this always happens when you look at the logs).

I also should point out that my static WOT RPM and my climb RPM are pretty much the same, and in climb I seem to lose 50 or so RPM as I climb out, that to me is wrong, but I cant seem to trace any cause and my CHTs and EGTs are cool...

At this stage the only thing I can think of doing is trying to swap out carbs, that it is some strange carb related issue... Can anyone help ? Please I have to say this has totally sapped my enjoyment of an otherwise perfect plane.
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Old 09-17-2017, 12:53 PM
rvbuilder2002 rvbuilder2002 is offline
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 8,054

Even though you said you cleaned the carb's, my first suspicion would be a small flake of some type of contamination in the left carb. intermittently causing a change in fuel delivery which makes that bank of cyl go lean.

I have seen this type of problem before, where it happened only intermittently and at totally random intervals and flight situations.
Any opinions expressed in this message are my own and not necessarily those of my employer.

Scott McDaniels
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")
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Old 09-17-2017, 01:11 PM
Azjulian Azjulian is offline
Join Date: Dec 2015
Location: Scottsdale, Arizona
Posts: 146

Ok I'll take that carb apart and go through everything with a magnifying glass, if there is something it's incredibly stubborn and yes very random and very frustrating.

It seems like a big problem at the time with the change in engine sound but when you look at the logs the impact seems very small in terms of rpm etc. but very scary.

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Old 09-17-2017, 02:50 PM
DHeal DHeal is offline
Join Date: Sep 2006
Location: Windsor, California
Posts: 773

Perhaps a loose flap of liner material within the carb fuel supply hose intermittently interfering with fuel flow? Are your carb bowl vent lines properly routed between the side of the float bowl and bale and not blocked or kinked? Remove the air filter, reach in with your finger, and see if the carb piston is free and has no sticking tendencies? Could a periodic vacuum or back pressure somewhere be influencing your fuel flow to that specific carb? Sounds like you have checked most everything. This is a toughy!

ps -- Have you run your EGT plot past Mike Busch at Savvy Aviation?
David Heal - Windsor, CA (near Santa Rosa)
EAA #23982 (circa 1965) - EAA Technical Counselor and Flight Advisor; CFI - A&I
RV-12 E-LSA #120496 (SV w/ AP and ADS-B) - N124DH flying since March 2014 - 740+ hours (as of Mar 2019)!
VAF donation through June 2019.

Last edited by DHeal : 09-17-2017 at 03:09 PM.
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Old 09-17-2017, 06:36 PM
Mich48041 Mich48041 is offline
Join Date: Apr 2008
Location: Riley TWP MI
Posts: 2,780

Weigh the new carb floats. Some new ones were bad.
Check the fuel tank vent.
Try using 100LL.
Joe Gores
RV-12 Flying
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Old 09-18-2017, 07:29 AM
Bob Kibby Bob Kibby is offline
Join Date: May 2008
Location: Arlington, Texas
Posts: 127
Default Similar Power Loss

Part of my similar power loss was attributed to sticking carburetor pistons. As I understand it, the pistons move to control carburetor inlet air as the diaphragm moves in response to air pressure changes. The piston motion can be checked by removing the air filter and sticking your finger in the inlet and lifting the piston up and down. The pistons should move freely with very little force. The carburetor air vent lines should end in the same relative location for each carburetor so that each carburetor is sensing the same ambient pressure is another suggestion. Not sure of all the things that can cause sticking pistons but excessive oil on the air filter and bad (old) auto fuel are examples I am aware of. I had EGT differences as high as 200 degrees and reduced power (4,300 rpm) during my episode.

I sent my carbs to Lockwood for the 200 hr service and haven't had any further problems. I also started adding Stabil to my premium auto fuel since I had the power loss problem.

Bob Kibby N712BK (355hrs)
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