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  #1  
Old 09-17-2017, 12:02 PM
Azjulian Azjulian is offline
 
Join Date: Dec 2015
Location: Scottsdale, Arizona
Posts: 111
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.

https://photos.google.com/share/AF1Q...JRR2l4cGNfQ1Vn
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  #2  
Old 09-17-2017, 12:53 PM
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 6,858
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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.
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  #3  
Old 09-17-2017, 01:11 PM
Azjulian Azjulian is offline
 
Join Date: Dec 2015
Location: Scottsdale, Arizona
Posts: 111
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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.

Julian
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  #4  
Old 09-17-2017, 02:50 PM
DHeal DHeal is offline
 
Join Date: Sep 2006
Location: Windsor, California
Posts: 609
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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?
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Last edited by DHeal : 09-17-2017 at 03:09 PM.
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  #5  
Old 09-17-2017, 06:36 PM
Mich48041 Mich48041 is offline
 
Join Date: Apr 2008
Location: Riley TWP MI
Posts: 2,509
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Weigh the new carb floats. Some new ones were bad.
Check the fuel tank vent.
Try using 100LL.
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  #6  
Old 09-18-2017, 07:29 AM
Bob Kibby Bob Kibby is offline
 
Join Date: May 2008
Location: Arlington, Texas
Posts: 117
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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  #7  
Old 09-18-2017, 08:49 AM
Steve Koziol Steve Koziol is offline
 
Join Date: Sep 2011
Location: Doylestown, PA
Posts: 36
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Do you have any curves and bends in your fuel line from your fuel flow meter to your carb that are not supported with a clamp that may be causing a bit of fuel starvation? Maybe a picture of your fuel line routing may be of help in this case. An unsupported hose can bend ever so slightlyin flight that would not be evident during a ground run fuel test.

Steve
RV9A
700 Plus Hours
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  #8  
Old 09-18-2017, 09:17 AM
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 6,858
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Quote:
Originally Posted by Bob Kibby View Post
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)
This could potentially be a cause, though I think it is a low likelihood since I think the OP previously mentioned that the airplane is rather low in total flight time.
A common cause of sticking slides is accumulation of fuel dye from using 100LL. That is one of the reasons that it is important to follow the 200 recommended disassembly/inspection interval on the carbs.
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  #9  
Old 09-18-2017, 11:09 AM
RFSchaller RFSchaller is offline
 
Join Date: Oct 2010
Location: Phoenix, AZ
Posts: 2,090
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Julian, was there any spiking of fuel flow or pressure that might indicate vapor lock?

Am I reading the graph right when it looks like your MAP is dropping? Is it a cause or an effect of the loss of power. I can't figure out what would cause it to drop as a cause unless the air flow was some how restricted and that doesn't seem feasible.

Rich
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  #10  
Old 09-18-2017, 02:06 PM
TS Flightlines TS Flightlines is offline
 
Join Date: Apr 2010
Location: Ridgeland, SC
Posts: 1,859
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Julian---not sure how far Tucson is from you, but Roger Lee is there and a guru on Rotax stuff. You might give him a call.
Tom
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