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Plagued by engine stumbling...

Azjulian

Well Known Member
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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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.
 
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
 
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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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)
 
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
 
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.
 
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
 
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
 
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

I have the same question - I don't understand how a fuel flow obstruction could cause such a remarkable drop in manifold pressure. It is my understanding that a loss of RPM due to fuel or spark should cause a rise in MP (the same way it behaves when you cycle a constant speed prop during runup).

UPD: oops, forgot it's turbocharged.
 
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Engine Stumble

I had a problem with engine stumbling that did not show up during any run ups or such...turned out I had a couple intermittent bad plug wires...might not be a carb problem. Only showed up in cruise flight after several minutes in the air.
 
Guys,
I own an S-LSA with a 912 that I fly on long cross countries fairly frequently. I have experienced stumbles several times over the past couple years. Most recently on my trip to OSH this year, I ended up having to abort and return to my home airport because the stumbling got to the point that I thought I may have to make an off airport landing. My mechanic took everything apart and cleaned it all out, changed the fuel filter and it seemed to run fine. I flew it in the AirVenture Cup race and into Fond du Lac and parked it for the week of the show. On pre-flight to leave, I found the right carb literally leaking gas out the back of the carb and the scat hose that goes to the airbox. I left the plane on the ground and had the guys from Leading Edge Airfoils come up from the Milwaukee area come up and take the carb off, replace all floats, do the complete rebuild with all new parts and replace all hoses. It ran great on the flight home from there to New York.

Bottom line is that despite the fact that I had replaced the floats per the SB earlier, the new floats were actually bad as tested by the LEAF guys. My inclination is to say it is a float related issue.

Carl
 
I have the carb stripped down on the bench, going through everything very carefully right now.

I'll update towards end of the week, when I've had a chance to do the work

I did measure the floats and they are good

I'm running 100LL have been since the issues started just to ensure a good fuel supply.
 
I experienced the same problem but with a low fuel pressure and hi fuel flow.
Changing the engine fuel pump solved the problem.
Jose del Peso 120633
 
Julian,

If you take the plane to Roger and need a ride back from Marana or you'd like a ride down and back to drop off your carbs give me a call.

I'd loan you mine, but I just spent two months and big $ getting them right, and they have AN fuel fittings as opposed to the banjo fittings.

Rich
 
Don't look past the restrictor that was removed from the return line. I've been told this solved several rough running engine problems. Rotax has it in there for a reason.
 
Not directly related so forgive me if this is thread drift.

During my recent carburetor woes with the shop swapping my choke assemblies I came across a blog by a Brit ROTAX Repair shop that suggested adjusting the idle mixture screw to 2 turns from full clockwise as opposed to 1 1/2 turns gives a smoother idle. Has anybody tried this, and if so did it work?
 
I?ve checked the return both by validating the hose is correct and that return fuel is seen in the filler neck.

I am running 100LL to help eliminate fuel contamination.

Hopefully I will be able to clean and test tomorrow.
 
SOLVED!

I have to give all the credit to Scott for making me recheck my work to clean the carbs. I dissembled and with a clean white rag in place blew compressed air into all the carb ports.

I stuck the air directly over the fuel inlet port and blew from both sides, finally I was rewarded with this on my rag !

OjGZsw5p5Q6at9Xq1


I have no idea what the origin is, under a magnifying glass it looks like wood....

Flew the plane to Payson and back from DVT yesterday afternoon and the engine was great the entire trip (9500?). Off topic but Payson was 19knots Peak gusting 25, it was bumpy coming in to land but after adding half the gust to my final speed very smooth touchdown.

Scott appreciate your advice on solving this one, crazy that was the 3rd time I blew out that port. I know that what I should have done is measure fuel flow to the carb bowl as well as all my line tests, that would have shown the restriction. I did visually check the flow there but I didn?t measure the volume as I had everywhere else.
 
I have to give all the credit to Scott for making me recheck my work to clean the carbs. I dissembled and with a clean white rag in place blew compressed air into all the carb ports.

I stuck the air directly over the fuel inlet port and blew from both sides, finally I was rewarded with this on my rag !

OjGZsw5p5Q6at9Xq1


I have no idea what the origin is, under a magnifying glass it looks like wood....

Flew the plane to Payson and back from DVT yesterday afternoon and the engine was great the entire trip (9500?). Off topic but Payson was 19knots Peak gusting 25, it was bumpy coming in to land but after adding half the gust to my final speed very smooth touchdown.

Scott appreciate your advice on solving this one, crazy that was the 3rd time I blew out that port. I know that what I should have done is measure fuel flow to the carb bowl as well as all my line tests, that would have shown the restriction. I did visually check the flow there but I didn?t measure the volume as I had everywhere else.

Looks like a biological to me.
 
Looks like a biological to me.

I have triplet granddaughters - 8 years old. Wife is constantly cleaning snots out their noses. They learned to run from her. I bet my wife can identify foreign material in your carb... :D
 
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