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Engine stumbled on take off

J

jdelpeso

Member
Hi all,
Some seconds after applying full power for take off the engine stumble, there were a low fuel pressure warning and hi fuel flow (10gal/hr). I haven?t calibrated the fuel flow sender yet.
I have performed some ground checks and after reaching 4,300 rpm happens the same: rough engine that quit if I don?t reduce power, low fuel pressure and hi F/F
This was going to be the second flight of this RV. Since the first flight I fixed a couple of fuel tank leaks and change twice the carb floats.
During days I have ground checked the engine with no issues. Yesterday I added 15 gallons of Chevron auto gas. I wonder if auto gas in bad conditions can cause this situation.
Open for suggestions
Thanks

Jose del Peso (120633)
Boca Raton, FL
 
Here are some things to check:
Obstruction in fuel line, especially upstream of electric pump.
Bad engine driven fuel pump.
Carburetor problems.
Boiling fuel aka vapor lock.
Missing return line orifice.
 
Thank you Joe for your answer
The engine has worked well until today
If the restrictor orifice for the return line is missed, this can be the problem (loss of power, low fuel pressure and hi fuel flow)
I have checked the page 46-09 of the KAI and I see no indication of where is the restrictor orifice and how to check if it is in place or not
Jose
 
It seems to me that the orifice is located in the hose and it matters if the hose is installed backwards. Someone with a better memory will answer.
 
Mich48041 said:
It seems to me that the orifice is located in the hose and it matters if the hose is installed backwards. Someone with a better memory will answer.
Click to expand...

Correct, it is in the fuel return/fuel pressure sensor hose.

If it is installed incorrectly, it is impossible to get proper system fuel pressure... OP said engine has worked fine up until this point so it likely is installed correctly.
 
Last edited:
rvbuilder2002 said:
Correct, it is in the fuel return/fuel pressure sensor hose.

If it is installed correctly, it is impossible to get proper system fuel pressure... OP said engine has worked fine up until this point so it likely is installed correctly.
Click to expand...

Scott, did you mean to say if it is installed INCORRECTLY?
 
Tomorrow I will go to the airport and check that the fuel return assembly is installed properly (brass fitting in the return line). Initially I don?t think this is the problem because the engine has been ok in the first flight and ground test.
My last thoughts are:
-At 4300 rpm the fuel pressure is in the green arc and the F/F was around 6 or 7 gal/hr. Tomorrow I?ll get the exact numbers.
-More than 4300 rpm the F/F increase to 10 gal/hr and the engine quits.
-Which is the path of this fuel?
-I think that the increase in fuel comsuption goes to the carburator and then to the combustion chamber with an air/fuel mixture that doesn?t explote.
-My question for the engine gurus is, Can the carburator work in a way that when the throttle is full open produces an incorrect increase in fuel flow?. I have just changed the floats, the good ones
-Can some debris in the carb produce this F/F increase?
Jose
 
Engine 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 to much oil on the air filter and bad (old) auto fuel are examples I am aware of.
Bob Kibby N712BK (255 hrs)
 
At 4300 rpm the fuel pressure is in the green arc and the F/F was around 6 or 7 gal/hr
Click to expand...
There might be two problems. First of all, the fuel flow indication is way off. The actual fuel flow at 4300 rpm is probably around 4 gal/hr. If it really was 6 or 7 gal/hr, then fuel must be leaking someplace. The "K" factor in the Dynon needs to be changed.
The second problem is the engine quitting. On page G4-1 of the PRODUCTION ACCEPTANCE PROCEDURES is a test to time how long it take to pump one gallon of fuel. Repeat that test. Years ago I had a vehicle that ran fine at slow speeds but stalled at higher speeds. The problem turned out to be a clogged fuel filter. Maybe there is an obstruction in the fuel system of your RV-12.
 
Check float bowl gaskets

Bob Kibby said:
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 to much oil on the air filter and bad (old) auto fuel are examples I am aware of.
Bob Kibby N712BK (255 hrs)
Click to expand...

Last fall, while doing some short (Young Eagle) flights, the engine on N73HR started running extremely rough, and wouldn't get above 3100 RPM. Thinking it might have been heat-related, we checked it again on a cool morning. Same problem. We had it out of service until recently (the world's slowest home-brew paint job), after which we were overdue for an annual. When I pulled the carburetors, everything looked very clean, but the right carb's float bowl gasket was severely askew. After replacing the gasket, things were back to normal. Reviewing the carb rebuild videos on Rotaxowners.com, there was one terse statement: Make sure the float bowl gaskets are making a good seal--otherwise the engine may run rough", or words to that effect. My guess is that the mis-aligned gasket finally developed a leak during the YE flights.
You might pull the float bowls and check the gaskets--worked for me.
Other folks comments/rebuttal would be appreciated.
 
After removing the cooling, the spring between the carb and the clamp in the
compesating manifold tube wasn?t attached. After fixing the spring the engine run well. I don?t feel totally confident that the spring was the only cause of the problem because I disattached the spring and the engine run perfectly.
For those of you that knows about engine and carbs my question is:
Can the vibration in the carb, without the spring, produces a movement in the floats and other parts of the carb that increases the fuel flow to 10gal/ hr and reduce the fuel pressure?
Jose
 
Just to finish the thread.
The problem was the engine fuel pump.
The lesson for me: low fuel pressure and hi fuel flow check the pump
Thank you
Jose del Peso (120633)
 
rvbuilder2002 said:
Correct, it is in the fuel return/fuel pressure sensor hose.

If it is installed incorrectly, it is impossible to get proper system fuel pressure... OP said engine has worked fine up until this point so it likely is installed correctly.
Click to expand...

Is this orifice an integral part of the hose assembly? Doesn't that hose have a banjo bolt fitting on one end and an AN fitting on the other? Is it possible to install it incorrectly?
 
tomkk said:
Is this orifice an integral part of the hose assembly?
Yes

Doesn't that hose have a banjo bolt fitting on one end and an AN fitting on the other?
The hose with the orifice is the one that connects to the distribution block, the fuel pressure sensor and the fuel return line fitting on the firewall.

Is it possible to install it incorrectly?
It is possible, but if the KAI is followed it should not happen, the installation step has you double check that the orifice is in the hose that connects to the return line fitting.
Click to expand...

If this hose is installed incorrectly, it should not be possible to get minimum fuel pressure with the electric fuel pump on.
 
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