My Citabria can't take off...

Swede

Swede

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Swede
:( From my intro thread... buddy and I rebuilt a 7ECA from the bones up. We installed an O-320 via STC, a pretty 0 SMOH engine. Added 10 gallons of fuel, she was ready to fly.

My A&P buddy who did 90% of the work taxied out, added power for the maiden flight, rolled about 100' and the engine rolled back, WAY back, coughed once or twice.

He did another high power runup... same deal. The engine would not sustain TO power. You could taxi all day, though. Thank God he did not get airborne. He'd have gone down straight ahead.

Our best guess was that the tanks were not venting properly. We checked the venting - it seemed OK - but just to be sure, we are going to use vented fuel caps for now to be on the safe side. The wings came with unvented caps.

Next step, checked fuel flow and cleanliness from the gascolator feeding the carb. Pretty bright blue fuel, seems clean.

Today, I am going to disconnect the fuel line from the carb and drain about 5 gallons of fuel, time it to see if fuel flow from the entire system is appropriate.

Any other thoughts? Besides fuel, it MIGHT be air. Inadequate air flow would result in an extreme over-rich condition at high power settings.

Could mag timing cause power loss at high power settings?

Right now, my bird is entirely unsafe to fly until we can correct this problem. Inputs HUGELY appreciated. Thanks!
 
Mixture control on the carb set way wrong. With the mixture full forward, you should be able to run the engine up to whatever the max RPM is specced for. I"m guessing 2650 or so. I'd look into the carb after checking fuel flow delivery and pressure.
 
Swede said:
Snipped.
Our best guess was that the tanks were not venting properly. We checked the venting - it seemed OK Inputs HUGELY appreciated. Thanks!
Click to expand...

You may have a vapor lock in one tank fuel line.

While you have the line off the carb, blow air back to each tank, allow the fuel to drain down again.

The Fairchild 24 was famous for doing this after the tank has been run dry and filled again.
 
Absent misfiring it's got to be fuel or air. Vapor lock or perhaps the fule line is constricted somehow. Another possibility is an air leak into the fuel system. Also check the induciton air in. Could be scat hose collapsing or a blocked air filter.
 
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Ten gallons for an engine that burns at least ten an hour at full throttle?

That airplane has a one-gallon unusable fuel level. It should have fuel outlets at the front and rear of each tank. If, for some silly reason, it has only outlets at mid-tank, it could be unporting during acceleration with so little fuel aboard. Unusable fuel would be a lot bigger in that case.

Vented caps don't belong on the Citabria. It has a vent system. Low pressure atop the wing, combined with the cap shape, could cause suction in the tank and hold the fuel back. You might be worse off.

Do the tail-low fuel flow test. Get the nose up into the climb attitude, hold the brakes and see if it develops full power and holds it. See the TCDS or the STC for the RPM range for that engine and prop combination. Might have some restriction somewhere, maybe a bunch of crud in the strainer already from the rebuilding/cleaning process.
 
Great stuff fellows, many thanks. The aircraft had 20 gallons of fuel before the initial attempt, so QTY wasn't an issue. Apologize for my initial misleading post.

My Buddy thinks we vented improperly during the rebuild. But we checked it... with the gas caps off one at a time, we blew gentle air into the vent on the left wing. We could feel air flowing freely into the tanks, by simply holding a hand over the open cap, checking the breeze. I can't imagine the vent check valves are at fault.

Taking those fairings off is a PITA; if the tanks are venting, there's no need.

We will try blowing air from the fuel line at the carb, per Tom-D's post. Vapor lock is possible. The Citabria fuel system is different, quite lengthy. Both tanks drain into a common manifold behind the baggage area, thence forward to the firewall gascolator. I drained that this morning, nothing but pretty blue fuel.

We'll keep plugging. TY all agin for the inputs.
 
I didn't want to bring this up but vapor lock is a function of heat and pressure. A fuel system on an aircraft, using aviation fuel is about the last place I would expect fuel vaporization at near sea level in temps hovering in the high 80s. The physics just isn't there to support this with fresh avgas.

I still vote for a metering problem. Main jet, mixture, fouled emulsion, etc.
 
Your mixture is too lean. Either you have the carb jetting set incorrectly or you have an intake leak somewhere.

Keith
 
I'll second the advice to not use vented caps. All that is going to accomplish is to put ugly blue stains on top of your pretty new wings and cause you to run out of gas way before you think you should.

I don't want to fault your mechanic but after a complete rebuild like this a couple of things:

That timed/measured gravity flow out of the main fuel feed to carb should have already been done as well as a full static power run up prior to anyone rolling down a runway.

What is the history on your carburetor? Recently overhauled? New?
 
TY all again, this is a great forum! Here's what we just accomplished - first off, the stock NON-vented caps were used in all cases.

1) All fuel test points were drained/checked; pretty blue 100LL, no dirt, no water. Including the belly drain, the lowest point in the system.

2) with caps ON, we detached the fuel line from the gascolator, and from there, drained ALL of the fuel using clear tubing. There was some very slight air in the line at times, but it was miniscule. All 15+ gallons drained within 10 minutes, meaning the fuel system itself is venting and delivering 5 to 10X what the O-320 would demand at full throttle.

Conclusion: Fuel system to gascolator was OK.

So from there, it has to be the fuel line to the carb, or the carb itself.

The carb is brand new, was attached and test run during the major OH. All went well there, so for the moment, I am assuming the carb is OK.

The fuel line between gascolator and carb: looked shoddy. Didn't even look aircraft, looked more like something from a semi truck. Blowing through it, and comparing the back pressure to a proper aircraft fuel line, there was a difference. The crap fuel line had significantly more back pressure.

So right now, we are fabricating a proper fuel line. If that's the problem, it speaks volumes... an 8" line that is incorrect can bring an airplane down.

If the new line doesn't fix it, the next step is the carb. We checked the throw of the mixture line, and it goes stop to stop on the carb body. Assuming the OH folk did their job, it should be set correctly. We'll see.

:)
 
Swede said:
Next step, checked fuel flow and cleanliness from the gascolator feeding the carb. Pretty bright blue fuel, seems clean.
Click to expand...

Swede said:
So from there, it has to be the fuel line to the carb, or the carb itself.

The carb is brand new, was attached and test run during the major OH. All went well there, so for the moment, I am assuming the carb is OK.

The fuel line between gascolator and carb: looked shoddy. Didn't even look aircraft, looked more like something from a semi truck. Blowing through it, and comparing the back pressure to a proper aircraft fuel line, there was a difference. The crap fuel line had significantly more back pressure.

:)
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:dunno:

Hard to help accurately when the description of the conditions change.

Hope you got it fixed, and it all works out. Enjoy the new plane.
 
Pull that flexible fuel line and inspect the inside. You may find a flap of rubber that is blocking the flow. That would explain running at low RPM but not high RPM.
 
docmirror said:
:dunno:

Hard to help accurately when the description of the conditions change.

Hope you got it fixed, and it all works out. Enjoy the new plane.
Click to expand...

I don't feel like I miscommunicated here. If so, sorry. The initial thought was that the tanks weren't venting. They are.

What we did was detach the AN-8 flare fuel line, 8" or so, between the carb and the gascolator. We attached a clear tube to the gascolator, and checked fuel flow, clarity, etc, by draining both wing tanks into waiting fuel containers. This was to check venting and flow. Both seemed fine.

We then added 5 gallons per side, 10 gal total, then drained another 5 gallons total to ensure no trapped air. This left 2.5 gallons per tank. The wing tanks were then topped of to 1/2 full each.

So, whatever is wrong, is between gascolator, and the engine. We're making a new fuel line. If that doesn't work, it's the carb.

bnt83 - TY we'll ensure there is no obstruction in the carb itself.
 
Well, you got distracted by venting, when that had nothing to do with anything fuel supply related.

What you wrote is that you sampled at the line feeding the carb. But what you actually did is sampled at the gascolator port feeding the line TO the carb.

Feeding "the carb" means that the carb is the next thing the fuel sees, not another piece of pipe/hose.

Anyway, glad you found it, and hope it all goes well from here.
 
I have gotten a bad carb from (a rebuilt) from Kelly Aerospace. This was confirmed by a third party carb shop we sent to have it checked out. So, don't rule that out. Mine would quite at a certain RPM.
 
The carb bowl has enough fuel to make full power for a while so venting isn't the issue. I'd drop the bowl to see if there's crud or water in there and to verify the float is properly operating and that fuel flows into the bowl when the float is down. And that the leaning mechanism operates to full rich. Easy stuff first.
 
Thanks fellas, I'm just glad to know that the aircraft is delivering fuel properly. The thought of tearing it down to get at the fuel manifold(s) behind the baggage compartment = major nightmare.

It's a lot easier knowing the problem lies somewhere between the firewall mounted gascolator, and the intake manifold. If it's not the flex 37 degree flare fuel line, it's the carb itself. We shall see.
 
docmirror said:
I didn't want to bring this up but vapor lock is a function of heat and pressure.
Click to expand...

If you unport a fuel tank on a Fairchild 24 or a 48 170, you'll find a different way of getting a vapor lock.
 
Swede said:
Thanks fellas, I'm just glad to know that the aircraft is delivering fuel properly. The thought of tearing it down to get at the fuel manifold(s) behind the baggage compartment = major nightmare.

It's a lot easier knowing the problem lies somewhere between the firewall mounted gascolator, and the intake manifold. If it's not the flex 37 degree flare fuel line, it's the carb itself. We shall see.
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you may have cured your problem by simply allowing full flow, this will eliminate the vapor lock (air in the system)
 
Tom-D said:
If you unport a fuel tank on a Fairchild 24 or a 48 170, you'll find a different way of getting a vapor lock.
Click to expand...

I suppose if the suction were enough from the pump, and the fuel system were completely sealed, it might be possible to reach the RVP of avgas on a hot day. It would take a lot of suction by the pump, and a well sealed fuel system to get the head pressure down that low.

Does 'unport' mean to seal the tank vent system?
 
Count me as another who had a problem with a newly overhauled carburetor. Tried everything to get it to work then called the overhaul facility. They said "Nope, that's not right, send it back". They sent me another carb and it worked perfectly. Never did find out exactly what the issue was with the first one but it can be as simple as a bit of missed debris lodged somewhere inside in the right place.

In your case, considering you say it tested fine initially but now you've found a shoddy fuel line I'd check the strainer and drain the bowl at least.
 
Good news... she flies! The problem was a partially blocked flex line from the gascolator to the carb. After removing it, I huffed air into it and compared it to a proper fuel line. The back pressure was obvious. It didn't even look like an aircraft-grade fuel line, more like something from a Mack truck. Into the dumpster it went.

I appreciate all the help here, fellas. By running a number of tests, it eased my mind that yes, the fuel system is OK, as is the carb.

Here are details and pics of the rebuild, on this forum.

http://www.pilotsofamerica.com/forum/showthread.php?p=1897196#post1897196
 
On point that concerned me about your original post, was that the TO roll was started...

A professional test pilot once told me he wouldn't take off with an airplane (for a test flight) that he hadn't done a ground run up at full power for at least three minutes. His reasoning was that at three minutes he could be in a position to land back at the airport.

I always do full power runup when test flying an airplane, for at least the minimum time I think I would take to get to a position where I could land back at the airport if I lost power.

Brian
 
Swede said:
Good news... she flies! The problem was a partially blocked flex line from the gascolator to the carb. After removing it, I huffed air into it and compared it to a proper fuel line. The back pressure was obvious. It didn't even look like an aircraft-grade fuel line, more like something from a Mack truck. Into the dumpster it went.

I appreciate all the help here, fellas. By running a number of tests, it eased my mind that yes, the fuel system is OK, as is the carb.

Here are details and pics of the rebuild, on this forum.

http://www.pilotsofamerica.com/forum/showthread.php?p=1897196#post1897196
Click to expand...

Thanks for the report. Have fun!
 
brcase said:
On point that concerned me about your original post, was that the TO roll was started...

A professional test pilot once told me he wouldn't take off with an airplane (for a test flight) that he hadn't done a ground run up at full power for at least three minutes. His reasoning was that at three minutes he could be in a position to land back at the airport.

I always do full power runup when test flying an airplane, for at least the minimum time I think I would take to get to a position where I could land back at the airport if I lost power.

Brian
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3 minutes... on the ground... at full power...

Hey, feel free to cook your engine. It's not like engine temps are important anyway.

Engine temps before the roll are directly related to the engine temps on the initial climb.
 
I'm no mechanic, but I'm guessing there's a happy medium between 3 minutes at full power, and simply taking it on the roll.

After replacing the fuel line, we did a static run up at about 90% for a minute... I was afraid of going full power on this without someone on the tail. I am new to taildraggers, and was a bit concerned and tentative.

When my buddy finally did launch, he did another run up. With no hiccups, off she went. My heart was in my throat for 500' or so!
 
Swede said:
... I was afraid of going full power on this without someone on the tail. I am new to taildraggers, and was a bit concerned and tentative...
Click to expand...

With the stick in your nutsack there is no possibility of nosing over unless you get a microburst from the tail.
 
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