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Engine starving for fuel

Doug Budd

MEMBER
Crawford Nebraska
I have a experimental cub with an o-200, with two 12 gallon wing tanks. It will starts running rough and quits then it will pick back up and as long as I don’t give it to much throttle it will maintain. I land in an alfalfa field or pasture or wherever and sump for water and there is none let it sit for a while it’ll start up and run good it does it when it’s hot it does it when it’s cold doesn’t seem to make much difference it’s very intermittent . any ideas? I’m not going to fly it anymore until I get it figured out.


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It’s hard to diagnose because it will run good for hours then start acting up then run good again.
 
Feed from both tanks or left/right selector? Are the goose necks on the caps open tubes or do they have valves in side of them? Fuel level make a difference?

Get a set of caps with no goose necks and go fly them. See if there is a change. Look for any fuel hoses that have sharp bends in them. Maybe kinking under the right conditions.

Web
 
Feed from both tanks or left/right selector? Are the goose necks on the caps open tubes or do they have valves in side of them? Fuel level make a difference?

Get a set of caps with no goose necks and go fly them. See if there is a change. Look for any fuel hoses that have sharp bends in them. Maybe kinking under the right conditions.

Web
Feeding from both tanks goose necks are open tubes. Fuel levels don’t make a difference. Last time I had 3/8 tanks today I had 3/4 each side. No header tank but the wing tanks feed from front and back
 
Straight and level flight or while maneuvering? Diameter of fuel lines?

Have you verified that it's a fuel problem? Maybe ignition cutting out under load?

Web
 
Pull carb heat when it acts up. That can give you a clue as to whether it's fuel starvation.

Jerry
 
Today I was straight and level when it happened pulled carb heat went full rich with mixture. It helped for a bit . I don’t have a primer so I can’t give it a shot to see if that helps. Sometimes it does it when I add power to climb, but not every time. Could it be the carb messing up?
 
Today I was straight and level when it happened pulled carb heat went full rich with mixture. It helped for a bit . I don’t have a primer so I can’t give it a shot to see if that helps. Sometimes it does it when I add power to climb, but not every time. Could it be the carb messing up?

If pulling carb heats makes things better then fuel starvation of some sort is probable. The two instances that I was involved in were vent and kinked line issues. In the cub with the tank vent issue it was obvious because the tank collapsed. In the kinked line issue the previous owner was trying to kill his ex wife. He kinked the fuel line out of the right header tank and disguised it by putting a long piece of fuel line to round the conner so it would not be noticed. I bought the cub in a divorce sale. It was one of her assests and the ex husband was the mechanic. In both cases carb heat made the problem better. Each instance caused an engine out.

I'm not sure on the carb being the problem but I would suspect everything that provides fuel and keep a runway close by when flying.

Jerry
 
Damn Doug. I hope you haven’t been sneaking over to the neighbor lady’s and your wife is the kind that can swing a wrench:eek:
 
Spare you and your plane some excitement and experiment by:
Level the plane to flight attitude, remove fuel line at carb, put a gallon (or bigger) plastic tub under fuel line, turn on fuel and let it drain for awhile.
see how long it takes to fill a gallon. Do some math and figure gph flow, it should be at least double what your max rated fuel burn is at wot
if there’s a vent issue may need to drain a couple gallons to create vacuum situation in tanks.
I assume fuel flow will slow down noticeably to indicate vent issue.
If not enough flow, work your way back up the fuel system, strainer, valve, tank strainers, vents etc..
Possible that issue is with carb too, strainer, crap in bowl..

this is will at least eliminate fuel issue if all checks out

Doug
 
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I'm building up a J-3 converting to a PA-11, I'm putting the bi plane fuel tanks in. Two tanks approx 12 gals each. I'm not sure how your system is plumbed, but on mine, I installed a fuselage quick drain for each line just behind the baggage area. I noticed that this was a low point in the system and could possible be a tap for water.
 
1/4” lines teeing into 3/8”
This should get your attention. Can you disconnect the fuel line from the carburetor, place it in a 5 gallon gas can and fill the can in 15 minutes or less? That would be 20 gallons per hour. 1/4" for a fuel line is likely way too small. Generally you will find 3/8" as a minimum.
 
I'm building up a J-3 converting to a PA-11, I'm putting the bi plane fuel tanks in. Two tanks approx 12 gals each. I'm not sure how your system is plumbed, but on mine, I installed a fuselage quick drain for each line just behind the baggage area. I noticed that this was a low point in the system and could possible be a tap for water.
That is where two of the drains are
 
Today I was straight and level when it happened pulled carb heat went full rich with mixture. It helped for a bit . I don’t have a primer so I can’t give it a shot to see if that helps. Sometimes it does it when I add power to climb, but not every time. Could it be the carb messing up?


I had this problem on a modified J-3 with no primer. It would quit on take off every time. Because it had an old fashioned glass sump we could see an air bubble in the top of the fuel sump, so we cracked the pipe plug open and bled the air out. End of problem.
 
I too had a similar problem on a Cub that i previously owned. Turned out there was debris (small broken pieces of pine straw) lodged in the fuel selector valve. The engine would only sputter occasionally. As mentioned previously, disconnect the fuel lines and figure out how much fuel flow you are actually getting at the gascolator. I believe the FAA requires 1.5 times the maximum fuel flow required at takeoff. Mine would run fine on the ground, but start to sputter on takeoff. I removed the valve and replaced it with a new one. I still have the old one. A picture below tells the tale. Good luck and be careful....


(Tried to attach a picture, but unable.)
~
 
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ummm...aren't you supposed to lean the mixture when you pull on the carb heat?

cafi
Normally yes, but in this case (suspected starvation) pulling carb heat and increasing mixture is being done as diagnostic measures to richen the mixture. If engine performance improves, you're on the right track.
 
Does it have tank strainers? A couple years ago a good friend went down in our old Breezy due to plugged tank strainers. He'd got a smokin' deal (free) on some avgas from an insurance claim, but put it in a crappy old drum and almost paid a very high price.
 
My Dad just had a Champ he maintains doing this. Had trash in the carb bowl and was stopping up the seat periodically. Had a Clipped wing Cub that did the same thing and someone had sloshed the nose tank and a flapper of the sealant would stop up the tank outlet. I'd look at the carb and if no smoking gun put it in level flight attitude and do fuel flow test and shake the wings while doing it.
 
Thanks for all your ideas! I’m leaning towards the carburetor having something in it because that makes the most since because it runs good for hours then acts up then runs good again. but I am going to check the fuel flow rate.


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I had something like this happen on one flight in my old C170,
and it turned out to be a clogged carburetor inlet screen.
I'd check that first, then while the line is off check the fuel flow rate.
 
... I’m leaning towards the carburetor having something in it because that makes the most since because it runs good for hours then acts up then runs good again.
When you pull the drain plug on the carburetor bowl, make sure that you let the gas run in a strong stream for a period of time. This will give you a maximum amount of flushing throughout the bowl cleaning out any little piece of contamination. That is unless there is something caught in the discharge nozzle.
 
When you pull the drain plug on the carburetor bowl, make sure that you let the gas run in a strong stream for a period of time. This will give you a maximum amount of flushing throughout the bowl cleaning out any little piece of contamination. That is unless there is something caught in the discharge nozzle.

Make an effort to catch what comes out of carb when you drain bowl. Be nice to have evidence to support that as cause.
 
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