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Thread: Engine starving for fuel

  1. #1
    Doug Budd's Avatar
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    Engine starving for fuel

    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 donít 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 itíll start up and run good it does it when itís hot it does it when itís cold doesnít seem to make much difference itís very intermittent . any ideas? Iím not going to fly it anymore until I get it figured out.


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  2. #2

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    Tank vents?


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    mvivion's Avatar
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    Quote Originally Posted by flynlow View Post
    Tank vents?


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    ents would be my first thought as well.

    MTV

  4. #4
    Doug Budd's Avatar
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    Quote Originally Posted by mvivion View Post
    ents would be my first thought as well.

    MTV
    Not vents, caps have goose necks and they are not plugged

  5. #5
    Doug Budd's Avatar
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    It’s hard to diagnose because it will run good for hours then start acting up then run good again.

  6. #6
    wireweinie's Avatar
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    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
    Life's tough . . . wear a cup.

  7. #7
    Doug Budd's Avatar
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    Quote Originally Posted by wireweinie View Post
    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 donít 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

  8. #8
    wireweinie's Avatar
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    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
    Life's tough . . . wear a cup.

  9. #9
    cubflier's Avatar
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    Pull carb heat when it acts up. That can give you a clue as to whether it's fuel starvation.

    Jerry
    If it looks smooth...it might be

    If it looks rough...it is!!

  10. #10
    Doug Budd's Avatar
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    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?

  11. #11
    Doug Budd's Avatar
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    1/4” lines teeing into 3/8”

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    cubflier's Avatar
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    Quote Originally Posted by Doug Budd View Post
    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
    If it looks smooth...it might be

    If it looks rough...it is!!
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  13. #13

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    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

  14. #14

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    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
    Last edited by Oliver; 08-27-2019 at 02:20 PM.
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    Drain the carb bowl and look for water. Even a little bit.
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  16. #16
    supercub's Avatar
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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.

  17. #17
    cafi19's Avatar
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    Quote Originally Posted by Doug Budd View Post
    Today I was straight and level when it happened pulled carb heat went full rich with mixture.
    ummm...aren't you supposed to lean the mixture when you pull on the carb heat?

    cafi

  18. #18
    skywagon8a's Avatar
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    Quote Originally Posted by Doug Budd View Post
    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.
    N1PA

  19. #19
    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by stewartb View Post
    Drain the carb bowl and look for water. Even a little bit.
    Yup. You water or junk in float bowl. Itís sucking it into main jet. Flush out, might have to blow backwards through main jet in carb


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  20. #20
    Doug Budd's Avatar
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    Quote Originally Posted by supercub View Post
    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
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  21. #21
    WhiskeyMike's Avatar
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    Quote Originally Posted by Doug Budd View Post
    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.

  22. #22

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    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.)
    Last edited by JWE; 08-27-2019 at 05:42 PM.

  23. #23
    www.SkupTech.com mike mcs repair's Avatar
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    Also verify the mixture arm on carb is going to full rich stop


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  24. #24
    PerryB's Avatar
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    Quote Originally Posted by cafi19 View Post
    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.
    After Monday and Tuesday, even the calendar says WTF !

  25. #25
    PerryB's Avatar
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    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.
    After Monday and Tuesday, even the calendar says WTF !
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  26. #26
    Steve Pierce's Avatar
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    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.
    Steve Pierce

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  27. #27
    Doug Budd's Avatar
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    Thanks for all your ideas! Iím 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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  28. #28
    hotrod180's Avatar
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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.
    Cessna Skywagon-- accept no substitute!

  29. #29
    skywagon8a's Avatar
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    Quote Originally Posted by Doug Budd View Post
    ... 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.
    N1PA
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    Quote Originally Posted by skywagon8a View Post
    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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  31. #31

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    Not unusual to see a little water and debris in bottom of float bowl.
    presence of some crap in the bowl may not indicate that’s the cause of your issues.
    Fly it carefully till you confirm you’ve found the problem.
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  32. #32
    Doug Budd's Avatar
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    Quote Originally Posted by Oliver View Post
    Not unusual to see a little water and debris in bottom of float bowl.
    presence of some crap in the bowl may not indicate thatís the cause of your issues.
    Fly it carefully till you confirm youíve found the problem.
    Oh I will it does get my full attention when it starts to stumble. Not so bad when Iím over pastures but over the timber it sucks. Itís been doing it for awhile and I was running out of things to try and it takes all the fun out of flying worrying if itís going to quit
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  33. #33
    Cub Builder's Avatar
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    Make sure you don't have an up and down area in the fuel line routing. I've seen this when the aircraft in question had an extra long flexible line between the gascolator and carb, so had some of the excess length of fuel line tied off to the engine mount to clean up the installation. The line was routed in such a way that it went up, then back down, then up again to the carb. That created an air trap that would intermittently stop the fuel flow and was the cause of multiple engine failures and forced landings. Just rerouting the line to eliminate the air trap was all it took to fix the issue.

    -Cub Builder
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  34. #34
    www.SkupTech.com mike mcs repair's Avatar
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    One other thought is a sticking intake valve or intake valve spring


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  35. #35
    skywagon8a's Avatar
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    Quote Originally Posted by mike mcs repair View Post
    One other thought is a sticking intake valve or intake valve spring
    I've had that happen. The intake valve suddenly jammed open which backfired through the intake snuffing out the engine. Gets very quiet.
    N1PA

  36. #36

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    While the cowling is off grab the airbox and twist to see if the carburetor bowl is loose. If the carb halves are loose---fix it. Marvel Scheblers can get weird if the parts are not tight and it seems no two carbs act alike. One might idle 200 RPM faster and refuse to quit at idle cutoff and the next one might lope at cruise. This is my first place to look.
    You can't get there from here. You have to go over yonder and start from there.
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  37. #37

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    So raise the tail to level flight and check the GPM flow at the carb fuel line , then sump then, carb bolt drain plug, do it with the left, right both. Rotate the fuel selector and study the flow. rule out the fuel system. Finger screens? GPM flow should be reasonably the same at all three stations, sump, fuel line , sump bowl. Sticking bowl float? wouldn't fly it till I found the smoking gun. I I would think you should be flowing 17 oz. per minute plus or over 8 plus gallons per hour by gravity.Max GPH for a O 200 ? got have the same in gravity flow rate.
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  38. #38
    skywagon8a's Avatar
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    Quote Originally Posted by Fortysix12 View Post
    So raise the tail to level flight and check the GPM flow at the carb fuel line ,
    You want to do a flow test with the plane in the most adverse position. That means tail very low at the very least. Preferably done on a slope with the nose high.
    For this particular purpose of flushing the carb, just all wheels on the ground will work.
    N1PA

  39. #39

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    Even better and I agree there Mr. Skywagon8a. Just thinking about establishing a base line for level flight. But you do make a good point.
    On a side note, Every super cub I have ever flown including mine will run out of gas at full continued climb tilt.

  40. #40
    Doug Budd's Avatar
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    Well I put a rebuilt carb on my cub . Two trips checking water hasnít quit . That fixed it ( I hope)


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