Carb leak '74 Archer

Found another squawk on my plane- Engine off- I advance mixture lever to rich, or, turn on electric fuel pump, fuel starts dripping out of the throttle bore, past the carb heat damper and into the bottom cowl. Is this typical of old Marvel-Schebler 0-360 carbs? Could this be a bad float bowl valve or fuel jet? Engine runs great in flight. I'm wondering if this affects the mixture and fuel efficiency in flight.

'74 Archer- New Owner, Student

Comments

  • Fuel pouring or dripping from a carb is not normal. Raw fuel under the cowling is a recipe for a fire. Have this problem addressed immediately.

    Jim "Griff" Griffin
    PA28 - 161
    Chicago area

  • definitely not normal, but I've read about this condition in other forums about other planes as well. What I have trouble understanding how an assembly that really doesn't have much stress or rotational wear on it's parts can lose tolerance enough to leak fuel and require a rebuild. These aren't auto carbs that are being opened and closed constantly.

    '74 Archer- New Owner, Student

  • Hi Yellowtail,
    If you advance the mixture to full rich with the engine off I would expect fuel to leak out of the carb as there is likely some head pressure from the tanks even with the fuel pump off. If I understand you the fuel is coming out of the air plenum and past the alt air door. With the engine on the air sucks this fuel into the intake and onwards to the cylinders.
    What position do you have the throttle in? If the throttle is closed but the mixture rich it is likely coming out of the idle circuit.
    With the pump on and the mixture full rich I would also expect more fuel out of the idle circuit. If the pump is on and the mixture is at cut off you should have no leaks but I would not be surprised if there is some leak.

    Eric

    Eric Panning
    1981 Seneca III
    Hillsboro, OR (KHIO)

  • Eric, we see some fuel drippage the day after flying after putting the plane away for the night. So that setting would be mixture at full lean, throttle full idle, pump off. We think that as we taxi back to the hangar at 3/4 rich and then shut off, there is some fuel that leaks out of the carb. Maybe engine heat expands the fuel in the bowl, or whatever, but more times than not after flying there are blue streaks on the white nose wheel pant.
    But more shocking is just pushing the mixture up, or turning on the electric pump when engine is cold, we get fuel drips. Can't believe an 1100 hr 47 year old carb needs to be rebuilt because of internal wear.

    '74 Archer- New Owner, Student

  • 47 year old carb? Wow, I wonder if that's a new record. And 1,100 hours in 47 years? That's an average of 23 hours per year. Hmmm, I'm actually surprised it flies at all. If you want it to fly well, put a hundred hours a year on it.

    Scott Sherer
    Wright Brothers Master Pilot, FAA Commercial Pilot
    Aviation Director, Piper Owner Society Forum Moderator and Pipers Author.

    Need help? Let me know!

  • Yellowtail et al;

    Our "updraft" carburetors are mounted below the engine and rely on engine vacuum to draw the air/fuel mixture into the intake.

    Fuel should never leak out of a carb unless something is wrong, or if someone pumps the throttle while the engine is stopped. When the throttle is pushed forward, the accelerator pump squirts fuel into the carb throat. If the engine is stopped, there's no vacuum to draw the fuel into the engine and the fuel drains out (as mandated by gravity). If another attempt is made to start the engine and it backfires, or if there is a spark source nearby (like an alternator, starter, or bad plug wire), that fuel can ignite. The procedure for a carb fire is to stop screaming, crank the starter and attempt to draw the fire back into the carb. If that doesn't work, evacuate everyone from the plane, get your fire extinguisher out, and put out the fire. If that doesn't work, call the fire dept and your insurance adjuster because you'll probably need to file a claim. After exhausting all these steps, it is perfectly acceptable to scream.

    Here's the bottom line: if the engine is not turning, it doesn't matter where the mixture control is, whether the electric fuel pump is on or off, or the position of the fuel selector. Nothing should be leaking fuel, especially the carburetor. It sounds to me like a float, needle valve/seat, or gasket issue. If anything on your plane is leaking fuel, it's telling you something is wrong.

    Lol, 47 years? Agree with Scott, that's gotta be a record somewhere! Can't imagine why a gasket or valve would go bad in that short of a time!

    Please let us know what you and your A&P/IA find.

    Jim "Griff" Griffin
    PA28 - 161
    Chicago area

  • sorry 1135.26 hours in 47 years!! Yes next job will be the carb. I understand and agree completely with the warning shots fired here. I guess the needle valve seat has some pitting or the float ain't floatin'. Luckily no external leaks in any fuel line, valve or carb gasket. Having fun with my hangar queen.

    '74 Archer- New Owner, Student

  • No worries! Any problem found on the ground is an "opportunity". :)

    Scott Sherer
    Wright Brothers Master Pilot, FAA Commercial Pilot
    Aviation Director, Piper Owner Society Forum Moderator and Pipers Author.

    Need help? Let me know!

  • Yellowtail, we're looking forward to seeing you and your hangar queen in the air!

    Keep us posted, and ask any questions you need answered. We're here to help!

    Jim "Griff" Griffin
    PA28 - 161
    Chicago area

  • You can ignore my prior post on this and follow Jim's advice. Aside from the accel pump the fuel is sucked out and the float bowl seal should seal against pressure from the fuel line.

    Eric Panning
    1981 Seneca III
    Hillsboro, OR (KHIO)

  • edited November 2021

    From Yellowtail18$$

    "What I have trouble understanding how an assembly that really doesn't have much stress or rotational wear on it's parts can lose tolerance enough to leak fuel and require a rebuild. These aren't auto carbs that are being opened and closed constantly."

    True that it isn't opened and closed as much as an automobile... we don't stop for cross traffic and we can't pull over to the nearest cloud when we have a flat tire or run out of gas! :wink::smile:

    What we do have is an engine that is rather large for the horsepower it makes and it has a very large rotating mass that is somewhat balanced when all of the parts were new.

    I say somewhat balanced because even though the prop is statically balanced when new I don't think all of the other parts firmly attached to the rotating mass are balanced. Plus when assembled all of the parts have a small looseness of fit that allows them to be assembled to their mating parts by hand without needing a press to force them together. This looseness or tolerance between all of the parts allows them to be assembled very slightly off from their individual centers of gravity creating an out-of-balance condition. Then with time and an AP's file, the prop is forced to become imbalanced out of necessity!

    The prop, because of its relatively large size can become very unbalanced by small amounts of weight being taken off out near the tips where most of the sand and rocks make their little stress riser nicks the AP has to dress out. yes they usually try to dress all blades equally but I have yet to see an A&P show up to work on my airplane with a NIST calibration certificate tied around his neck!

    I say all of this because it is probably the vibration that over millions maybe billions of cycles beats moving parts and pieces into submission. Given enough time with its increasing intensity this vibration will increase the wear and tear on everything on the airplane... everything!

    Everything attached to the engine takes the most beating from vibration. Think about your exhaust system with those long pipes being constantly shaken until they crack releasing that sneaky and deadly carbon monoxide. More isolated from the vibrations but still affected are those 'very expensive' instruments in your panel. Even the pilot and passengers are affected.

    Three weeks ago I and a friend dynamically balanced our Props. My airplane is remarkably smoother. My friend's RV-10 that was not as out of balance as my Archer was also remarkably smoother. His wife, who has over 1,200 hrs as a passenger, said he should have had this done years ago, she was never so happy for his monetary expenditure on an airplane!

  • I researched prop balancing and that is now on my refurb list. Removed the carb today and noticed a little leakage out the throat when I pushed the mixture level to full rich. So he's going to the shop for new internals this week.

    '74 Archer- New Owner, Student

  • If you have a gas powered tug take the fuel cap off sometime and you will see a complex set of vibrations on the surface of the fuel. The float seat and float "see" these vibrations too as it works to average out fluctuations and consumption to keep a consistent float height.

    I have a Glastron GX185 ski boat and I am replacing the carburetor with a throttle body kit for better top end mixture control as well as hot/cold idle. We also sometimes go at higher elevation lakes too and I have no patience for rejetting. The kit is an emissions free adaptation of a GM throttle body with an adapter plate for the intake manifold as well as a riser and port for closed loop O2 control.

    The stock Volvo fuel pump (Carter) is also junk and I have never has one last beyond a season even if it is pickled in alcohol free and stabilized fuel. The big trade off is no electrical power = no go. However, the stock system also needs power. If there is not 12V on the alternator output then the pump shuts off and since the tank is low you are done. The cost of a system like this is ~$1100.

    Here is what the aviation (experimental only) system looks like:
    https://www.flyefii.com/system-32/

    Eric Panning
    1981 Seneca III
    Hillsboro, OR (KHIO)

  • Eric;
    Are you considering the FItech or Holley Sniper for the boat? Do either of them make a throttle body injector for the Volvo engine? Just casual interest.

    Jim "Griff" Griffin
    PA28 - 161
    Chicago area

  • Hi Jim, I bought the kit from Performance Fuel Injection Systems. Also includes a brand new distributor. I have not installed it yet but everything they have sent me is nicely made. They are a smaller company but great to work with. We spent some time confirming the intake manifold.
    https://pfisys.com/

    Eric Panning
    1981 Seneca III
    Hillsboro, OR (KHIO)

  • Nice!

    Jim "Griff" Griffin
    PA28 - 161
    Chicago area

  • Follow up to the leaking carb- I had a local shop rebuild the carb and IRAN'd the mags, new wire harness, retimed engine, new plugs--- engine runs like new, climbs like a fighter! No leaks, engine starts great in cold or hot. 6x primer cold, 2x primer hot start. From now on, I always watch for any fuel in the bottom of the cowl. Plane is currently in annual and we're not seeing any leaks anywhere in the fuel system except an occasional drip at the sump valves, which is caused by a little sediment getting caught in the o-ring. Solution to that is siphon down the tank to 5 gals., remove valve and let the fuel drain out any sediment using a fine screen filter in the funnel, then clean valve w/ new thread sealant, then pour fuel back. Check o-ring for wear and replace valve if necessary.

    '74 Archer- New Owner, Student

  • I am glad you sent your carburetor out for repair. There are a number of dead pilots and passengers of Piper airplanes from carburetor issues. Anytime fuel leaks from the carburetor with the engine off without excessive pumping of the throttle, you are at risk of the engine stopping in flight, due to excessively rich mixture.

    Regarding thread sealant in fuel systems, use MIL-G-6032D AM Type 1 (current SAE spec.) call fuel lube.

    John Schreiber
    A&P, IA, CMEL
    A 978mhz.com

  • You didn't mention as to whether the fuel selector was in the on or off postion. This said, your carb should not leak. Previous posts were correct in their analysis. As to how much fuel you are losing through this process would require you to tape a zip lock bag to the carb throat and measure it. I would suspect a needle valve seat, or a brittle/loose carb gasket/seal. Depending on where you park your aircraft, your fuel tanks may be slightly higher than your carborator and turning your fuel selector to off after engine shut down should cut the source of fuel to your carb. On engine start, try starting with the fuel mixture leaned a tad. You did not mention what your airport elevation is, but I have found that a PA28-235 with an O-540 engine needs to start out lean, especially in the summer, and a PA28-140 with an O-320 at sea level also needs to be leaned out slightly.

  • Not sure this is helpful, but I own several old British cars that use SU carburetors. When the float is not doing its job the needle valve will not shut off the fuel flow and the bowl will overflow.

  • like I said, not seeing any fuel leaks out of the carb throat after shut-down. But, no, I was not turning off the fuel selector after engine shut-off. I believe the float needle valve was worn and allowing fuel to leak into throat when the gas sat and warmed up after a flight, and it might also be possible it was running overly rich too. But that is all fixed now. My airport elev is 645' msl

    '74 Archer- New Owner, Student

  • I had a float failure on a Glastron ski boat. Was absolutely flooding the engine at idle. I was able to limp home by disconnecting the always on low pressure pump, starting and then periodically filling the bowl up again with the pump every x seconds. Glad I had some crew up for the adventure. I replaced that carb but will be converting it to throttle body injection in July with a kit designed for the engine.

    Eric

    Eric Panning
    1981 Seneca III
    Hillsboro, OR (KHIO)

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