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Thread: Engine Stumble source?

  1. #11
    Norm, when you do the flow test, place the tail in a ditch. This will simulate the take off attitude. CHEERS don

  2. #12

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    Shocked

    Quote Originally Posted by dpellegreno View Post
    Norm, when you do the flow test, place the tail in a ditch. This will simulate the take off attitude. CHEERS don
    (Norm formerly of EAA, is my older brother)

    More info. At a three point attitude & a 60% full cowl tank, I get only 8.6 gph out of the carb drain. I instrumented the gascolator housing with a thermistor and found that the temperature would peak at 122 degF on climbout.

    With the cowl tank now refilled up to 80% full, the EGT suggests the mixture is adequately rich on takeoff as there is about a 100 degF reduction from peak. After a few landings and taxi backs, the EGT was noticeably higher on climb out - but no stumble.

    All data was taken on a day of about 78 degF air temp.

    My initial conclusion is to retain the instrumentation, insulate the gascolator, and try more takeoffs to see if the rising EGTs present itself again.

    I'm glad I didn't know the fuel temp was reaching 122 DegF

  3. #13

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    This might be of interest.
    Attached Images Attached Images  

  4. #14

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    We had an incident recently where an Ercoupe engine failed on takeoff.
    The owner told me that mechanics had inadvertently reversed the fuel line/vent or return line during maintenance.
    It would run fine on the ground, but not enough fuel flow for a full power climb.
    Last edited by Bill Berson; 09-06-2013 at 08:51 AM.

  5. #15

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    I should add that I'm also going to try adding a pitot tube to the cowl fuel tank vent, now that I've got an indication using the EGT for showing impending fuel flow starvation. That should add another 3 to 4 inches to the effective fuel head at 60 mph airspeed.

    Another trick under consideration is to replace the fuel line end fittings and hose with ends that have a smooth sweep instead of the present -6 JIC 45 deg el at the gascolator, and 90 deg el at the carb fuel inlet. Theoretically it promises only about 0.6 inches of fuel head improvement in total though at 8.6 GPH.

    There must be other J4s with an 85 hp engine. How do they do it?

    There are no Tees in the fuel tank outlet - but that is a very interesting consideration. THX

  6. #16

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    Update - Insulation with pipe wrap reduced the gascolator temps to a little over 100 degF. I also added a 1/4 dia pitot to the fuel vent at the same time & found that the EGTs now show no evidence of fuel starvation at full power. I have to get the fuel level down some more to verify that one or more of these fixes work tho.

  7. #17

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    Old thread, I know, but new member here.

    I had a problem identical to this one with my continental A-65 which plagued me for almost a year. Turned out that the gascolator had been mounted backwards while remaking a leaky fuel line connection to it found during the annual inspection.

    There should be no place anywhere in the fuel system for small bubbles in the fuel lines to form big ones, which can cause blockages with symptoms identical to vapor lock. Small bubbles should just flow up after forming, right into the fuel tank. This cannot happen with the gascolator reversed. Fuel should be flowing into the glass or aluminum bowl through the large central hole in the top of the gascolator, not the smaller hole above the screen which is outlet only. Remove your gascolator from the firewall bracket and blow through the two holes to determine which is which.

    No one could figure this problem out, and it resulted in the replacement or overhaul of numerous components inside of the cowling including the carb, plugs, magnetos, harness, fuel lines, mag switch, and P leads, among other things. The good news is now the old Cub runs perfectly (as it did before) and everything under the cowlings gleams like new pennies! A gent who had been wrenching on Cubs for 45 years mentioned this possibility, and the problem was finally fixed in 30 minutes. the joy is now back in flying the old Cub!
    Last edited by 43-29080; 05-18-2015 at 09:42 AM.

  8. #18

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    What I find interesting about this thread, no one mentioned to do minimum fuel tests to see what the minimum fuel flow is. Doing fuel flow test with full tanks tells nothing, well it does tell something but is only part of the test. Imagine taking off from your homebase or field and flying somewhere else. You go ahead and take off for home knowing you have enough fuel to make it back with what is in the tank but now your flow is reduced and you have an engine out on takeoff. Why because you did not do a minimum flow test.

    If someone mentioned this I missed it.

    Tony

  9. #19

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    I am getting ready to do flow test on a new to me airplane. It has hundreds of hrs flying, but I put a new tank in her. I changed nothing but the tank. I am still doing a flow test, minimum and max fuel flow test. I need to know what the least amount of fuel I can fly with. I will then post the info in the airplane.

    Tony

  10. #20

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    A quick update on my J4A fuel supply with a C-85 and cowl tank.

    I added a 1/4 dia forward facing pitot tube to the fuel cap using epoxy, added an aluminum doghouse around the firewall-mounted gascolator. I also added fire sleeving to the fuel line between the gascolator and the carb inlet. This dropped the max temperature rise (over outside ambient) of the gascolator bowl from 60 degF to about 10 degF. All three were done at the same time so I have no way to know the comparative effects of either modification. The carb is a new Marvel Scheibler, and the above fuel flow tests were done at ~ 1/3 full cowl tank.

    No engine interruptions have been encountered since, although a lean roughness has been noted at moderate throttle on a cool dry day, verified by it smoothing out w carb heat. Before that I had leaned out the idle adjustment substantially to minimize exhaust soot - probably too much. The EGT does seem to run higher than I'd like on climb out yet. Still using ~ 1/3 100LL 2/3 Mogas.

    I think the gascolator is properly plumbed but will definitely verify this.

    From Harry Zeisloff's comments many years ago, he said vapor lock is initiated by more than simply a few bubbles - he compared it to more like letting soda pop or champagne fizz off.

    "You can never know too much about fuel and fuel systems."

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