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Thread: First flight of my J4A since 1975

  1. #11
    Check your fuel tank to make sure it is correctly vented. If not properly vented it will restrict the flow of fuel. Judging from positioning of the 25 gallon fuel tank in the 3-view drawings of the J4A, there is plenty of gravity to provide fuel to a C85-12. You might do a fuel flow test on the ground at various attitudes by jacking the main wheels to simulate a nose-high attitude. You might also check the carb if you haven't already, there is a small screen where the fuel hose threads into the carb. Make sure its not clogged. I would recommend getting to the bottom of this before further flight - good luck,
    -Joel Marketello, restorer of a J5A (N397PA).

  2. #12

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    The vented fuel cap was checked. I have a 12 gal cowl tank and no wing tank. I also have a 9 gal tank behind the cabin but it is placarded for "level flight only".

    Checking the carb inlet screen is a good idea although the Marvel Schebler carb was brand new. There could be some debris washed into it however.

    I did an informal full fuel flow test while draining the cowl tank for lube service to the fuel valve. I got flow rates of ~16 gph at normal ground attitude.

    The TCDS specifies a 7.5 inch minimum fuel head for a C-85 with that carb, and I would be very close to that with the flight conditions during the above flight test. Apparently there isn't any margin in the TCDS sheet specs. I don't of course know what the actual climbout attitude was.

    The engine originally came with a Bendix carb but it wasn't until I got to studying the TCDS I realized I had to replace it with M-S. How many others out there are operating with a Bendix carb on cowl tanks?

    I was really wondering how common this experience was with other people.

  3. #13

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    A very belated update - I ended up epoxying a 1/4 inch od curved pitot tube to the vent of the cowl tank cap. I also added a metal box around the firewall-mounted gascolator after I found the aluminum gascolator shell was reaching 122 degF on climbout. At the same time I added fire sleeve to the engine compartment fuel line to serve as insulation. No more fuel flow problems to date, but I still have my fingers crossed.

    Like all (?) carburated engines, the float bowl vent (and overflow vent) is ported to the cowl's intake opening. This means there is extra back pressure (from the static pressure) on any fuel flow limitations of the fuel delivery hardware. Adding the fuel tank cap pitot tube only balances this effect out.

  4. #14
    cub builder's Avatar
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    One other thing to look at that I have seen, especially after a restoration or change to a different engine in these antiques is a longer than normal loop in the fuel line either between the fuel tank and the gascolator, or between the gascolator and fuel tank. If this loop goes up a bit to where the top of the loop is higher than the level of the fuel in the fuel tank during an aggressive climb, the fuel flow will stop if you somehow get a bubble in the line (like a vapor bubble from a hot gascolator). Anyway, make sure your fuel lines don't run uphill higher than the bottom of the fuel tank in a nose up position then back down hill as that can create problems when your fuel levels are low. The last time I saw this was on a Taylorcraft that had been converted to a C-85. It had a big loop of line between the fuel tank and gascolator. Periodically, if the engine was hot, it would start and run just long enough to run all the fuel out of the carb and half of the fuel line, then would simply die and refuse to start again until it cooled down a bit. Sometimes that was during take off roll. The problem was addressed by simply lowering the top of the loop of fuel line. If I recall right, the J-4 only has an loop of fuel line from the gascolator to the carb, so check that it doesn't make a loop going up high enough to create a problem.

    Hope you're having a blast with your J-4. It's a sweet airplane.

    -Cub Builder

  5. #15

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    Happy

    Quote Originally Posted by cub builder View Post
    One other thing to look at that I have seen, especially after a restoration or change to a different engine in these antiques is a longer than normal loop in the fuel line either between the fuel tank and the gascolator, or between the gascolator and fuel tank. If this loop goes up a bit to where the top of the loop is higher than the level The last time I saw this was on a Taylorcraft that had been converted to a C-85. It had a big loop of line between the fuel tank and gascolator. Periodically, if the engine was hot, it would start and run just long enough to run all the fuel out of the carb and half of the fuel line, then would simply die and refuse to start again until it cooled down a bit. Sometimes that was during take off roll.

    Yep, the older CONVENTIONAL-gear planes that some of us old "stinkers" (family-friendly thread) have flown have seen this problem. IIRC Luscombe 8-A's even have an AD (or is it a change to the type cert?) requiring full Carb heat on takeoff to "suck?" or force more fuel from the fuselage tank in a nose-high attitude apparently due to lack of head pressure from the tank behind the cockpit.
    Just an added data point .. MAYBE applying Carb Heat will help alleviate the "sag-off" ???
    Last edited by 233507; 07-02-2015 at 12:01 PM.

  6. #16

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    Quote Originally Posted by cub builder View Post
    Hope you're having a blast with your J-4. It's a sweet airplane.
    I can't find the original thread that pointed it out, but someone noted the nice soft J-4A oleo gear & that I should appreciate it. Wow! Have I ever learned in spades as this old 172 driver had to learn to land a tailwheel airplane!

  7. #17

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    Update - Still suspicious but no further engine stumbles after adding ram air to the cowl tank and baffling to the gascolator and fuel line insulation. I do see a change in EGT with cowl tank fuel level so fuel pressure is the likely variable here.

  8. #18

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    .......and after almost 6 years we all hope your tail wheel instructor is doing as well as your J-4.

  9. #19

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    You better stop while you are ahead. You realize of course, that if you have that cute lady with the nice smile teach you to fly in that cool little slice of history, that its all downhill after that, most other planes are going to lack something. Its kind of like after a years as a school boy eating at your cafeteria, going to a 4 star restaurant!
    I have a J3 Cub, great trainer and all around fun plane.
    There are a few simple concepts to learning tailwheel flying, please send me a P M if you'd like to talk about it with me. Good luck,. and once you master the Cub, go look for a Stearman to get some dual in, that is advanced training.

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