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Thread: Engine Stumble source?

  1. #21
    crusty old aviator's Avatar
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    Perhaps you should consider replacing the big 85 with an A65, as it was designed for. The fuel cap ram air seems innocuous enough, but if you have some overly anal, greedy, by the book IA who knows old Pipers perform your annual, he'll ground her for it...unless you've paid him handsomely to fill out and send in a FAA Form 337 for it.

  2. #22
    cub builder's Avatar
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    Quote Originally Posted by nrpetersen View Post
    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.
    After rereading this thread, it reminds me of an issue I have run across twice in the last year or so since this thread originated. Since your J-4 now has a C-85 and MS Carb, I assume the fuel line between the gascolator and carb was replaced. Check to make sure the fuel line between the gascolator and carb doesn't loop up high enough to be higher than the bottom of the tank header. If you get just a bubble of air in a fuel line configured like that, it will stop or significantly hinder the fuel flow. Most recent case was on a T-craft with an identical configuration to yours. When they made up a new fuel line to the MS Carb, it was a bit long so looped upward just enough to create an intermittent problem with fuel flow when the header tank was below 50%. A ram air to pressurize the tank would have worked as a workaround. However the fix was to simply lower the height of the fuel line between the gascolator and carb so all of the fuel line was below the height of the bottom of the tank.

    -Cub Builder

  3. #23

    Join Date
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    The C-85 really puts new life in the old girl. I can't imaging the original J4s having a 65hp (even some with 55 hp) with full old electric (6 V yet). I wonder what the empty weight that was assigned by Piper was, although I have a metal prop. I ended up with an empty weight of 920 lbs.

    I agree that the fuel tank pitot isn't factory original, but from pictures there seems to be a lot of precedence (sp?) in other J-4s as well as other airplanes. If a fussy IA objected, I'd point to the almost non-existent factory service manual, the unfinished Piper assembly documentation with no revision control, and then make him fly it around the patch with low fuel. It sorta came under the guide of don't ask, don't tell..... For certain, I wouldn't ride along.

    My -6 JIC fuel line is only 14 inches long and has only a downward loop to it with a 45 deg fitting on the gascolator and a 90 deg ell on the carb. It would be better if it was a -8 fitting and hose etc but there are so many points in the system where things are 1/4 inch NPT so it isn't practical to convert.

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