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Thread: Carb heat mystery?

  1. #1
    TurboBeaver's Avatar
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    Carb heat mystery?

    Working on a PA 12 /0320 that has an 18 airbox on it so inlet flange on the right side instead of the left. Box has no
    dump hole" tube "out of the bottom, just the 2" hole. Original 12 muffler and shroud. Taking inlet air from left rear baffling and feeding
    into top of muffler on left side. Exits left bottom over to inlet flange on far side of airbox so has to do a 180 to get connected. Shows ZERO drop on runup at 1700 and zero drop at cruise. Other side of muffler has normal cabin heat and preasure. Have removed shourd to check for a
    Constrictsion......none. Preasurized with shop vac seams
    normal ????? If you come in and land the scat tube feeding carb heat is cold but inlet hose is hot! Like its pumping hot air backwards???? Any ideas as we are all ears. Airbox looks decent with normal damper clearences.
    Not perfect but serviceable, not one with 1/4" gaps on top n bottom. Zero side to side play in it. Sooooo simple but still a mystery why we cant get ANY hot air up into the carb????Click image for larger version. 

Name:	20181016_153106~3.jpg 
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ID:	39494Looks just like this system except the airbox inlet is over on right side
    Instead of straight in the back.?????
    Last edited by TurboBeaver; 10-25-2018 at 08:05 AM.

  2. #2

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    Airbox presure higher than the rear baffle outlet pressure? What else could it be?

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    Is the door opening/closing in the proper direction/backward controls? Sometimes the simple stuff gets ya.
    DENNY

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    TurboBeaver's Avatar
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    Denny the damper is closing correctly but thank you for responding. And Steward your correct but still trying to sort it all out as we got nothing.......

  5. #5
    S2D's Avatar
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    couple actual pictures of the setup might help
    And is there a divider on the muffler to keep the cabin and carb heat separate?
    I may be wrong but that probably won't stop me from arguing about it.

  6. #6
    S2D's Avatar
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    Simplest cure would be to make a 2" bottom hole, then the ram air would have a place to dump and not pressurize the airbox.
    I may be wrong but that probably won't stop me from arguing about it.

  7. #7

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    I've had the butterfly in the box work normally unless you held it in place and operated the control, then it slipped on the shaft and no carb heat. It drove me nuts for a while.
    Thanks Redwagon thanked for this post
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  8. #8
    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by Carey Gray View Post
    I've had the butterfly in the box work normally unless you held it in place and operated the control, then it slipped on the shaft and no carb heat. It drove me nuts for a while.
    I was just going to mention that!!


    Sent from my iPhone using SuperCub.Org mobile app

  9. #9
    TurboBeaver's Avatar
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    Carey;
    The damper works freely and holds fine when you deflect it, though I realize it is normally a culprit. But I hear ya. Tnx

    SD2. It does have the divider around the middle, and cabin heat side works fine. And it does have a 2" dump hole in bottom of airbox already; but thinking I will fab up a short piece of 2" tube and attach it to that dump hole as a "downspout" incase that is where it is getting preasurized? And also take a few pics. Tnx.
    And I am thinkin Steward is onto it: this airbox has to be getting
    backpreasure somehow ???? Inlet scat hose feels just like the outlet hose should feel???? Very very strange. This 12 has less than 50hrs on it since the 0320 was hung using the Kenmore STC but we have found alot of things they did that someone should have their ass kicked for.........
    Getting it all fairly well tiedy up, but this is sorta the last
    thing to correct on it.
    Last edited by TurboBeaver; 10-26-2018 at 03:32 PM.

  10. #10
    PerryB's Avatar
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    As to the dump tube/airbox pressure , I think it's moot. When you're in carb heat mode, the lower cowl pressure doesn't matter. The hot air from the muffler shroud is being fed into the carburetor. In other words it's being sucked from the muffler and lower cowl pressure has no bearing on it. Even if you have some odd airflow anomaly that's causing a reverse flow when not in heat mode, as soon as you pull the knob and flip the butterfly over, the engine should be sucking hot air like crazy. I think you must have a problem with the butterfly/air door, or a basic plumbing issue. All the dump hole does is allow airflow and cooling through the shroud when not in heat mode. Your cabin heat valve should be of the dump style also. It makes for a cooler muffler and cooler firewall.

    Afterthought -- you don't have a big flappy piece of silicone baffling thats folding over and blocking the inlet in flight do you ? I would imagine that to cause a choking problem, but figured I'd ask.
    Last edited by PerryB; 10-26-2018 at 05:06 AM.
    After Monday and Tuesday, even the calendar says WTF !
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  11. #11
    S2D's Avatar
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    Quote Originally Posted by TurboBeaver View Post

    SD2. It does have the divider around the middle, and cabin heat fine works fine. And it does have a 2" dump hole in bottom of airbox already; but thinking I will fab up a short piece of 2" tube and attach it to that dump hole as a "downspout" incase that is where it is getting preasurized? And also take a few pics. Tnx.
    My bad. I thought in your original post you said it didn't have the hole in the bottom.


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    I may be wrong but that probably won't stop me from arguing about it.

  12. #12
    cubpilot2's Avatar
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    Quote Originally Posted by TurboBeaver View Post
    Carey;
    The damper works freely and holds fine when you deflect it, though I realize it is normally a culprit. But I hear ya. Tnx
    This 12 has less than 50hrs on it since the 0320 was hung using the Kenmore STC but we have found alot of things they did that someone should have their ass kicked for.........
    Getting it all fairly well tiedy up, but this is sorta the last
    thing to correct on it.
    Ive done a couple of the Kenmore conversions but many years ago. If the exhaust is like your parts book photo then they did not follow the STC. (Going on memory)

    The STC required the installation of a PA-22 exhaust, which is a cross over style like the 18, only with a larger muffler.
    It also stipulated PA-18 balanced tail tail feathers.

    As I recall the feed for the carb heat exits on the right side as it does on the PA-18. This make for a short connection to the carb, with less heat loss.

    Unless they got a separate field approval for the original exhaust, you may have an expensive problem.
    Ed

  13. #13
    TurboBeaver's Avatar
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    Thanks Ed. I am going to post a few pics right away to see
    If someone picks up on something obvious that we may have missed. Like Denny says " its the little things that can get ya".

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