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Thread: engine runs rough

  1. #1
    Senior Member Slyfox's Avatar
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    felts field, spokane
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    Default engine runs rough

    new engine installed 4th month of 2009. has 750 hours. 912uls.
    My kitfox has been running poorly for quit some time, first sign was rough running on take off. thought it was fuel pump, nope new one, went through carbs, nope still there. what it will do is act like it's running on 3 cylinders for about 2 seconds. todays flight it did it randomly for 3 times. got back to the airport and did 2 touch and goes, on the second one it seemed a little rough in the mid range on down I ran the runway and took back off and it did it after hitting the throttle full throttle for more than 2 seconds. came back around cus it quit doing it and landed. on taxi back to the runway was running poorly, pulled out the enrichment and didn't get better. pulled out more and the engine wanted to quit. I feel this is not fuel. I turned on my electric fuel pump and it will do this with it on.
    Now when did this really start. Thinking back I put in the earthx battery with the b and c regulator. first flight went out 15 minutes, on the way back seems more vibration than normal. next flight it reared it's ugly head. my thought is the earthx or b and c or both. my thought now is to put my original regulator back on and fly. if that doesn't do it, put the odyssey battery back in. thoughts
    steve
    slyfox
    model IV 1200-flying
    912uls
    IVO medium in-flight
    RV7A-flying
    IO-360
    constant speed prop

  2. #2
    Senior Member Dave S's Avatar
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    Default Re: engine runs rough

    Hi Steve,

    I can offer a couple thoughts although diagnosis via e-mail leaves a lot to be desired.

    Considering that engine running troubles can be roughly divided into 1) cylinder health (compression), 2) Ignition and 3) fuel/carburetion the symptoms don't sound much like compression (barring a sticking valve or broken valve spring) and you have pretty well gone through the fuel/carburetion. Might be good to think about the ignition system.

    The random symptoms don't really sound like what a fuel problem would do, assuming the fuel flow is not interrupted from time to time for whatever reason.

    On the ignition, I'd not suspect the modules since they either work or don't, or they don't work when cold. My first thought would be to check the cheap stuff first - grounds for the modules need to be solid. A cracked wire inside the insulation can hide so manipulating the ground wires to search for an internal break is important. If I recall, the original 912 installation in a IV moved the carbs/manifolds to opposite sides and some sort of provision needs to be made to re-establish a separate ground from each module to the manifold. It seems that some instructions had both grounds going to single fabricated bracket which if it cracked could interfere with both modules. P leads and switches are something to look at. Integrity of the harness to flywheel pickups as well as solid and correct mounting of the pickups another possibility.

    Don't know if that helps, but I'd start looking over the entire ignition system.
    Dave S
    Kitfox 7 Trigear (Flying since 2009)
    912ULS Warp Drive

    St Paul, MN

  3. #3
    Senior Member rv9ralph's Avatar
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    Default Re: engine runs rough

    Another area to check is the carb boots for cracks. They are part of the 5 year rubber replacement recommended by Rotax. (although I am more of a believer of "replace on condition".)

    A crack in the boot will result in an induction leak causing a lean condition in those two cylinders.
    Ralph
    Kitfox 3 flying
    Building Kitfox SS7 (RockFox)
    915iS Engine
    Building Partner Victor V

  4. #4

    Default Re: engine runs rough

    No bueno to hear another 912 is on the struggle bus. I will state that I had a similar issue and in speaking with Hal he pointed at the ignition boxes. If you have the old ones that have maybe 3 inches of wire coming out of them and directly mounted over the engine, it is possible that one of the wires has an internal short / break that is intermittently connecting and disconnecting when vibration sets in. I had chased these issues around the plane for months and finally had enough of it - swapped over to programmable ignition modules and never looked back. The new modules also have a few feet of wire on them to allow for mounting in cab away from the heat - bonus.

  5. #5
    Senior Member Slyfox's Avatar
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    Default Re: engine runs rough

    tell me more about this. willing to spend whatever to fix this. about a year ago I changed one module to a newer soft start with extra wiring for the soft start. the other one only has a cold start up problem that works itself out after I start the engine, generally will start working when the rpm's are over 2000. I left it alone with 1000 a pop. I've run on one mag while flying and no change going between the two. But a programable module does sound interesting. give me the web site or something for this. thanks
    steve
    slyfox
    model IV 1200-flying
    912uls
    IVO medium in-flight
    RV7A-flying
    IO-360
    constant speed prop

  6. #6

    Default Re: engine runs rough

    If you’d like, I have one of my old cdi boxes that I can send your way for you to swap out with your others to see if the problem goes away. That way we can narrow it down to the ignition before you go spending a ton of money. It will be the one that doesn’t have the broken internal wire - promise! Haha. If that solves the problem and you want to go the programmable cdi box, we (AvDES) sell them along with a mounting kit to bring them into the cab. That said, I’m not a paying sponsor on the site (yet), so I cannot openly discuss the kit in the forums. Touch base with me in pm and I’ll shoot you my phone number if you’d like to discuss your problems more.

  7. #7
    Senior Member Slyfox's Avatar
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    Default Re: engine runs rough

    right now my plan is to rip the coil-module assy out and check all wiring and do a test on each coil with heat. if that passes than yes I'll consider it. thanks

    might be a few days, been working hard and I'm going fishing this weekend. lol
    steve
    slyfox
    model IV 1200-flying
    912uls
    IVO medium in-flight
    RV7A-flying
    IO-360
    constant speed prop

  8. #8

    Default Re: engine runs rough

    Quote Originally Posted by Slyfox View Post
    right now my plan is to rip the coil-module assy out and check all wiring and do a test on each coil with heat. if that passes than yes I'll consider it. thanks

    might be a few days, been working hard and I'm going fishing this weekend. lol
    sounds good! And lucky man! I’ve been trying to escape to a fishing hole for months now. Good luck!

  9. #9
    Senior Member Slyfox's Avatar
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    Default Re: engine runs rough

    a little update on this. I cleaned the plugs, they were quit black. started it up, and it ran very good on the right mag. left mag, ran like it was not on all 4. problem is in the ignition. I had my spare cdi and installed it on the left side. switch that is. not sure at this time which coils that one runs. will find out though. I have the rotax heavy duty manual so I have the wiring diagram for this. I replaced that cdi about a year ago with a soft start. the original cdi quit working when cold. installed that cdi and ran the same, so it is working. I am starting to think coil at this point. I need more time to dig in and find the problem. let you know.
    steve
    slyfox
    model IV 1200-flying
    912uls
    IVO medium in-flight
    RV7A-flying
    IO-360
    constant speed prop

  10. #10

    Default Re: engine runs rough

    Good to hear the troubleshooting is going well. If it ends up being a coil I HIGHLY recommend reaching out to Hal. He has a stash of them and will save you big bucks.

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