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Thread: Rotax 912 runs rough

  1. #1
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    Default Rotax 912 runs rough

    I've been fighting with my 80 HP rotax 912. Here is the problem, when I do a mag check at 2500 or 3000, I seem to get pretty much the same rpm drop on both sides, about 150 rpm. If I do the mag check at 3500, one side drops about 150, and the other side drops about 500 rpm. I tied it off, and ran it to full throttle, and maybe 100 rpm drop at most on either side. Any ideas? Does it sound like ignition when it runs so smooth at full throttle compared to 3500 where it's very noticeable that it's not running right. It's not at all smooth at that rpm with both mags on. JImChuk

  2. #2
    Super Moderator desertdave's Avatar
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    Default Re: Rotax 912 runs rough

    Check your spark plug wires condition and if they are on the correct cylinder. Don't ask my how I know.........
    Dave
    KitFox 6 Taildragger
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  3. #3
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    Default Re: Rotax 912 runs rough

    I did put all new spark plug wires in, and today I checked for resistence from the wires that come from the same coil. All of them read good. That in a way would indicate that they were going to the right spot as well. I unplugged from spark plugs that were supposed to be driven by the same coil and showed the same amount of resistence on all 4 sets of wires from the four coils. I'll double check though just to be sure. Thanks for the suggestion. JImChuk

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    Senior Member Delta Whisky's Avatar
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    Default Re: Rotax 912 runs rough

    Maybe carb balance? It might be that they are slightly out of balance at the troublesome rpm and the mixture or power difference is affecting the rpm drop.

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    Default Re: Rotax 912 runs rough

    Thanks for that carb balance thought. I had balanced the carbs, but not at that rpm as I remember it. That is also pretty easy to do to see if it will make a difference. I'll try it today. JImChuk

  6. #6
    Senior Member Dave S's Avatar
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    Default Re: Rotax 912 runs rough

    Jim,

    With smooth operation high & lo RPM and good mag checks at that range, I believe that indicates the ignition is probably OK.

    One suggestion based on an old timer's lo tech, cost free troubleshooting toolbox may help to determine if the engine is running rich or lean enough to cause the problem at that RPM. Then it is a matter of trying to figure out why that is the case.

    If you have the standard choke ("starting carb" or whatever rotax likes to call it) try this - make sure the engine is reasonably warmed up first. Set the RPM at the troublesome range, then slowly pull the choke control and see how the engine responds - if the engine smooth out and mag check becomes better or OK with some degree of choke that indicates the mixture is lean at that RPM range (you may need to grow another hand or two to do that!).

    If the engine slows down and becomes rougher with some degree of choke that indicates the engine is running too rich at that RPM.

    Can't say I fully understand the carbs on this engine; however, most carbureted engines have overlapping systems for idle, full throttle and partial throttle. Normally the leanest range (For multiple reasons) is partial throttle but don't hold me to that on the Bing!
    Dave S
    Kitfox 7 Trigear (Flying since 2009)
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    St Paul, MN

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    Default Re: Rotax 912 runs rough

    Thanks Dave, the choke idea is very easy to check as well, and I've used it often with two strokes. I'll have a look at that as well. I keep going back to ignition, but with it running so good at full throttle, I was thinking carbs as well. I'll maybe have a better idea by this evening. JImChuk

  8. #8
    Senior Member jrevens's Avatar
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    Default Re: Rotax 912 runs rough

    Jim,

    Do you have the springs on your throttle arms?
    John Evens
    Arvada, CO
    Kitfox SS7 N27JE
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  9. #9
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    Default Re: Rotax 912 runs rough

    I do have springs on the throttle arms. They pull about 1/3 as much as the stock ones do though. I ran it up today, and tried pulling the choke. Engine was well warmed, and pulling the choke caused it to rev up about 300-400 rpms from idle up to 3500. Pulled the tops off the carbs and raised the mid range needles one notch, so the clip is on the third groove down from the top. They had been in that position before, and I had moved them to the factory setting of 2nd groove from the top. When I first started flying the plane, the EGTs were barely 1200 in cruise, so that's why I lowered the needles. At the factory setting, they were running at bout 1425. Raising the needles today did make it run better, although it still has the dead spot on the one mag at the 3500 rpm area. Reason I didn't go after the idle circuits in the carbs was I know they have the # 35 jets, and they have been out and cleaned recently. Also the air metering screws have been adjusted not long ago. I had also put new carb boots on, so didn't think that was the problem. I'll revisit all of those items in the next few days though. I didn't try to balance the carbs, it is 23 above for temps and about 25mph wind and didn't feel like standing in the prop blast to balance the carbs. I got plenty cold enough just changing the jets out. My thinking now is this. Raising the needles just helped some, but it's not the solution. I would guess an air leak somewhere, but the 1425 EGTs at the factory setting in cruise argue against that. I think I'll reinvestigate the idle circuits in the carbs and carb boots and intake manifold o rings where they attach to the heads. I do appreciate the suggestions, so keep them coming. JImChuk

  10. #10

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    Default Re: Rotax 912 runs rough

    I had this happen a couple of times with the 912 uls 100 hp. Turned out to be trigger coil gap both times, When you get the 500 rpm drop look at the egts. If there not all close to the same temp, i would check the gaps on the trigger coils. There is a reason Rotax want you to check Mags at 4,000 rpms i guess.

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