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Thread: 914 ULS backfiring

  1. #11
    Senior Member jiott's Avatar
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    Default Re: 914 ULS backfiring

    Amer, a couple more thoughts: How do the present set of spark plugs look? Are they a nice light brown color with no deposits? If there are black deposits, they could glow and cause dieseling after shutdown; could also indicate a mixture/carb problem.

    Also, I would never recommend running the engine at idle for a few minutes before shutdown. Idling will cool the engine down and quickly cause carbon deposits to form on the plugs. You want to shut the engine off when still nice and hot. I use the method suggested by.....can't remember: When done taxiing immediately rev it up to about 2500 rpm, then pull the throttle clear back to idle, as you do this shut off one ignition, count 2 seconds, then shut off the other ignition. It stops smoothly and immediately.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  2. #12

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    Default Re: 914 ULS backfiring

    Hi Jim

    The plugs when taken out are black and sooty as is normal for the Rotax I think. They have been like this for us all.

    We have 3 Kitfoxes in our flying group. 2 x Rotax 914, 1 x 912 100hp.

    I think we have got into the habit of letting the 914's run at about 1800 for a few minutes to let the turbo spin down, then reducing to 1600 then switching off one mag then 2 secs later the last mag.

    As I said my engine is the same as his, same settings for fuel and mixture, same fuel and same shutdown procedure and no backfire. However, I am just at 22 hours from new, so I haven't changed my first set of plugs yet. Maybe I will have the same set of strange plugs in that Garry had? Hope this backfire thing is not contagious?

    I will pass on the message to Garry about shutting down as you have described, and I will do the same as well from now, and then give you some feedback on if this helped with the backfire.

    Thanks

  3. #13
    Senior Member Dave S's Avatar
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    Default Re: 914 ULS backfiring

    Regarding Johns two questions

    1) Shutting down the engine by shutting off the fuel valve I would reserve only for a non standard circumstance such as busted p lead or observed fuel leak firewall forward.

    2) Shutting the fuel valve off after engine shutdown is something I do as a matter of a normal procedure to prevent a leak in the hangar should the carb decide to leak and to exercise the shutoff valve to assure the fuel valve seals are working and to assure that the fuel shutoff valve will work in an emergency situation. I have observed rental aircraft where nobody ever shut off the fuel valves in the entire fleet and three quarters of them were frozen and totally incapable of being rotated not so good if you really, really need to shut off the fuel flow.
    Dave S
    Kitfox 7 Trigear (Flying since 2009)
    912ULS Warp Drive

    St Paul, MN

  4. #14
    Senior Member jiott's Avatar
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    Default Re: 914 ULS backfiring

    Black and sooty sure is not normal for all of us flying the 912us/s. I don't know about the 914 but it sure doesn't seem right to me. I'll bet it is too much idling.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  5. #15

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    Default Re: 914 ULS backfiring

    Quote Originally Posted by jiott View Post
    Also, I would never recommend running the engine at idle for a few minutes before shutdown. Idling will cool the engine down and quickly cause carbon deposits to form on the plugs. You want to shut the engine off when still nice and hot. I use the method suggested by.....can't remember: When done taxiing immediately rev it up to about 2500 rpm, then pull the throttle clear back to idle, as you do this shut off one ignition, count 2 seconds, then shut off the other ignition. It stops smoothly and immediately.
    A 914 needs to spool the turbo down and get some cooler oil into it, you absolutely need to idle it a couple minutes before shutdown.

    About 900 hrs behind 914's, never shut off the fuel, and always killed both ignitions after two minutes idle. Never had any shutdown dieseling or backfiring, just a smooth shutdown with no gearbox rattling.

    I would look at the last thing changed (the plugs)or altered.
    James T
    Valrico, FL

  6. #16

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    Default Re: 914 ULS backfiring

    Don't know if it's been suggested, but I would put the old plugs back in ( if you have them) and see if you get the same result. It seems that the problem started with the plug change.

    Clark

  7. #17
    Senior Member jrevens's Avatar
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    Default Re: 914 ULS backfiring

    Quote Originally Posted by amerkarim View Post
    ...
    I think the point about switching off with the choke out is a valid point and is something he will try today after his flight to see if the mixture is too lean for any reason...
    Amer,

    My understanding is that backfiring is caused by excessive un-burned fuel in the exhaust system that ignites. I would think that “too lean” wouldn’t be the problem. Maybe excessive carbon build-up in the exhaust system. That could be the result of a too-rich mixture. I agree with the spark plug change for some reason also being a possibility.

    Thanks, Jim & Dave for the feedback... I’ve been doing it (shutting off the fuel valve after shut-down) as you suggested Dave, & that is how I was analyzing it also. For shut-down I use the “2500 rpm, then retard throttle while simultaneously turning off both ignitions” technique. It seems to work very well.
    John Evens
    Arvada, CO
    Kitfox SS7 N27JE
    EAA Lifetime
    Chap. 43 honorary Lifetime

  8. #18
    Senior Member Esser's Avatar
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    Default Re: 914 ULS backfiring

    My lawn tractor will always backfire unless I pull the choke when I shut it down. If it is rich, there is too much fuel and it doesn't ignite. Maybe a bandaid solution but pre-detonation ect is usually cause by too lean of a mixture that auto ignites from something hot as an ignition source. That is similar to what they are experiencing, so I would say either starve the engine for fuel (lean it out to the point where it wont auto ignite) or enrichen it so that it won't auto ignite. Either way you have the perfect fuel air mixture right now that is causing auto ignition so you need to change the mixture one way or the other. At least that's the way my amateur view sees it. You may also have carbon deposits in your exhaust that are glowing hot when you shut down that is your ignition source but it may even be the hot turbo.

    I'm really surprised on everyone's view about fuel starvation for shutting down an engine. The only time I would care about that is on an injected system where the fuel lines are dead ends, ran dry, and had to be primed again(On my EFI set up all my injectors are on a loop so this isn't an issue). For most of my carbureted things such as my motorcycle, I routinely turn off the fuel petcock and let it starve of fuel so that unstable auto gas isn't stored in the carb bowls and make a mess if I don't ride it often.


    Also, people with vapour lock issues on starting should be starving their engine of fuel on shutdown in my opinion. The rotax runs so hot that when you shut down and close your fuel valve, your still hot engine heats under the cowl when there is no air flowing through it that it causes fuel vapour in the lines. This increases the pressure in the lines. With no where for the vapour to go, the increased pressure pushes fuel into your carbs and you have a flooded condition which is why you end up have hard starting problems. If you starved your engine of fuel on shut down, when you went to start again you could open your valve and fill your bowls with fresh cool-ish fuel. This trapped heat under the cowl is why you see lots of rotax guys open their oil doors on their cowls to let out excess heat.


    Take all this with a grain of salt. I am in no way a professional about these these things. Just my personal observations that are worth as much as free advice is worth.
    ------------------
    Josh Esser
    Flying SS7
    Rotax 914iS
    AirMaster Prop

    Edmonton, AB, CWL3

  9. #19
    Senior Member jrevens's Avatar
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    Default Re: 914 ULS backfiring

    Quote Originally Posted by Esser View Post
    ...
    Also, people with vapour lock issues on starting should be starving their engine of fuel on shutdown in my opinion. The rotax runs so hot that when you shut down and close your fuel valve, your still hot engine heats under the cowl when there is no air flowing through it that it causes fuel vapour in the lines. This increases the pressure in the lines. With no where for the vapour to go, the increased pressure pushes fuel into your carbs and you have a flooded condition which is why you end up have hard starting problems. If you starved your engine of fuel on shut down, when you went to start again you could open your valve and fill your bowls with fresh cool-ish fuel. This trapped heat under the cowl is why you see lots of rotax guys open their oil doors on their cowls to let out excess heat. ...
    That's the reason for the fuel return line that Rotax now recommends... you'll have no excess pressure in the lines, and you'll have more cool, fresh fuel circulating when running, right up to the distribution point where the line branches off to each carburetor.
    Not just Rotax guys, but many airplane owners (of RVs for example) open their oil doors for the very same reason.
    John Evens
    Arvada, CO
    Kitfox SS7 N27JE
    EAA Lifetime
    Chap. 43 honorary Lifetime

  10. #20
    Senior Member jiott's Avatar
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    Default Re: 914 ULS backfiring

    I open my oil door not because of vapor lock, but because I don't want to fry my expensive ignition modules.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

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