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Mixture adjustment?

in 912 / 914 Technical Questions 14 years ago
So, here at Santa Fe, NM we're running around a 10,000 foot density altitude most afternoons. I've been getting an excessive mag drop at runup (sometimes as much as 400-500 rpm) which goes away if the power is held at a higher level for a minute or two. My exhaust stack is also quite sooty, as are the plugs when we pull them.
A runup after landing usually produces a 100 rpm drop.

It looks like we're running rich. Is there any way to lean things out? Any way to know for sure if the mixture is ok?

Thanks

  • Re: Mixture adjustment?

    by » 14 years ago

    Hi Michael,

    I know Santa Fe so you most likely are running a little rich, but a mag drop of 400-500 rpm shouldn't be a mixture issue if you have everything else setup properly.That's a pretty big mag drop. I was thinking more in line with 100-150. You may very well have another issue.
    You may have a plug or plug wire problem so I wouldn't just try to lean this out without looking at some other causes. Is is smooth at cruise rpm? Does it vibrate or shake at idle rpms?

    Here's how to lean and in your case more info is needed.
    If this is a consistent problem and you know that the carbs are balanced then an adjustment could be made for the higher elevation of Santa Fe and high density altitude flying. First though what rpm are you using to check for the mag drop? Are the carbs balanced? The idle rpm black sooty plugs are not indicative of high rpm running. If the plugs are always a black dry soot at idle then a little tweak on the mixture screw on the bottom of the carb will help. If it's only 1-2 plugs it may not be needed. What are your EGT, CHT and oil temps like at cruise (5000-5300 rpm, EGT especially) If you do need to make an adjustment after some of these question's answers come back then you could move the clip on the needle from the third position from the top to the second position from the top. This will lean it out some for the higher rpms, but won't help idle. This lowers the needle down into the main jet a little farther.

    p.s.
    You need to check the needle and see if someone has moved it out of the third notched position.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
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  • Re: Mixture adjustment?

    by » 14 years ago

    The drop of up to 400ish is at 4000. If it happens we run it up to 4800 or so for about a minute and then we get a normal mag drop when we go back to 4000. In cruise it is smooth and mag drops are small. After landing a 4000 rpm runup always results in a 100 or so rpm drop and very smooth operation throughout.
    The problem really only started when it got hot.
    That is way I thought it not something else, but admittedly things can be a little rough at lower rpms.
    I'll work on some numbers for the cruise temps.
    Thanks!
      
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  • Re: Mixture adjustment?

    by » 14 years ago

    Hi Micheal,

    If it is only loading up at idle that is causing the big mag drop then keep the idle up while on the ground. I can cut mine back to 1750, but typically let it stay at 2000+ if I'm just waiting. 100 at 400 isn't bad. Tweaking the mixture screw on the bottom of the carb will help with the loading up at idle combined with an 1800+ idle.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
    Report

  • Re: Mixture adjustment?

    by » 14 years ago

    I am having a similar issue. Ever since I purchased this plane it has seemed to run rough at all RPMs. I synch'ed the carbs after dissasembling them to clean out all the gunk that was in them for sitting many years. I had hopped this would cure the problem. When I had the mag issue I assumed maybe the rough running was do to the intermitent mag. But since I fixed that yesterday I flew it for an hour and it still has quite a vibration at all RPMs. I am generally flying at 2-3,000 ft. alt. Field elevation is 1500MSL.

    The exhaust stack is coated with black soot as are the plugs. Oh, new plugs (DCPR8E gapped to 0.22)were installed also. This is the plug and gap suggested by 912 Xtra with their high compression kit. Maybe I need to go back to the DCPR7E plug since I think it is a little hotter. Any ideas?

    When I had the carb apart I think the clip was on the third groove put will have to recheck. Would moving this clip up one grove help this issue?
    Thanks,
    Keith
      
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  • Re: Mixture adjustment?

    by » 14 years ago

    Hi Keith,

    You mention syncing them.
    Did they sync at idle and up to 3500-4000 rpm? If you sync them at 2500 rpm like some that won't help you. I recommend that all carbs be synced at least at 3500 - 3800 and up to 4000 rpm depending on what's being checked.

    It still sounds like you have a carb issue and may have missed something. It could be a few things; it could be the carb diaphragm is in the wrong position, you're right the needle clip should be on the 3rd slot (leave it there that isn't the problem), choke circuit installed backward, the jets still clogged (these may need to be rodded out as flushing won't clear them after a long sit time)(be careful and use something that won't enlarge the jet holes), one of the passages are still clogged inside carb body, if the carbs are fairly old the "O" rings may be so bad as they leak like a sieve.

    I would leave the plugs DCPR8E alone, but would widen the gap to .023 - .027. This isn't your problem, but I would still widen your gap.



    I believe it is still the carbs.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
    Report
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