Kitfox Aircraft Stick and Rudder Stein Air Grove Aircraft TCW Technologies Dynon Avionics AeroLED MGL Avionics Leading Edge Airfoils Desser EarthX Batteries Garmin G3X Touch
Results 1 to 3 of 3

Thread: low idle rpm and hesitation

  1. #1
    Birdseyeview's Avatar
    Join Date
    Sep 2017
    Location
    Oregon, Ill
    Posts
    191

    Default low idle rpm and hesitation

    My 912 ULS seems to be having carb issues again. I'm beginning to dislike the Bing carbs as being too darn touchy.

    During todays preflight mag check I noted some minor rough idle at and below 4000 rpm. The rpm drop is at around 200 rpm for both L and R mags (but on previous flights is was more 'normal' at 150 rpm). The ambient temp has been in the 50s lately and everything was running smooth when the ambient had been in the 70s. The engine still runs smooth at WOT and cruise. However, after engine warm up the min idle has also dropped low into the tach yellow zone (less than 1800 rpm) and it wants to die there unless I throttle up a little (2000 rpm ish). The min idle had previously been set at 1800 rpm with very smooth operation there. No external adjustments to either carb or their throttle cables have been made that would account for this drop in idle speed. There is also some hesitation in the engine response when throttling up quickly from idle to WOT during takeoff or in a go-around (never seen before).

    I checked the float bowl fuel levels and spark plug appearance after my last flight and all looked very good.

    It seems to me that the idle jet in one of the carbs may be fouled, or maybe a needle jet is fouled. The clips on the needles are both in the position #3 groove per the manuals. I'm at 60 hours total time on this engine and it was running great after a new set of plugs a few hours ago. I plan to check the jets for fouling and I'll also check the carb sync. Is there anything else that makes sense to check?
    Larry Olson
    Kitfox Series 6 - 1st Flight Oct 2021
    Tri-gear, smooth cowl
    912 ULS

  2. #2
    Birdseyeview's Avatar
    Join Date
    Sep 2017
    Location
    Oregon, Ill
    Posts
    191

    Default Re: low idle rpm and hesitation

    Good news - When I removed the carb float bowls and checked the various jets for fouling I discovered that the o'ring seals on both fuel/air jet/screws were damaged. Also, the o'ring seal on one of the low idle jets was also damaged. These three tiny o'rings were replaced by me a number of hours ago and I can only guess that I must have damaged them during installation since I don't remember lubricating them before installation. Replacing them now caused my issue to be completely resolved. As a result, I'm now surprised that the engine ran as long as it did and as well as it did since that last o'ring replacement. Since I can't expect the carbs to run well with damaged o'rings like this I have to retract my previous complaint about the Bing carbs. I'll redirect my frustration back at myself for mis-installing the o'rings in the first place.
    Larry Olson
    Kitfox Series 6 - 1st Flight Oct 2021
    Tri-gear, smooth cowl
    912 ULS

  3. #3
    Administrator DesertFox4's Avatar
    Join Date
    Aug 2008
    Location
    Arizona
    Posts
    3,505

    Default Re: low idle rpm and hesitation

    Good find Larry. Thanks for following up.👍


    DesertFox4
    Admin.
    7 Super Sport
    912 ULS Tri-gear


Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •