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Rotax 912ULS Cylinder Num 3

in 912 / 914 Technical Questions 11 years ago
I have a Rotax 912ULS bearing serial number is 5651792.


And I have a small issue with cylinder number 3 the spark plugs are white.. basically like new, the engine idles and runs smooth, however when removing all the plugs on cylinders 1,2 and 4 these are all in black colour exactly like the below photo. (And I think this is how they should all look right?) considering that this has been on idle for more than 3 to 4 minutes.


While number 3 Cylinder spark plug looks new!! photo below!!





I have noticed this issue while changing the oil and plugs filters etc.. on the newly purchased aircraft, the said aircraft has 680 hrs on it.

I did try to search the net and I did read that normally cylinder num 3 plug reading is different from the others but in my opinion its too much..

Your urgent attention is much appreciated.

BR
Michael
912ULSplugs.jpg (You do not have access to download this file.)
Cylinders1.pdf (You do not have access to download this file.)

  • Re: Rotax 912ULS Cylinder Num 3

    by » 11 years ago

    Hi Micheal,

    Using idle isn't a good way to judge plugs and can be misleading. Run the engine up to operating speeds and then check. Secure the plane on the ground and run it up around 4500 rpm or go fly and observe the temps. EGT's would help if you have them. As soon as you land don't sit and idle. Shut the engine down right away. Then look at the plugs. Make sure you engine has had the carbs synced at idle and the higher rpm's. I prefer 3500 for the high end. My guess is your plugs will be okay. Remember the plug gaps should be between .023 - .027 gap. At idle the air/fuel mixture can be a tad off between cyl. because of the intake manifold. Cold air intake from outside the cowl verses air filters inside the cowl will make a difference. Inside the cowl air filters have the mixture running richer causing black plugs.

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

    Thank you said by: YEN NIEN YU
      
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  • Re: Rotax 912ULS Cylinder Num 3

    by » 11 years ago

    Hi, Roger

    Thanks for your reply.. yesterday I again went to check the intake manifold for leaks etc.. but all was good..

    I have just finished a 700hr inspection and a full rebuilds of the carbs the engine is running like a Swiss clock!!! no vibrations sound is perfect..

    Yes we did sync the carbs at 2000rpm and 3500rpm all good it’s just the num 3 plug reading!! However I have been told by my engineer that I should test fly it, and then try to shut down the engine without letting it idle for long.. I will try this and let you know..

    No I don’t have an egt gauge I think I must get one! A 4 ch one any ideas?

    Mic
      
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  • Re: Rotax 912ULS Cylinder Num 3

    by » 11 years ago

    If the intake manifolds leaked you would run lean and shouldn't have the black dry soot.

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

      
    Report

  • Re: Rotax 912ULS Cylinder Num 3

    by » 11 years ago

    Hi Roger,

    I know the funny thing is that only cylinder 3 is working lean (or we think)!! as I said yesterday we did some serious ground run and the engine runs smooth and synced..

    one question as I am new to Rotax 912 I previously had a 582.. is it normal for the 912 when starting it sort of give a kick back on the prop and you have to crank it again??


    Mic
      
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  • Re: Rotax 912ULS Cylinder Num 3

    by » 11 years ago

    Hi Michael,

    You DO NOT want any kickback. It will ruin the sprag clutch on the starter and that's a PITA to replace. Several things can cause the prop to kickback. It could be a weak battery, it could be a loosely shimmed gearbox or for many just poor starting technique or low rpm setup. If the battery is old or weak replace it, you can check the gearbox just like it is described in the Line Maint. Manual when you do a regular inspection.
    This may also be a simple fix. If it is a starting setup issue then try this. Have your idle rpm set for 1700-1800 rpm. Use full choke on first start with the throttle closed and see what happens. If you have a low set idle (1650 or lower) then crack the throttle slightly on first start with the choke. If your idle is set too high and you choke it it is possible that you may have a slight flooding issue.

    You have 700 hrs. On the gearbox. Has it been inspected or re-shimmed?

    For many it is just getting the throttle, rpm and start procedure setup properly.

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

    Thank you said by: YEN NIEN YU
      
    Report
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