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One Cylinder Running rich

in General Tech Discussion 13 years ago
I just pulled and inspected the spark plugs on the Rotax 912ULS installed on my Sport Cruiser. The plugs on cylinders 2, 3 and 4 are all the normal grey/white color. However the plugs from cylinder 1 are sooty black indicating, I understand, that the mixture is to rich. Any idea why this is occuring since both clyinder 1 and 3 are on the same carburetor? The plugs have 100 hours on them.

  • Re: One Cylinder Running rich

    by » 13 years ago

    Hi Joe,

    One thing that can cause that is allowing the engine to idle for a while before checking the plugs. one or two and possibly all the plugs will appear black after periods of idling. Also if the carbs are slightly out of sync, the engine runs well at cruise but one side can load up at idle. You might also check the compression on all cylinders and see if that one differs from the rest.

    Bill.
      
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  • Re: One Cylinder Running rich

    by » 13 years ago

    I had a similar thing. 3 & 4 were black but 1 & 2 were fine. I reset the idle from 2100 to 1860 then leaned the idle mixtures till i got the highest rpm. Went back and forth till I got 1860 again. I think the flight mixture was opening a bit at 2100 rpm. Once I was working only on the idle jets it cleaned up and smoothed out.

    Hank
    Thank you said by: Dave
      
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  • Re: One Cylinder Running rich

    by » 13 years ago

    Joseph, it's possibly a misfiring plug, a break down of HT lead, cracked distributer cap or more serious, sticking/broken piston rings, leaking valves.

    John
      
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  • Re: One Cylinder Running rich

    by » 13 years ago

    Hi Joseph,

    Your dry black sooty plugs are probably normal. On the majority of the engines you may have dry sooty plugs at times and it may not always be the same plugs. The air flow and feed into the engine intake manifolds aren't equal lengths. If you check the plugs after idling this is when they may be black. You will not have that situation when running over 3500 rpm like in normal cruise. Your black plug(s) will go away when you advance the throttle to the higher rpms. If you don't believe it run your engine up to 3500-4000 rpm for a couple of minutes to clean the plug up and shut the engine down without any long idle. This dry black soot can build up when we taxi in or just sitting at idle. Personally I would leave it alone. The only thing you can do is tweak the mixture screw a tad. If you tweak the screw then remember it will affect both cyl. on that carb.

    If you ever have a black wet plug then I would want to know the cause.

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

      
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