I have changed all the spark plugs.

The old ones are pretty dark. Only three of the spark plugs have dark brown porcelain.

What is the probable cause?


1. flights 1- 2 hrs: cruising rpm ca 4500: circuit ca 3500 rpm
2. ca 100 hrs in total
3. have to tape off the oil cooler for ca 60%
4. have to tape off the water cooler for ca 30%
5. altitude airfield 2700 ft
6. ultralight FK9ELA 80 hp Rotax 912

Best regards Jan

    by » 13 years ago

    Hi Jan,

    Several factors at play.

    Many Rotax engines will have black sooty plugs at times from idling at the lower rpms. That goes away at cruise rpms. That mixture can be adjusted slightly, but it usually isn't that big a deal. The carbs should always be synced at idle and the upper rpms (i.e 3500), this is very important and don't neglect it. I sync all 912 engines every 100 hrs. If you have a ground adjustable prop it should be set to get at least 5500-5600 rpm WOT flat and level and at your average cruise altitude. Then you should be cruising at 5000+ rpm and not the mid 4000's. All this comes into play. The length of service time on the plugs and the gap makes a difference too. The plug gap range is .023 to .027. Use the smaller gap for the cold winter months and the wider gap for the summer months. Don't leave plugs in service too long. I change mine at 75 hours and others at 100 hrs., but I wouldn't go over 100 hrs. If you can't get up to operating temps then use a piece of 2" wide aluminum tape across the coolant and or oil radiator. If that isn't enough because it's real cold add more until the temps come up. Unless there is another problem at play most of the time these few little items can make a difference.

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

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