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STARTING PROBLEMS

in 912 / 914 Technical Questions 11 years ago
Hi all,

ROTAX912 UL: 200 hrs. Carb synch OK.

I have encountered the following engine starting behavior.

1. when starting the engine it runs rough as if not all cylinders are firing or some are flooded. :(
2. switching off the engine. Strong fuel smell when opening the upper engine cowl. :sick:
3. waiting a while and starting the engine again. No problem. Sweet running. :unsure:
4. Normal flight. Idle 1700 rpm very smooth.

I would welcome any idea for my diagnostics?

Thanks.

Greetings Jan

  • Re: STARTING PROBLEMS

    by » 11 years ago

    Jan,

    Does this happen often or was it isolated problem?

    Is your fuel tank higher than the carbs?

    Do you have an airbox on the carbs?

    Doesn't sound like a serious problem. May have some ideas with a little more info.

    Bill
      
    Report

  • Re: STARTING PROBLEMS

    by » 11 years ago

    Bill,

    Thx for the response.

    It is just happens recently 3 times now.

    The carbs are below the (wing) tanks. The aircraft is an ultralight FK9ELA.

    There is no air box just conical air filters.

    Your ideas will be very much appreciated.

    Jan
      
    Report

  • Re: STARTING PROBLEMS

    by » 11 years ago

    Jan,

    Just a guess, but with high fuel tanks, a worn float valve tip or trash in the float valve can cause fuel to flow through the fuel pump and overfill the carb bowls while the engine is off. as the bowl overfills, the floats will exert more pressure on the float needle until it closes.
    When you start the engine, that carb will run extremely rich until the fuel level in the bowl goes down and the vibration from rough running can splash fuel out the overflow pipe.

    An easy way to check the float valve is to carefully remove the carb bowl so as not to spill any fuel. Set the bowl on a level surface and lift out the floats. Measure the distance from top edge of the bowl to the fuel level. It should be about 13mm. When you replace the floats in the bowl, they should float with the float pins just touching the fuel. A leaky float that sinks deeper into the fuel will also cause a high fuel level in the bowl and an overrich condition. Just shake the floats to see if any fuel is inside.

    Hope this helps,

    Bill.
      
    Report

  • Re: STARTING PROBLEMS

    by » 11 years ago

    Hi Bill,

    Thx.

    Your guess is very plausible. I must check fuel overflow when opening the fuel valves of the wing tanks.

    My habit to prepare the aircraft for flight is to open the fuel valves which action is at the top of the checklist. So fuel can already result in overflow and as you said enriching the carbs. The float valves (more likely the viton springs) may not be designed for this condition.

    Anyway I will only open the fuel valves just before starting the engine and may be avoiding the problem. I will perform the level measurements as well and check the components for proper functioning.

    Will report back.

    Jan
      
    Report

  • Re: STARTING PROBLEMS

    by » 11 years ago

    Jan,

    I doubt you would have to change your checklist sequence. These leaks are usually tiny and can take hours or days to overfill the bowl. A properly seated float valve will not allow the bowl to overfill even if you leave the tank valves open overnight. If you run the engine and it runs well, then immediately check the bowls, the levels will probably be OK. I would let the plane sit overnight and check the bowls before starting.

    As for the overflow tubes, a rough start can cause fuel to spew from them momentarily even with a good carb. Even taxiing over a rough field can cause an overflow from the tubes.

    If it is a bad leak, the engine would run rich and possibly roughly all the time and would not clear up. A good indication of a too rich carb is unusually low EGTs on one side.

    Bill.
      
    Report
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