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RPM drop on take off.

in 2 Stroke Technical Questions 14 years ago
I have a blue top 582 in a Mainair Blade microlight, and twice on take off have had to abort because of a drop in RPM. It runs to 6800 rpm OK, then drops off to around 5400 rpm. I checked both carbs, fuel pipes ,changed all plugs, blown through tank vent, checked carb sliders and choke operation, cooling water. I have had an indication of mag drop of 800 on front coilo and only 150 on rear, but engine revs are similar to listen to it.Engine idles fine ,but did back fire once during circuit . Air filter is clean and all cables have been lock wired. Any thoughts would be gratefully received.

  • Re: RPM drop on take off.

    by » 14 years ago

    Hi Kevin,

    Part of the mag drop sounds like one plug isn't firing, but the sudden rpm drop on take off sounds like a loss in fuel flow. Have you checked the fuel flow? Make sure sure all plugs are firing.

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

      
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  • Re: RPM drop on take off.

    by » 14 years ago

    Kevin, try replicating the problem while static, brakes on, for instance, or nose against a wall. When/if engine revs drop, switch off and check carb bowls for fuel level. If ok, get someone to wriggle ignition wiring whilst at full revs.
    John
      
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  • Re: RPM drop on take off.

    by » 14 years ago

    The 1st thing I thought of and you dont mention it is your fuel filter,

    Have you cleaned or replaced it and what type is it,

    I wonder do you have one of the squeezy hand primer pumps.

    Nick.
      
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  • Re: RPM drop on take off.

    by » 14 years ago

    Hi - the fuel filter has been changed for correct Rotax type and there is a hand primer fitted along with a by-pass line. All plugs are new and correct type and gap. I intend to do a full power run tomorrow with cocks obviously to replicate the fault. If it happens I will check bowl levels. Carb rubbers are in good condition as well. Will let you know what happens -thanks to all.
      
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  • Re: RPM drop on take off.

    by » 14 years ago

    Kevin, did you change the filter before or after this problem started? Have you checked your fuel pump?

    I note you said you had a bypass line around the primer bulb but I had a very similar experience to you after fitting a brand new primer bulb (no bypass)which was purchased from the aircraft manufacturer. With 2 up I lost power on take off run. No problem getting max rpm on static full throttle. While flying a circuit solo I would have an rpm drop at cruise but a momentary reduction of throttle pressure would bring rpm response back to normal.
    Dumping the primer bulb and replacing with a better quality model solved my problem.
    Also my flying mate had a similar problem with a new fuel filter and on a previous occasion he had the problem caused by a faulty fuel pump.
      
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