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Thread: Low static RPM

  1. #1

    Low static RPM

    We are doing our annual inspection and noticed we are not getting the
    static RPM we should. We are seeing just under 220 RPM and the POH
    says we should get between 2250 and 2350. Our tach is good (just had
    prop balanced and tach was less than 25RPM off).

    Our A&P is going to check timing and throttle and mixture movement
    limits. Also, check was done without cowling on aircraft. Would that
    make a difference?

    Our A&P and our engine guy both think it might be time to have carb
    overhauled. It's never been done and has about 3500hrs on it. Does
    that sound reasonable, or should we be looking elsewhere? Engine is
    at about 1300 hrs SMOH.

    If it is the carb, any recommendations on where to get it fixed. I
    found rebuilt exchange carbs for $685. Its a Marvel Schebler HA-6.

    Thanks in advance.





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  2. #2

    Low static RPM

    You are only 50 rpm off (assuming that you meant to type 2200 instead of 220). I would think that having the cowling off would cause that much problem. Also, the POH figures are for an airplane getting ready to take off, out in the clear, pointed into the wind or for no wind. If the wind is blowing from the tail toward the nose it could cause that much drop. Also, if the mixture is a little too rich and you didn't lean for max rpm. I am not an A&P, but I would try a few things before I started spending a lot of money. Checking the timeing, mixture and throttle are very good things to do even if you don't suspect something is wrong.

    Good Luck

    rgschmi <rgschmi@execpc.com> wrote:
    We are doing our annual inspection and noticed we are not getting the
    static RPM we should. We are seeing just under 220 RPM and the POH
    says we should get between 2250 and 2350. Our tach is good (just had
    prop balanced and tach was less than 25RPM off).

    Our A&P is going to check timing and throttle and mixture movement
    limits. Also, check was done without cowling on aircraft. Would that
    make a difference?

    Our A&P and our engine guy both think it might be time to have carb
    overhauled. It's never been done and has about 3500hrs on it. Does
    that sound reasonable, or should we be looking elsewhere? Engine is
    at about 1300 hrs SMOH.

    If it is the carb, any recommendations on where to get it fixed. I
    found rebuilt exchange carbs for $685. Its a Marvel Schebler HA-6.

    Thanks in advance.






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  3. #3

    Low static RPM

    The RPM increases as we lean the mixture as it should, but we have
    noticed we have to pull the mixture control much further back to get max
    RPM than we used to. The engine temp was barely in the green as we had
    the cowl off and were ground running it. Mags are going to be ckecked on
    Monday.

    Since the cowl was off, we had no filter for engine intake air, which
    should yield the highest manifold pressure we can get.

    We were outside the hangar, pretty much in the open, facing into the wind.

    We will do some checking on Monday and post the results. We are hoping
    it's timing or turbulance(?) caused by having the cowling off.

    Jury still is out on whether to overhaul carb in any case, since it's
    never been overhauled


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  4. #4

    Low static RPM

    Just as an aside, folks should not be doing full-power run-ups with the
    cowling off. It WILL cause hot spots in the cylinders, as there is almost
    no airflow down through the cylinders without the pressure box in place.
    Cylinder expansion will be very uneven. Associated problems may not show up
    for some time. Overhaulers break-in rebuilt engines without a cowling, but
    they have a strapped-on pressure box, and are running what is called a "test
    club" That is a four-blade, short-bladed, very wide-chord wood prop that
    moves a ton of air right into the front of the pressure box. Idling in
    order to check for leaks is OK without a cowling, but not full-power
    testing.





    _____

    From: musketeermail@yahoogroups.com [mailto:musketeermail@yahoogroups.com]
    On Behalf Of Bob Schmidt
    Sent: Saturday, December 03, 2005 8:07 PM
    To: musketeermail@yahoogroups.com
    Subject: [musketeermail] Low static RPM



    The RPM increases as we lean the mixture as it should, but we have
    noticed we have to pull the mixture control much further back to get max
    RPM than we used to. The engine temp was barely in the green as we had
    the cowl off and were ground running it. Mags are going to be ckecked on
    Monday.

    Since the cowl was off, we had no filter for engine intake air, which
    should yield the highest manifold pressure we can get.

    We were outside the hangar, pretty much in the open, facing into the wind.

    We will do some checking on Monday and post the results. We are hoping
    it's timing or turbulance(?) caused by having the cowling off.

    Jury still is out on whether to overhaul carb in any case, since it's
    never been overhauled


    Join BAC today and be a part of the ONLY Type Club for the Musketeer series!

    www.beechaeroclub.org






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