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I have a 912 ULS Serial #5645194 on a Dragonfly towing hang gliders. After turning off the engine, if restarted immediately, the engine turns over fine and starts right up. If any time elapses, like a minute or two.. or ten, the starter will not turn over the engine. It engages, and will sometimes move the prop a couple inches, but that's it. We then have to turn over the engine with the prop by hand, and sometimes only a couple blades will allow the starter to do it's job. Sometimes it takes enough turns that I get tired and another guy turns until it goes.... or we wait until it really cools down and it again starts just fine.

Jumping the batteries makes no difference.

The engine turns over about twice as hard hot as cold. The guys from LEAF claim it can't be tight pistons caused by cylinder cooling as the pistons and cylinders are the same material.

I hate to pay the $300 for a new starter if it's going to act the same.

It does have the newer heavy duty starter.

Got any ideas?
Danny
  • Re: Hard cranking - hot

    by » 12 years ago


    Danny,

    Had the same problem. Mine turned out to be a bad ground connection. The bolt that attached my ground to the engine was tight but had corroded underneath the bolt. The next time it wont start, feel all the ground and hot wire connection points. If one is warm, could indicate a bad connection. The quick and dirty way is to loosen and inspect all the hot and ground wires. This would be the hot wire to the starter, the ground wire to the block, the battery connections, and the starter solenoid connections. Could also be a bad solenoid. I'd try everything before buying a solid gold starter.

    Bill.

  • Re: Hard cranking - hot

    by » 12 years ago


    Danney,
    Sounds like a tight crank; perform the test in the Line maintenance manual chapter 05-50-00 page 20.
    If it fails the crankcase has fretted and has lost clearance on the crank journals.
    Rob

  • Re: Hard cranking - hot

    by » 12 years ago


    Thanks Bill and Rob,

    I will be at the airport this weekend with the proper tools to check both possibilities. Any other things to check while I am at it? Anyone? :silly:

    Danny

  • Re: Hard cranking - hot

    by » 12 years ago


    The engine turns over about twice as hard hot as cold. The guys from LEAF claim it can't be tight pistons caused by cylinder cooling as the pistons and cylinders are the same material.


    I have a low hours 912 UL (~150 hours, serial # 4405138) that's exactly like that. The engine turns over quite freely by hand when cold. When hot, there is very noticeable resistance, the engine is quite tight though it will still start. I wouldn't bet any money on this but I've always thought this is a result of reduced piston/cylinder clearances when hot.

    I'm not sure I agree with the LEAF guys as I would expect the piston to cool more slowly than the cylinder due to the more effective convective cooling of the cylinder.

    I wouldn't touch the starter yet. The hot engine may be exposing a weakness in your starting circuit. Bob Nuckolls of the AeroElectric Connection makes the point that a couple of Ohms resistance in a low voltage high current circuit can have a very significant effect on the voltage at the load. The first page of the attached article makes this point.

    I would go through your starting circuit, from battery ground, to battery, to battery contactor (if you have one), to starter solenoid, to starter, and then engine block ground strap to airframe. Take apart each connection, clean it up and remake it with dielectric grease to keep it corrosion-free. Look for poor crimps or poor solder joints. Also, if your cables (battery to starter solenoid, starter solenoid to starter) are long and/or undersized, consider installing 4AWG wire (Nuckolls recommends 4AWG welding cable).
    WhenIsAGoodGroundNot-BobNuckolls.pdf (You do not have access to download this file.)

  • Re: Hard cranking - hot

    by » 12 years ago


    Okay, a short progress report and another question/observation.

    First, we did find 1) a corroded battery terminal and 2) a loose battery terminal.

    Second, we did "perform the test in the Line maintenance manual chapter 05-50-00 page 20." We found an interesting result that we can't explain. Basically, the torque reading was at the very limit of the specs when turning the prop when the engine had set for 30 seconds or more. BUT, if the prop was "broken loose" and then a torque reading was taken immediately, it took no more than 30 pounds foot of torque. In my terms, after sitting still for as little as 10 seconds, the prop took two hands to turn, but then would continue turning with one finger. It seemed to "freeze up" if allowed to sit for any length of time.

    Anyone got a clue? Normal?

    Still haven't started the engine to test the results of our exploration.
    Danny

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