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Does this sound like a sprag clutch problem?

in 912 / 914 Technical Questions 9 years ago
Hi,
A friend of mine has an 80hp 912 powering a skyfox Gazelle
The engine has 1200hrs and has developed symptoms identical to the engine in this YouTube link.

https://m.youtube.com/watch?v=wOVwB5EmtRg

I thought it may have been the sprag clutch... but my friend was advised by local mechanics and the local Rotax dealer that Rotax 912's don't wear out sprag clutches ?
He has since been sent on an expensive wild goose chase including

Gearbox rebuild
New ignition modules and coils
New trigger coils
Carbs have been re built and synchronised
New battery

The engine has the exact symptoms as the engine in the YouTube clip ...BUT it will eventually start and when it does start it runs beautifully ..

Your help will be greatly appreciated because he is about to set fire to his aeroplane :/

Thanks in advance

Jason

  • Re: Does this sound like a sprag clutch problem?

    by » 9 years ago

    Does this engine have kickback when trying to start? (prop kicking in a backwards rotation during a start)

    The starter sprag usually gets damaged by either rotating the prop backwards by hand too much or from starting kickback. This stretches the sprag spring out. If an engine is really hard to start and it is a constant battle to start then yes the sprag will have a shortened life and your friends engine already has 1200 hrs.
    If he has the older black starter (1200 hr. tired starter) it may behoove him to upgrade to the higher torque gold colored starter. If he has a primer type choke system he would be better off hooking up the regular enricher (choke) system.

    Your friend needs to find out if he has a fuel delivery issue or an electrical issue. The video looks to have a poor prop speed for a good start.

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

      
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  • Re: Does this sound like a sprag clutch problem?

    by » 9 years ago

    Hi Roger

    Thank you for the reply

    I have seen the prop go backwards a little bit,a couple of times ,but mostly it will just lock solid ......it will turn over like the engine in the video clip with the accompanying "clunking" noise and just stop solid as if the starter hasn't got enough grunt to turn it over .

    I'll check which starter is fitted.

    The engine idle is set at 1800rpm and he has an enrichener choke system ....fuel delivery and electrical system checks out and the engine never gets turned backwards by hand.

    Once again Roger thank you ...I'll let you know how we go.

    Jason
      
    Report

  • Re: Does this sound like a sprag clutch problem?

    by » 9 years ago

    Just an update........the engine has had a recently "refurbished" Black starter motor.
    Also, I mistakenly thought the engine had 1200hrs it actually has 1700hrs

    Jason
      
    Report

  • Re: Does this sound like a sprag clutch problem?

    by » 9 years ago

    The newer style starter (gold in color) has more torque than the black starter and can make a difference. It did on an old plane of mine that I had trouble with starts. Does the engine turn over by hand equal to the other one on your field when cold and hot or is it harder to turn by hand?

    Was this easier to start within the last 100 hrs. before the starter was rebuilt?

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

    Thank you said by: Jason Savill
      
    Report

  • Re: Does this sound like a sprag clutch problem?

    by » 9 years ago

    Roger,the starter motor was rebuilt recently as part of the trouble shooting process and failed to make a difference.
    The engine is being removed this weekend for a sprag clutch replacement....I'll keep you posted
      
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