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Cranking issue

Category: 912 / 914 Technical Questions 2 weeks ago

Hello all,

I have been stumped with a recent 912ul to 912uls engine swap. What I thought, would be a simple 2 day swap, is now on day 10 and I still just can not get this new to me motor running. Maybe someone here can help out, TIA.

Here's what's going on, I had a 912ul that seize on me about 2 weeks ago. I was able to source a used 912uls which was rebuilt in winter of 2016-17. Logs says the new to me motor was running spring of 2017. Ran/flown for an hour, then removed from the aircraft.

Flash forward to last week. We installed the uls engine on my plane. Added oil and antifreeze. Purged the oil system. With plugs out. Turns over no problem. Oil pressure is indicated, no gas or water leaks, as it sits cold. Put the plugs back in and turn it over and the dang thing won't turn past the 1st compression stroke. Like the starter gets hung up or something. I've never really come across a problem like this.

This uls engine has the longer heavy duty starter and still very little cranking is possible. I was told to by service tech at one of the major rotax suppliers to bypass all the other electronic systems and go straight to the start motor and see what happens. So I did this, with good solid car jumper cables. Post on the starter with hot and ground to the side of starter motor. Well what happens is basically the same thing except maybe 1% of the time the starter motor might get to the 2nd compression and stop. I know that the starter can be burned out if not careful. I have not tried to keep craking while it wouldn't turn over. Less than 5 sec bursts, no more than 5 or less at a time, total times tried maybe 30 or 40. Starter outside of engine spins free and clear.

It currently has new gel motorcycle battery with 170cca a little less than half of what is called for (330cca) i know. I have connected a car battery direct to starter and still have the same issue. It just won't crank past 1 mostly and sometimes 2 compositions. The car battery has over 600cca, should be more than enough. There are some notes about a possibility of bad cdi box but im not sure how that would effect the starter. I can't even get it to crank over fast enough for a start yet. Another AP i was talking to said look at the grounds, but how could it be this even if the starter was grounded directly?

I have looked at and tugged on all grounds that I can see/ find. Even tested continuity and by that, all seams to be good. Leakdown test was within 1 or 2 psi. I can't do another to verify till I can get this one fired up and warm. I did do a compression test and all cylinders got up to 90 psi tho 1 and 4 seamed to take twice as long to get there. Not sure if that means something or not. I'm open to any and all suggestions. Thanks again. 

  • Re: Cranking issue

    by » 2 weeks ago


    How hard/easy is it to turn the prop by hand when you have the plugs in?


  • Re: Cranking issue

    by » 2 weeks ago


    Seams to be a normal amout of pressure to me. It might be slightly more than the ul but I have no way to measure. All I have is max compression at 89-91 psi on all cylinders. 


  • Re: Cranking issue

    by » 2 weeks ago


    As Tyler is hinting at; It might have fluids in the cylinders, can you turn it by hand?
    "Burp" the Oil Tank.  It should take about 3 full Slooow turns by hand.
    If that does not clear it, pull the bottom plugs and then power crank it for 10-15 seconds. (Mags OFF!)
    That will clear any excess fluid that may be in the cylinders.
    Place some clean paper towels under the Open plug holes to see if it sprays anything.


    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Bill.Hertzel@Yahoo.com
    Clicking the "Thank You" is Always Appreciated.


  • Re: Cranking issue

    by » 2 weeks ago


    Assuming that the above suggestions above all check out and you still have a problem, can you swap the starter for the one off your old engine which you know was cranking?

    Another option would be to get yourself a current clamp, make sure it is DC not AC! Something like the UNI-T UT210E will measure up to 100A DC. Clamp it around the starter cable and crank it, that will give you a guide as to how much power is being drawn. Unfortunately I don't have any reference values for comparison.

    The installation manual states that the starter solenoid should be capable of 75A continuously and that the standard starter is 0.7kW which would draw a current of about 60A. The HD starter is 0.9kW which would draw 75A. In reality the currents are probably less due to the volt drop through the circuit when under load.

    I would imagine that if the current is relatively low then there is a fault with the starter. If the current is relatively high then I would imagine that there is excessive resistance from the engine.

    If the engine turns by hand with compression strokes that feel normal the problem must be in the starter mechanism. Could there be a problem in the starter (sprag) clutch mechanism?

    Out of curiosity, what went wrong with your old 912UL?


  • Re: Cranking issue

    by » 2 weeks ago


    Sounds like a battery or starter motor issue. Get a local mechanic's opinion.


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