I have a 912 UL fitted on my Rans S-12 Airaile. I live in the Philippines (tropics). When I start the engine from cold (after burping) with full choke and closed throttle, the engine fires and idles smoothly. Warm up takes 'forever' but engine runs real sweet throughout the whole rev range. So, that's the easy part.

After returning from a flight and shutting down, I cannot get the engine to restart. The best I can manage is for the engine to fire and run so roughly that it is shaking engine and airframe apart. If I advance the throttle, the engine will not increase rpm and either stalls or continues to rattle. If I persevere (which I am loath to do because of the damage the vibration must surely be causing) the engine will, after about 45-90 seconds, suddenly spring to life and run smoothly. I have tried choke; no choke, throttle; no throttle - it makes no difference.

When the engine has been rattling itself to death, I have run a mag check; this doesn't affect the engine rough running in any manner but neither does the engine stop (ruling out an ignition problem!)

I've had the carbs stripped, cleaned and tuned; still no improvement! Battery is good; proved by the engine starting okay from cold!

I'm not an engine mechanic but my A&P guy (who has received some Rotax training) is also at a loss. We don't want to spend on replacement carbs or any other parts until we can definitively determine the cause.

The problem now has me grounded so I am desperate for help.

Thanks in advance to anyone who might have some ideas! :(
  • Re: Cold starts are easy; warm starts are impossible

    by » 12 years ago

    Hi Terry

    The first place I would look is the gearbox. How many hours has the engine done and has the gearbox been serviced.

    What type of prop have you got fitted and does it have a slipper clutch fitted.

    Cheers Mark.

  • Re: Cold starts are easy; warm starts are impossible

    by » 12 years ago

    To the best of my knowledge, there is no slipper clutch in the gearbox. The engine is about 10 years old but only has a couple of hundred hours on it. Prop is a 68" warpdrive 3-bladed.

    This morning, we checked the ignition cables. I read that it is possible that the cable isn't properly connecting with the pin (in the connector) and fires with a weakened spark. cecks confirmed this is not the case.

  • Re: Cold starts are easy; warm starts are impossible

    by » 12 years ago


    Do you have a fuel return line installed? Sometimes in hot weather, vapor lock can be a problem when starting a hot engine. A fuel return line can help, and usually cure that problem. Insulated fuel lines can also help.


  • Re: Cold starts are easy; warm starts are impossible

    by » 12 years ago

    I'm kind of with Bill. This sounds a little like heated fuel lines. Are they in fire sleeve? Are they away from a heat source like the hot exhaust pipe? Do you have a return line? This doesn't sound electrical, but a fuel issue. One last thing. After you shut it down go out and turn the prop by hand. Is it easy or really hard to turn? (Just curious on this one)

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

  • Re: Cold starts are easy; warm starts are impossible

    by » 12 years ago

    Thanks; Mark, Bill and Roger.

    Will try all of your suggestions.

    Just as a bit of background, we bought this engine which had been fitted to an air-boat. I know when we got the engine that the previous owner said it ran like clockwork apart from a rough idle. So we were expecting some problems. In truth, the idle is fine; it purrs away at 1200rpm and is smooth throughout the range. And as i have said, it start fine when it is cold.

    I'm trying to find the words to describe what starting the engine is like when it is hot. To me (and I am no engine expert) it feels like it isn't firing on all cylinders and continues to thump and thrash while inducing severe vibration in the airframe (and engine mounts). When I advance the throttle, nothing happens; that is, the rpm will not increase (although sometimes this will make the engine stop).

    I can persevere (but I am worried about the damage this is doing to the engine and the mounts) and it will eventually suddenly 'all come together' and run sweetly.

    I'll have the fuel lines checked tomorrow morning.

    By the way, it doesn't make any difference whether I have the fuel pump on or off.

    Thanks again for all your help.


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