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Frequent engine-outs at launch. Cause?


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I've got the ROS 125 motor and had 3 flights today. I've been having engine-outs and blips in power recently at launch but since I'm a new pilot I figure that maybe I'm not warming up the engine enough before flight. I have no issues during normal flight. 

Since it takes about 10 seconds at full throttle for me to climb to a safe height at launch, I figure that a reasonable warm-up technique will be to stand on the ground and run the motor at full throttle at 10 seconds before each launch. 

- I let the engine idle without a prop for 10 minutes as I got other stuff ready. 

- Turned the engine off to put on the prop.

- Right before the first flight, I go full throttle while on the ground but at about 5 seconds it goes out and I have to restart it. I go full throttle again and it does the full 10 seconds without issue and the climb out has no issues. 

- Landed and turned off the engine for about 10 minutes. 

- Warmed up engine at full throttle for the second flight. There must have been a bubble or something because after about 4 seconds the engine power died momentary and then quickly went back up.

- The same blip occurred while climbing, giving me a bit of a scare.

- Landed and turned off the motor for 15 minutes.

- Filled the fuel tank to half from quarter full.

- Did the same warm up on the ground and this time the motor died twice before it could do the full 10-seconds at full throttle. 

- During climb out there was another sudden loss in power but it seemed to last a bit longer than last time. I was already thinking about my glide down but the engine thankfully kicked back to life.

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@ full throttle the engine requires a big amount of fuel. The problem lets me think to a lack of fuel, and this can be caused by a problem on the fuel line (bubbles, chokes).

Please carefully re-check all the fuel line looking for loose connections, dirty filters, chokes and so. Be sure there are no air bubbles in the fuel line.

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Warming a 2 stroke at idle is not good practice. I believe the ROS has a clutch?  If so, the engine will tick over with a prop fitted, without the prop turning.

To warm the engine properly, it should be under load (driving the prop). With the paramotor on your back and after checking the area and thrustline are clear, start the engine and run at a little more than idle (enough to engage the clutch and turn the prop) until it runs smoothly. This allows for it to clear any excess fuel from starting.

Next, slowly increase throttle so that the engine builds RPM smoothly until you are at full throttle and, as you say, hold it there for a few seconds. You can close the throttle more quickly, but best not to just 'snap' it shut. Closing the throttle quickly means that the engine is still spinning fast, but drawing against a closed carb. The engine only gets its oil with the fuel, so high RPM plus closed throttle equals insufficient lubrication. It can also cause you to lose balance as the torque and thrust drop away.

With the engine at idle, prepare yourself for a burst of thrust and torque, then open the throttle quickly. If all is well, the engine should pick up smoothly. Hesitation or stalling indicates either an insufficiently warmed engine or an issue with its set-up.

At the heart of all this, the key is to warm the engine evenly. Too fast and the parts will expand at different rates, leading to leaks and possible seizure.

I have had an occasional momentary loss of power, which I soon traced to accidentally hitting the kill switch (on the throttle grip). :oops: 

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Idling the engine once it is warm is not so bad, but warming it by idling is an invitation for trouble:

The cold plug is more likely to foul.

Fuel (and lubricant) is more likely to condense on the inside of the crankcase, causing lean running and reduced lubrication.

Condensed fuel and oil may enter the cylinder when the throttle is opened, with the potential to cause plug fouling or stalling.

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  • 2 weeks later...

I sound like a broken record. Fuel primer? Do you have a squeezy bulb, if it feel hard to squash then the little internal 1 way valves could be sticking. They are open when idle to mid throttle but under full throttle they get sucked up and close, causing a dip in power through starvation. Seems obvious but very hard to find when your trouble shooting an engine. Eliminate or replace the bulb, its a cheap and simple way to tick it off the possible causes.

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