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912 ULS float valve overflow overflew

  • Ukpylot
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8 months 1 week ago #18675 by Ukpylot
912 ULS float valve overflow overflew was created by Ukpylot
Short story: my float valve overflow line was full of fuel and leaking out at idle, engine vibrating a lot - what causes this? The engine has 3 hrs on it.

Due to some travel my 912ULS hadn't been started in about 5 weeks, so I went out and drained the gas and put new gas, went to start it up and it was hard. But got it started and did some taxiing - with a lot of vibration under 2000 rpm, everything good higher than that. Came back to the hangar and put throttle to idle and revs dropped to 400 and then the engine quit. Not good.

Went back next day with someone who knows more about engines than me and we discovered that the throttle cable on one of the carbs was buckled a bit and so we weren't mechanically in sync any more. We took care of that, started the plane up (not first time but not as hard as the previous day) and it was running great, idling at 1650 and then the vibration started again and fuel started to sprinkle a little on the windshield.

I stopped the engine, took a look and the float valve overflow lines were full of fuel and had been spitting out in the direction of the windscreen. At that point we decided we didn't know enough to continue and I should post here.

I did go back out again yesterday to see if running it for a while would see the problem through but I couldn't even start it (part of me wonders if the battery isn't fully charged after such a long time without starting).

So I'm curious if people can suggest why the float valve might overflow? Everything had been working great before the break, start first time, purr like a kitten. And, more importantly, ideas for getting it running properly again?

Thanks,

Paul

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8 months 1 week ago #18680 by RobSeaton
Replied by RobSeaton on topic 912 ULS float valve overflow overflew
the carbs are shaking so the float valve cannot seal.
You need to pneumatically sync the carbs and set the idle stops.
Start by watching the carb sync video series www.rotax-owner.com/en/videos-topmenu/rotax-minute/408-cs-4247

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8 months 1 week ago #18687 by Roger Lee
Replied by Roger Lee on topic 912 ULS float valve overflow overflew
Listen to Rob about carb sync first.

Here are some more common vibration causes:

www.rotax-owner.com/en/rotax-blog/item/57-vibration-help

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell
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8 months 1 week ago #18727 by Ukpylot
Replied by Ukpylot on topic 912 ULS float valve overflow overflew
Thanks everyone for the replies. Today I did both carb syncs and redid the throttle cables and things are a lot better. No vibrations and a steady idle at about 1950.

The leak from the float valve overflow ddown not stop so I took a look and bent the tab a little to close the valve more strongly. After that the leak stopped and I think I’m ready to go back to taxi tests.
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8 months 1 week ago #18730 by Roger Lee
Replied by Roger Lee on topic 912 ULS float valve overflow overflew
Hi Paul,
Sounds like you're on the right track. You can set that idle down a bit.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell

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8 months 1 week ago #18736 by Ukpylot
Replied by Ukpylot on topic 912 ULS float valve overflow overflew
Hi Roger,

What would be a better setting? The video said between 1800 and 2200.

And what would be the right way to lower the idle now that the carbs are synced?

Thanks,

Paul

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8 months 1 week ago #18737 by Roger Lee
Replied by Roger Lee on topic 912 ULS float valve overflow overflew
Anytime you do any adjustment to the carbs the gauges should be on. One carb may be more influential for what you are trying to accomplish. Trying to just adjust the idle lower without the gauges will most likely put them out of sync.
Many people have varying likes and dislikes for setting their idle rpm so some of this become personal preference. Too high an idle can make you drag your brakes constantly during taxi or too high an idle may make the starting sequence different to keep from flooding. You can set it for something like 1650 so long as you know not to ever just sit there at that low rpm in a 912ULS with its 10.8:1 compression and let the gearbox bang around. Always best to warm up or sit and idle at 2000 or better. The lower 1650 rpm is better suited for pulling back to idle for landing (personal preference here because the stick actually controls the speed not the throttle) or just when you get ready to shut the engine down pull it back to this lower idle and then turn the mags off.

So you may get differing opinions here on a good idle rpm number, but it's not an exact number, but it is within an rpm range that suits your needs, likes and specific aircraft.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell

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8 months 1 week ago #18738 by Pacheco
Replied by Pacheco on topic 912 ULS float valve overflow overflew
I agree with Roger
Something like 1650 to shut the engine down is good but don´t forget to maintain the engine running at about 2000 or more during warm up and taxing to preserve the gearbox.

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8 months 1 week ago #18739 by kenryan
Replied by kenryan on topic 912 ULS float valve overflow overflew
Do these comments that caution not to idle the 912 ULS too slowly apply equally to the lower compression 912 and the 914?

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8 months 1 week ago #18740 by DHeal
Replied by DHeal on topic 912 ULS float valve overflow overflew
Paul -- I concur with Roger's suggestion of a 1,650 rpm idle speed. That is what Van's Aircraft recommends for the RV-12 with the 912 ULS and it works very well for my operations. As Roger says, don't let it go much below 1,800 - 2,000 rpm for any length of time. Active observation and management of the throttle / RPM by the pilot is a very useful habit with the ROTAX. -- David

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8 months 1 week ago #18741 by Ukpylot
Replied by Ukpylot on topic 912 ULS float valve overflow overflew
When I left the hangar yesterday everything seemed great, engine sounded good at all speeds, no leaks but when I went back today to recheck the idle speed everything started up nice, 2000 rpm for a couple of minutes then 2500 to get the temps up.... after about a minute there was fuel venting out of the float valve again and some small vibration that wasn't there yesterday.

I suppose I have to put it back on the gauges but it all looked good when we left, seems odd that it would change overnight though I wonder if it is more to do with how warmed up the engine was yesterday by the time we were done as opposed to the 'cold' start today. I say "cold" because it is 92F out.

Any thoughts on the float valve? Still a vibration issue?

I may have to see if the FBO mechanic can do something but I doubt they've seen a Rotax any more than I have.

Paul

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8 months 1 week ago - 8 months 1 week ago #18742 by Roger Lee
Replied by Roger Lee on topic 912 ULS float valve overflow overflew
Pull the bowls and weigh the floats even if you have fairly new ones. Then you may need to pull the carb and turn it upside down and make sure the float armature that the floats ride on is at its proper height. It is possible that the needle valve is bad, but more likely the other two items may be the offender.
You can play around for another week, but the sooner you check these items to sooner the problem gets fixed or at the least you rule them out.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell
Last edit: 8 months 1 week ago by Roger Lee.

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8 months 8 hours ago #18784 by Ukpylot
Replied by Ukpylot on topic 912 ULS float valve overflow overflew
I weighed the floats in the leaking carb and they came to 11.7g so I definitely need to replace those. Will order them this week and I expect I'll be able to have them in and try again next weekend.

I'm curious how they can get to weigh so much - how does the fuel get in?

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8 months 2 hours ago #18786 by Roger Lee
Replied by Roger Lee on topic 912 ULS float valve overflow overflew
When the floats sink that allows more fuel to flow and makes idle rpms too rich and flood the carbs. You may have fuel coming out the vent tubes.
The floats are made of millions of tiny air bubbles and each seal themselves from fuel absorption. If the material composition is wrong then fuel can penetrate the float over some time and they loose the ability to float and slowly start to sink and fail to control the fuel level in the bowl.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell

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7 months 3 weeks ago #18919 by Ukpylot
Replied by Ukpylot on topic 912 ULS float valve overflow overflew
Thanks for the explanation Roger, makes sense.

This weekend I got my replacement floats and after swapping them out I'm happy to say that the engine seems back to normal, no vibrations and no fuel out of the overflow. Ran it through a range of RPMs and all seemed good, will do some taxi tests to verify before taking it back up.

Thanks to you and everyone who made suggestions, all very helpful

Paul

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