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Periodic inspection of the float buoyancy for ROTAX® Engine Type 912 and 914 (Series)


WmInce

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I wonder how may floats are actually defective?  Is this a rare problem, or is every float manufactured during the specified period defective?

 

Next question, Rotax says to check every 25 hours but, if a float has been sitting in fuel for a year and meets specification (either according to the volume method or, the seemingly better weight method) is it going to continue to meet specification, or will it fail at any time?  

 

(Yup, who knows...)

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I have had two fairly new aircraft with approximately 60 hrs on them with a sunk float. Most of these bad floats are on newer planes and have very little time on them before symptoms show up. If you have had no issue and you have more than 100 hrs. you most likely aren't going to. That said any float can get a pin hole and absorb fuel. The floats are a hard foam with thousands of little air pockets then covered in a fuel proof coating. When a carb float absorbs fuel your first tattletale sign is usually fuel smell and fuel venting from the carb vent line and may easily have rough running because of a flooded carb. When one float sinks the other can no longer control the fuel level and the carb simply over flows. The easiest way to check since all of us here has had our floats in fuel already is to pop the bowl off and see if the brass pins coming out the side of the float is equal to the fuel level and both floats are the same. If a float has absorbed any fuel the pin on the float will be under the fuel level or all the way on the bottom of the bowl.

You can weigh them, but you need a very sensitive scale that can read in tenths of a gram and if it has absorbed fuel it will have sunk and the pin will be under the fuel level. 

 

If you don't have symptoms then I wouldn't loose sleep over it until you get it checked. If a float sinks you WILL have symptoms.

 

 

I posted a video here a little while ago to remove, inspect and clean a float bowl. That technique works well here.

 

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Just idling would take a long time to drain the bowls. You may have to have pizza delivered before it shut down. 

Floats aren't typically replaced on a rebuild. They usually hold up for many many years.

I honestly would not worry about a sinking float. It does happen, but isn't usually that prevalent.

The fact that we don't see talk about sunken floats all over the forums should be testament that they are pretty resistant.

 

You should be turning your fuel off though every time you park it for the night. In a high wing that's just constant pressure against the system 24/7 and if it did have an issue it will leak through.

 

Personally I won't loose any sleep unless I developed symptoms. Worrying about it is like putting the cart before the horse.

 

Fuel venting, fuel smell, rough running engine.

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Turning off the fuel is a good idea. Especially in a hangar.

 

Something else that may be a good idea is placing blocks under each axle when in your hangar. If you have a tire go flat with fairly full tanks this will prevent a mess on the hangar floor. I use two 4X4 blocks stacked on each side. They stay put such that I rarely have to adjust them. I have not done any full tank flat tire tests but wouldn't object to someone out there trying it. :ph34r: If it accomplishes nothing else I will never have a flat in the hangar - just like having a generator protects you from having a prolonged power failure at your house. <_<

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I'd like a little more information on the construction of the floats. I understand they are solid, but based on Roger's reference to a "pin hole" one assumes they are covered with some material that inhibits the entrance of gasoline. Does this mean the interior of the float is a substance that readily absorbs and holds gasoline? What is the outer covering substance? Is the interior substance closed cell in structure, so it only absorbs right where the "pin hole" is or can it soak through the entire interior?

I guess I need to cut one of my floats open and look at it.

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The float insides are a high density foam that has an outer coating. The coating can flake off or get a pin hole. Once the outer shell is breached the fuel will slowly begin to saturate the foam until it starts sinking. I did an annual on a CTSW today and inspected the carb bowls for debris. The hose change was done by me earlier this year. The bowls were spotless, but one float had a spot that where the coating was worn off and it would flake on my fingers. This float set got replaced. It was on its way out. This plane is head up north to the Page fly-in and having a flooded carb over some of those areas would be heart stopping.

 

I cut it in half and was going to take a picture of the foam inner core, but the foam is so dense I didn't think it would show up in the photo.

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I appreciate the response but it begs the question of whether the inner material is closed cell - apparently it is not and the gasoline either goes through the cell membrane or there is no real closed cell structure. If I find a bad one, I'm going to cut it open.

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As a little history lesson...

 

I've been futzing with BING carbs like this since I bought my 1986 R80RT:

 

15490108102_7f5f4077c6_z.jpg

 

Floats and float settings have always been problematical, as have floats absorbing gas.

 

In any event, this is what the floats used to look like:

 

15303882717_cb0794b385_z.jpg

 

Just foam, no apparent coating or at most a very thin, transparent one.

 

I think BING had to redesign them for aircraft use, since a slip or skid could affect the mixture. Hence, the move to the split floats.

 

And that new, split float design was offered as an upgrade, which I did to my R100GS after having a wet foot from flooded carb one too many times:

 

15303955037_8a8176e5de_c.jpg

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