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JonathanMilbank
Joined: 14 Apr 2012
Posts: 388
Location: Aberdeen area
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 Posted: Sat Aug 08, 2020 12:14 pm Post subject: Floats that sink and floats which can't! |
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Recently I posted something on Matronics entitled "Carburettor icing with heated carbs" and now I must put my hand up and admit that the diagnosis was probably wrong, although I still find it difficult to let that idea go!
Today's weather was about as perfect as it gets in North East Scotland, so three flights were undertaken by a sharing pilot friend and the first two flights went without a hitch. But on the third climb-out passing 800' AGL, the engine misfired briefly which prompted my friend to retard the throttle before cautiously advancing it while he levelled the aircraft. Thereafter it ran as smoothly as anyone could wish until he landed.
Having already found one sunken float after the previously reported occurrence of brief 1 - 2 second violent shaking due to one bank of cylinders failing momentarily, I removed both float bowls again and at first all 4 floats seemed to be buoyant. Then I commenced to press each float down in turn to see whether they bobbed back up to regain their half-submerged levels in the fuel bowl. It quickly became apparent that one float could only be depressed slightly with my finger, as though it was already almost resting on the bottom of the bowl.
Here it comes, now wait for it, wait for it, the smoking gun! The brass sleeve which lines the vertical holes through older floats, had unstuck and slid down the pin which guides the float vertically, until about half of the sleeve was projecting beneath the float. Of course this constitutes an obstruction to prevent downward movement of the float when the fuel level is dropping, particularly during high fuel demand such as during the climb-out after take off. So when fuel is most needed, the float chamber needle valve can't open enough to let more fuel into the bowl at a sufficient rate.
Recently I acquired some replacement floats after one of my old ones had sunk and all of the replacement floats were supplied without brass sleeves lining the vertical holes. I wonder why? Not! Clearly this phenomenon of insecure brass sleeves must have become known to Rotax/Bing, or why else would they have discontinued installing them? If anyone else has been made aware of this, it certainly passed me by.
You won't be surprised to know that all 4 floats in my carburettors are now of the new type without the inserted brass sleeves.
PS: I've just been informed that the latest floats do have brass sleeves inserted, only they're shorter and not so easy to see. Hopefully they're now also not able to slide out of the hole through the float.
http://www.dogaviation.com/2017/10/switching-to-newest-improved-carburetor.html
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Matt Dovey
Joined: 13 May 2017
Posts: 48
Location: st albans, england
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| Posted: Wed Aug 12, 2020 1:50 am Post subject: Re: Floats that sink and floats which can't! |
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If you read the article in your link the brass sleeve are still in the new floats. They are just shorter for the reasons given.
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JonathanMilbank
Joined: 14 Apr 2012
Posts: 388
Location: Aberdeen area
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| Posted: Wed Aug 12, 2020 2:05 am Post subject: Re: Floats that sink and floats which can't! |
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What is also not so easy to see in the cutaway pictures of floats provided in the article, is that the brass sleeves in the old style of floats have slight flares at each end to prevent them sliding out as happened with mine. I still have the defective float, kept as evidence that the brass sleeve which caused the fuel starvation problem has no flares at the ends.
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