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The Rotax notice about carb floats absorbing fuel can be fixed by lightly coating the floats with a novolac (Rhino 9700 epoxy/phenolic) which is impervious to fuels and ethanol. This is the material used in integral fiberglass fuel tanks to keep the glass from being effected by ethanol mogas. It's also the same coating used to protect concrete floors from chemical attack and provides that hard, shiny surface. The novolac has low viscosity and can be brushed on the floats with a fine artist's paint brush.
We weighed the floats before coating application at 85 grains (5.5 gms) per pair and 95 (6.2 gms) grains after coating, within the Rotax limits of 7 gms per pair. We are submerging the floats in mogas with 10% ethanol and will reweigh them in a week to look for weight gain. If none, then the coating should extend the life of the old floats by blocking fuel absorbtion.
This is not an approved Rotax fix but I believe it will work well. Will report in a week on results.
Bob H
  • Re: Carb Float Repairs

    by » 10 years ago


    Bob,
    Where can I buy some of this in the US?

    Mike

  • Re: Carb Float Repairs

    by » 10 years ago


    The float issue isn't an outer coating problem. It is the closed cell structure inside the float. Putting a coating on it is a band-aid and if it gets any kind of a pin hole or gets the skin abraded from rubbing on the side of the bowl from vibration it will still have an issue. You'll have to worry about the outer skin flaking too. Better to just keep a good float if you have it and if you do have one sink just replace them. These floats aren't failing all over the map. This was a good cautionary SB from Rotax for public knowledge on the possibility of a sinking float. It by no means means all these floas are going to sink.

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


  • Re: Carb Float Repairs

    by » 10 years ago


    You can certainly pay $216 for 4 new floats and make Rotax happy. Or you can use the novolac coating which is impervious to fuels. Rhino 9700 is available from the RV guys and maybe Lancair in small quantities. The coating is very hard and does not scuff/spall off by rubbing contact. I normally use it inside glass fuel tanks to separate fuel from glass/epoxy.
    We sumberged a pair of coated floats for 2+ wks in 10% ethanol mogas and weighed each on a very sensitive bullet reloading scale within .01gm. After 2 wks, there was no weight gain which means that the coating blocked any fuel absorbtion. I applied the coating with a fine, soft artist's paint brush, keeping the coating thin. It took 2 coatings to completely cover the floats.

    Amature builders can do as they wish but since I worked in aerospace M&P for many years, I like to find straightforward solutons to technical problems and this one seems to work.
    Bob H

  • Re: Carb Float Repairs

    by » 10 years ago


    The Bing floats don't rely on the outer coating to keep them from sinking. It has more to do with the closed cell construction during their MFG. If the process was bad then fuel can permeate the cells and cause it to sink. An outer coating could work, but you may be just as likely later to get a sunk float if the float abrades on the bowl wall. Then the coating is gone, but so long as the cells are in good shape it shouldn't make a big difference.

    That said any time you have the MFG original coating flaking off it is time to replace.

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


  • Re: Carb Float Repairs

    by » 10 years ago


    I have not placed the coated floats in carbs for testing yet, so that's required for complete validation.

    Roger- The MFG float material is a urethane foam around 2 pcf density that is self-skinning and closed cell. To control that density, you place exact portions of the 2-part urethane into a mold cavity. The foam expands according to it's expansion ratio and if you calculate it correctly, you can hold the required final density. Mistakes occur when an improper amount of the mixture is put into the mold such that too much material makes for higher densily foam that sinks in carb and too little material makes too light a foam which is good for floating but lacks necessary resistance to absorbtion. It has to be done right.
    Another way to make floats is to take pre-expanded foam of the correct density and machine out the floats from that block. However that eliminates the skin which acts as a fuel barrier. They probably do it the first way.
    Bob H

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