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Last June I started noticing a fuel smell followed by a 2-3 sec engine sputter during takeoff climb out.  Since the carbs were 5 hrs from the 200 hr check I decided to rebuild them complete and replace the floats with the newest Rotax floats with R on the side.  I replaced all other components and set float brackets to 10.5 mm.  Reassembled, installed synchronized and have been flying without issue the last 10 months.  Yesterday the same issue returned.  First thought was heavy floats, but didn't notice any fuel in the trays, but did notice a few drops of air filter oil and gas mixture on bottom of the filters and dip trays.  I changed the floats in both carbs because I had sets of new ones.  Test flew and had the same problem.  Also noticed the same oil fuel on the bottom of the filters. I removed the right carb and disassembled it and found nothing that looks out of order.  I just don't understand why the issues returned after 10 months of service.  I will check the float bracket height again, but it still visually appears parallel with the base.  I hope someone can help me find what l'm missing.

  • Re: 912 uls bing 64 issue

    by » one month ago


    Did you weigh the floats?  <7.0 grams per pair.  You might consider the new Marvel Schebler epoxy floats.  I have MS floats and so does a friend - both flying RV-12 with no more carb problems.  Also, look here.... https://vansairforce.net/community/showthread.php?t=194385&highlight=vapor+lock


  • Re: 912 uls bing 64 issue

    by » one month ago


    To add to this story  I have found the new floats too heavy  they over 3 grams each new  the old ones were 2.65 grams  i have been working on rotax and rotax 912 motors for over 20 years if i weigh the floats and fond them  3g or more i remove them and fit lighter floats. the issues i have found over the years  is fuel pump pressure   for reliable running i have found that you dont want more than 4 PSI fuel pressure and about 3 to 3.5 psi at WOT  this gets the best power smoothest idle and  no flooding.  I have also found there are 3 different fuel needles  one is silver  it has the softest spring and is the best for a reliable float level and n flooding at low speeds as the soft spring cushions the floats and they remain closed even if the idle is rough   then there is the gold  ones  they harder than the silver but still pretty reliable  they can flood at low idle speeds or with vibration  and lastly i have found the black ones   they are very hard and they can even cause inflight flooding  i always throw them away. I always check the CO readings and make sure they are as per Rotax recommended readings 

    What i would check  is  1) the float weights  and  2) the fuel pump pressure      what i have found if you have a fuel pressure of 5 to 6 psi  youhave to have floats that are under 2.8 grams  if it is 4 to 5 psi the 3 gram  float will work.

    When you  have the floats floating in the bowl the fuel needs to be inline with the bottom of the float pin  if the pin is not above the fuel  or just touching the surface  the float is too heavy. often  you can mix and match the floats  if you have a scale that measures to 1/1000  of a gram  then you can average out the total weights  dont have two heavy floats in the same carb. 

    The new fuel pumps can have there pressure changed by changing the pressure control spring  this tends to fix most issues. 


  • Re: 912 uls bing 64 issue

    by » one month ago


    The new Rotax floats weight checks at 5.9 to 6.0 grams.  The floats I used 10 months ago and the ones I used two days ago are virtually identical.  Same Rotax pn and weight with R on the side.  Have read about the marvel floats, but would like to stay with Rotax parts.  I also installed a new fuel pump last year shortly after the carb rebuild and fuel pressure has always been 5.4 - 5.6 psi.  This is with both electric and mechanical combined.  Vans says the electric pump should always be on in the RV-12.  I have doubts that this is a float or fuel pressure problem because everything functioned perfect for 10 months.  Today I plan to check float bracket and float needle, but visually they looked OK on the right carb.


  • Re: 912 uls bing 64 issue

    by » one month ago


    Fuel pressure has always been 5.4-5.6 psi even after fuel pump replacement which also occurred last year shortly after carb rebuild.  Did check floats in the bowls and both are exactly as you describe.  


  • Re: 912 uls bing 64 issue

    by » one month ago


    I see 3.5 psi - 5.5 psi as the norm and it's never an issue. Using the electric pump usually adds about .5 psi. The new float weight after coming out of fuel is around 6.3 - 6.5 grams. That's fine. The older style floats can be around 5.9 - 6.4 grams after coming out of fuel. You are supposed to weigh floats at every annual. I also take the time to clean everyone's idle jet since it's right there and easy to get to. I find most at least partially clogged. It only takes a couple of minutes.

    It may be worth going back into both floats and make sure all is good. Have you done a good carb sync? I mean no disrespect, but are you sure they were properly re-assembled and something inside hasn't  either come loose or not in quite right.? It sounds odd that you're seeing fuel at the air intake if something isn't amiss in the carb.


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


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