For a large portion of time during the construction of the DOG Aviation RV-12 there have been some issues with the Rotax 912 ULS engine’s carburetor floats. The root of the problem revolves around some carburetor floats absorbing fuel and sinking in the float bowls. Apparently, the material the floats were made from (looks to me like some sort of special foam plastic with a hard outer coating or sealant) tend to adsorb fuel which causes them to get heaver so they begin sinking in the carburetor bowls. The sinking floats allow more fuel into the carburetor bowls than desirable … which begins to create a rich mixture as the engine becomes flooded with excess fuel.
A few years back when this began to become problematic, Rotax changed
the manufacturing process and tested the floats. The newer tested floats were
given dimples for identification so they can be distinguished from the older non-dimpled
floats and were supposed to be “the fix” for the fuel absorption issue. Rotax issued a service
bulletin suggesting that all non-dimpled floats should be replaced with ones that
are dimpled as pictured below. And, if not replaced, the non-dimpled floats required being
checked at regular intervals (25 hours or 60 days) by using one of two methods ….
either remove the floats and weigh each pair (the pair must weigh less than 7
grams) or pass a displacement test where a measured amount of fuel is injected
into the carb with a syringe while looking at the overflow orifice until fuel
flows out of it. Depending on the amount of fuel it takes to see fuel exiting
the overflow orifice, one can determine if the floats are soaked with fuel or
not. Either way, it is a pain in the butt.
Going back a few years, per a Rotax service bulletin, older non-dimpled
carburetor floats in Rotax 912 engines are to be replaced with floats that are
dimpled such as the ones shown in these photos.
However, as time went on, it was
realized that even the newer “dimpled” replacement floats were developing the same
problem of adsorbing fuel and sinking in the carburetor bowls … it just did not
occur as often. Now Rotax has changed the floats yet again and given them
yet another new part number of 861-188. It is my understanding Rotax has also changed
the material the floats are made from and is now making the floats from a denser material. The “newest”
floats can be identified by what appears to be the loss of the brass sleeve or bushing
that passes through the float. In fact, there is still a brass bushing but it is
much shorter and centered in the hole making it difficult to see. This next
statement is purely a guess on my part: I suspect because the material the newest
floats are made from is denser, it is very likely also heaver … so to keep the
measured weight for each float the same as the old floats, the amount of brass
used for the bushing was reduced.
The old style float is on the left and the “newest” style 861-188 float
is on the right. The maximum weight remains the same … both floats weighed
together must weigh less than 7 grams. (They typically weigh slightly under 3
grams each out of the box).
Admittedly, I was not currently experiencing issues with the floats in
the DOG Aviation RV-12 but decided to make a preemptive strike and just replace
the floats with the “newest” 861-188 floats now that they are available. When
ordering the newest floats, the DOG Aviation procurement department also
purchased a float bracket gauge (part number 877-730) so the float arms could
be accurately adjusted to parallel. To use the float bracket gauge the main jet
is removed and the gauge is screwed to the base of the carburetor in place of
the main jet. Glad the gauge was
purchased because the float bracket arms were adjusted by sight the last time
the carb floats were changed … but the gauge reveled that although close (think
it was around .024"), the float bracket was out of spec which is listed between
.016" to .020" measured between the gauge and the float bracket arm
using a feeler gauge. With the 877-730 gauge in place, the float bracket arms
were adjusted to .018" which is the sweet spot per the maintenance manual.
The Rotax 877-730 float bracket arm gauge in position after removing the
main jet. The float bracket arms are tweaked so the gap between the float
bracket arm and the gauge is .016" - .020" … I chose to adjust the
arms to .018" as can be seen here.
Hopefully, by switching to the newest style 861-188 carburetor floats at
this time, it will eliminate the likelihood of carburetor float issues in the