Hi Henk & Bart,
While I am not flying yet, I naturally have an opinion on vapor lock and how
to minimize it with the Rotax fuel setup.
I believe that the fuel return is in fact a vapor return and that the
location and orientation of the lines which make up the return and carburetor
connections are important, although not well documented.
There are 5 connections (assuming a fuel pressure gauge is fitted) required
---From the fuel pump to the carburetors and return line. The gauge and return
lines should be fitted to opposite sides of a "T". The bottom of this "T"
should go DOWNWARD to a second "T" to which the fuel pump connects. The down
line then continues to a third "T" which runs to each carb.
Rotax has a 5 way manifold which facilitates these connections -- the logic
of connection to the manifold is similar although they introduce the fuel at
the bottom since they aren't constrained by use of "T"'s -- note that the
return is at the top of the manifold.
The goal is to have fuel go into the downward going line to the carbs while
the vapor rises in this line and goes into the return line; that is, the
downward line functions as a fuel/vapor separator. Since vapor can go
through the return line restrictor much more rapidly than fuel, the system
should quickly purge any vapor which arrives at the first "T" and fill the
downward line with fuel. Fuel at the lower "T" is pumped to the carbs
although heat pickup in the carb lines could cause vapor which must be
separated by the carburettors via the float bowl. Insulate all lines,
including those from the "T" to the carbs.
Vapor can theoretically occur in the carbs themselves due to boiling of fuel
in the float bowl so it is wise to open the access doors after stopping the
engine if a restart is anticipated. Probably a good idea in any case in
order to minimize heat deterioration of rubber components under the cowl.
Of course, all of this is just theoretical since I'm still building... Any
comments or corrections are welcome since this theory will eventually become
practice here.
Regards, John A044
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