[FlyRotary] Re: nonEFI fuel
systems
Hi, Ken....I also have been
looking at the Weber carb. Without going into full details, how
difficult is it to add a mixture control to the 48 Weber?
Thanks in advance for your reply. Paul
Conner
There are two way to do it,
one is with an air bleed system that bleeds air form the intake to the
float bole, this is what they use on some snowmobiles and I have not
tried this yet on a Weber.
The system that I use is an air ram
that changes the pressure differential between the float bole and the
Venturi.
The ram tubes in this photo are
positioned about 1.5 inches from the velocity stacks, when the ram is
closed the air is drawn only from the air cleaner which is where the
float bole vent is, when you open the air ram it increases the air
pressure on the Venturi's and the float bole vent pressure that is in
the air cleaner remains the same so it draws less fuel.
If I start opening the ram at cruise power
setting setting the tac will start climbing until it reaches perfect
fuel mixture and then it will start to drop as it goes in the lean
side and for economy cruise I drop the rpm about 100 rpm on the lean
side, depending on altitude at say less than 5000 ft if I open the ram
wide open if will kill the engine but at over 12,000 ft it can take
all the air without killing it, at 16,000 ft the ram will raise the
tac 1000 rpm which is enough difference between holding alt and coming
down and I would have never made it up there without the
ram.
One other thing about the Weber is
that its very high tec compared to crude aircraft or American
carb's in that it is very adjustable and it is very sensitive to
pressure changes, if the air is thinner there is less draw one the
main jets so there will be less fuel drawn in.
This can be adjusted for the
particular engine and application by changing the air correction jets
and the emulsion tubes in relation to the main jets, you can go to a
smaller main gas jet and a smaller main air and the mixture on the top
end will remain the same but it will lean the bottom end and mid
range, the emulsion tubes mostly effect the mid range.
By jetting down to a smaller air
correction jet and smaller main gas jet makes it more sensitive air
density and to be honest I don't think the combination the I am
currently running is the best as I only have 100 hrs flying on the
48mm where I had 1000 hrs on the 51mm so I had it dialed in much
better
Ken
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