Darzeelon, I will answer your questions.
I understand perfectly how a two-needle carburettor works.
What I don't understand is your claim in post #39 that the main needle controls 90% of the
carburettor's function...
Would you care to explain how does it, for example, tailor the fuel curve for the engine's
decreased fuel needs at 1/4, 1/3, 1/2 throttle?...
Oh, there is no explanation for this?
That is because it doesn't!
I am not certain you "understand perfectly how a two-needle carburettor works". In order to
understand how this carb works, you must take into account how the main needle and the
nozzle/spraybar work. I have explaianed this already, but perhaps you did not read the
information.
FBD:
You have seen how the barrel inside the carb slides back and forth as you
move the throttle arm ? There is a screw on top of the carb body that threads
into the body. On the end of that screw is a portion that is not threaded. That
non-threaded portion fits into an angled slot in the barrel. When you open and
close the barrel, the carb barrel moves sideways too, because of the angled slot.
Look at your main needle. It is referred to the main needle because it's adjustment
controls about 90% of the carb function, hence the name "main needle". See the thin
taper on the main needle ? This taper, as it rides inside the nozzle, controls all the
function of fuel metering, except for the very small amount in the "idle range" that is
controlled by the low speed, or idle needle. When the main needle is in position, it fits
inside the nozzle/spraybar. As you open the barrel to allow air flow, the nozzle pulls
away from the taper on the needle.
As we increase the amount of air going into the carb, the taper on the main needle lets
more fuel in as well. The taper has been carefully designed so ratio of fuel/air is good
throughout the full range of operation.
The needle/nozzle combination controls
all the mid-range fuel metering except
the idle setting, and the wide open/full speed setting. Without understanding what the taper
on the needle does, it is not possible to understand how the carburator works. 1/8, 1/4,
1/2, 3/4, and 7/8 throttle settings are controled by the high speed needle.
As the carb barrel opens, and slides sideways, it pulls the nozzle away from the HS needle.
As the fat part of the needle moves out, the thinner part of the needle permits more fuel
to flow past itself, and through the spraybar, and out the end of the spraybar. The fuel is
pulled through by the vacuum of the engine, and is helped by the fuel pressure fron the fuel
tank.
Like I said, the taper on the main needle controls 90% of the function of fuel metering.
You said:
Darzeelon: That is because it doesn't!
You might want to have another look at a carb or read up on them to gain the understanding
on how they actually work.
FBD.
