My posts from this point on will be a tutorial for those who want to learn. It is important for
those who wish to learn about this carburetor operation not to get sidetracked worrying
about other types of carbs, lets stick to this one basic style first. Once we learn the basics,
we can branch out and look at variations and exceptions.

Lets look closer at the parts of the carb, shall we ? When we examine the parts carefully,
we can see how they work....and how they interact with one another. The interaction of the
parts is simple, and at the same time very clever indeed.

The parts we need to concern ourselves with are the:

1. carb barrel, the part that rotates inside the carb
2. nozzle, the round brass tube that is pressed into the barrel (1). I will refer to this tube
as the spraybar when we talk about it's function in the carb.
3. main needle.
4. low speed needle.

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 amount of air going in at any given throttle
setting remains the same, but we can adjust the amount of fuel by turning the main
needle in, or out. We can adjust the over-all "mixture" of fuel/air.

When we have the barrel all the way open, or in full throttle, the amount of fuel we let
in is still controlled by the taper on the main needle. Once we get the fuel flow adjusted
at full throttle, the taper on the needle takes care of all the "partial throttle settings", except
for the very bottom, or "idle range". The adjustable main needle serves as an adjustable
"main jet" would in other applications.

Lets think about the other end of the nozzle/spraybar now. The low speed
needle is screwed into the carb barrel as well, on the opposite side from the main needle.
It moves out of the spraybar as the barrel is opened, so the amount of fuel changes anytime
the barrel is rotated. Think of the low speed needle as a "plug"....an adjustable plug. The LS
needle will act as a plug, in fact, if you screw the LS needle all the way in....it will completely
plug the nozzle, and no fuel will flow.

The small taper on the low speed needle works just like the taper on the main needle, except
the distance of travel is very small. This distance is called the "idle range". The statement in
the Webra manual is correct when it said; "the tapered tip of the idle needle influences the
idle range in conjunction with the fuel supply opening on the spraybar". The "fuel supply" is
the small amount of fuel, or trickle of fuel that I mentioned earlier. This small amount of fuel
is allowed to flow past the main needle, even if the barrel is closed. There will always be this
small amount of fuel, otherwise we would not have any fuel available for the engine to idle with.

The term "influences" means, of course, "to adjust". Keep in mind that the taper on the LS
needle keeps the mixture correct, just like the taper on the main needle does, but it only
does is in the idle range. When the LS needle is pulled out, or clear of, the spraybar....it no
longer does anything. The main needle is metering the fuel....all the fuel. The LS needle is
now just along for the ride.

This is why Webra states; "the idle needle only affects the "idle range". It is true also that
the high speed needle only affects the high speed range. It is also true, as the Webra manual
says: "it is important to grasp these principles" if you want to understand how the carb works.

FBD.