| This
is generally how Model Engine Carburetors work |
| A
model carburetor usually has two mixture controls. High speed and low speed or
idle. These mixture controls alter the fuel/air mixture at the various carburetor
openings. Usually the needle valves will meter the amount of fuel allowed to mix
with the air. A needle valve is a tapered needle that is moved into a hole to
provide a very precise adjustment. Less
expensive carburetors have an air bleed for the idle adjustment. This is just
that, it bleeds in extra air at idle to obtain the correct mixture. The only issue
with air bleed is you usually won't get a good crisp transition to high speed
or a good mixture just above idle or mid range. |
|
|
| A very popular design
uses two needles, one on each side of the carburetor. The rotating barrel moves
in and out as it is rotated. As the barrel is closed to an idle the idle needle
moved into the spray bar as shown below. This will then restrict the fuel to set
the idle mixture. As you can see the idle needle is tapered and it is going into
the end of the spray bar. |
| |
It
is very important with this design to never bottom out or force the idle needle
into the end of the spray bar. If you force the idle needle into the end of the
spray bar you can bell mouth or flair the end of the spray bar.
If this happens you will never be able to get the proper idle adjustment, mid
range or transition. This is why we do not suggest "turns out" for idle
needles. Because to find turns out you will usually bottom the idle needle and
screw up the spray bar. See How to adjust aero carbs
or marine carbs |
|
| Shown
below is a variation of the same principle as described above. As the barrel closes
and moves inward the slot is covered thus restricting the fuel flow at idle. With
this design the possibility of ruining the spraybar is eliminated. |
| |
|
| Another
way of metering the fuel mixture at idle is done without the in and out motion
of the barrel but with the actual rotation of the barrel. This is the way the
mixture disk works on the K&B carb or a Perry carb. As the barrel is rotated
the outer sleeve rotates with the barrel and the inner tube remains stationary
as it is attached to the carb body. This type of design requires very little movement
to the idle adjustment. The adjustment is made by rotating the inner tube only
a few degrees in either direction so the slot is open or closed more at idle.
See How to adjust a K&B carb with the brass
mixture disk |
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