flow through the diaphragm assisted
float needle valve varies
from the conventional float needle. With the conventional
type as shown on the left in Figure 2, fuel flows
from the inlet filter and inlet channel up through the needle
seat orifice past the float needle valve and spills over into
the float bowl. With the diaphragm type float needle valve,
fuel from the inlet filter enters the channel above the float
valve tip. When fuel level is low in the bowl, the needle
valve is off its seat and fuel flows down past the float
valve tip into a fuel channel which leads upward through
the bowl casting to a point above normal liquid level
and spills over into the float bowl.
diaphragm type needle valve differs in operation from
the conventional float needle in that a larger needle seat orifice
can be used to provide greater fuel flow to the float
chamber and yet allow the use of a small float. This is accomplished
through a balance of forces acting on the float
needle valve and diaphragm against fuel pump pressure.
Fuel pressure entering the float needle valve chamber tends to force the
needle valve closed. However, the
same pressure is also acting on the float needle diaphragm.
The diaphragm has a slightly larger area than the float needle valve
head, therefore the greater pressure acting on the diaphragm tends to push
the needle valve off its seat.
The force of the float arm acting on the needle stem,
as the float bowl fills, overcomes this pressure difference and closes
the needle valve. Therefore, the float's function
is to overcome the pressure difference and it does not
have to force a needle valve closed against direct fuel pump
pressure as does the conventional needle type.
If desired, certain Quadrajet models may
be converted from the diaphragm type needle and seat to the conventional
float needle and seat by installing
a service modification kit. Refer
to 9C Parts Section of the Delco Carburetor
9X Manual for specific applications and part
conventional float needle seat on some Quadrajet carburetors
has holes or "side windows" in its side whose purpose is to supplement
fuel delivery past the float needle valve
when it opens, whereas the "side windows" have been removed on some
float needle seat applications so that all fuel
will be discharged over the top of the needle seat assembly. Removal
of the "side windows" improves operation of the float needle on some applications.
conventional float needle seats are nickel plated and use a double angle
seat. The double angle seat, along with
use of a less resilient viton tip float needle, aids in preventing
the float needle from possibly sticking in the seat
due to fuel gum formation.
carburetor float chamber is internally vented on all models
through a vent tube or tubes located in the air horn. The internal
vent tube(s) leads from beneath the air cleaner to
the float bowl chamber. The purpose is to balance air pressure
acting on the fuel in the bowl with air flow through
the carburetor bores. In this way, balanced air/fuel