Holley model 4150 vacuum secondary carburetors
are easily identified by these features dual feed hung float bowls
a primary metering block and a secondary metering block
and no accelerator pump on the rear bowl most distinctly
a vacuum unit will be connected to the secondary throttle shaft on the
passenger side the carburetor
4150 models are designed to fit to a square bore
intake bolt pattern they can also use an adapter
to bolt to spread bore intakes there's a lot more to a 4150 vacuum secondary
carburetor
come in for a look inside let's start out with the throttle body
this component is also referred to as the base plate and might have a few
vacuum port configurations
most Holley carburetors have at least one large port that's intended for
drying in crankcase fumes from a PCV valve
and one smaller port that's meant for hooking up any accessories that require
full vacuum
if your base plate has two large ports one is meant for PCV
and the other could be used for connecting to power brake booster
keep any unused ports plugged with a cap to eliminate any vacuum leaks
on the base plate of a 4150 you'll notice an arm the will operate the accelerator
pump of the front fuel bowl
model 4150 vacuum secondary carburetors
only have one accelerator pump located on the front fuel bowl
the throttle plate assembly also has a curb idle adjuster screw
on the driver side which sets the idle speed of the engine by opening and
closing the primary throttle plates
which allows more air to enter the engine on the passenger side
you'll find the fast idle cam adjustment screw for models with electric chokes
the main body of a 4150 will have a choke horn
and choke plate that controls the airflow amount during the engine warm-up
under the choke plate you'll find the two primary venturi bores
and boosters you'll also find an accelerator pump discharge nozzle
that sometimes called a squirter on a vacuum secondary 4150 model
there's no accelerator pump for the rear bowl so in turn there's no discharge
nozzle
for the secondary venturis the volume and duration of fuel flow through the
primary squirter
can be customized with the Holley accessory kit you can swap to a
different size squirter
and you can switch to one of many different accelerator pump cams
that are changed out on the throttle plate assembly back on the body
the front and rear venturis have 4 small holes that are the
air bleeds for the primary idle and main fuel metering systems
the outer 2 are for the idle circuit and the inner 2
are for the main circuit these holes should always be kept free of debris
and can be flushed with carburetor cleaner as maintenance
like all Holley carburetors the 4150 has a primary metering block
that will have two primary Jets to control normal fuel flow
and a power valve the will act as an auxiliary fuel supply during
acceleration
idle mixture is set by adjusting the idle mixture screws located on each side
of the metering block
on a model 4150 just above the passenger side
idle mixture screw you'll find a small vacuum port
this is called the timed or spark advance vacuum port
at idle this port will show little or no vacuum to keep the distributor
from advancing prematurely
if you have a mechanical advance distributor you'll need to plug up this
port
all 4150 vacuum secondary carburetors come with dual feed
Center hung float bowls inside a dual feed bowl
there's a float that's fixed in place with a center mounted hinge
that will press on a needle and seat assembly thats positioned above
the float and seat work together to control the fuel level in each
fuel bowl on top of the bowl for some models
you'll find an adjusting nut that will control the level of fuel
inside the bowl on the bottom exterior of all primary fuel bowls
is an accelerator pump the accelerator pump arm
is affixed to the base plate and presses against the diaphragm inside the
accelerator pump
the pump is activated every time the throttle is pressed
and will send fuel immediately to the squirter that we looked at earlier
4150 model carburetors also have a secondary metering block
in the rear block a vacuum secondary model
you will find Jets but you won't find a power valve
also there will not be an accelerator pump mechanism
on the rear Bowl of a vacuum secondary model however
the rear Bowl may have a provision for one of these accelerator pumps but no actual
unit installed
now let's go over to the passenger side of the carburetor control
of the opening and closing up the secondary throttle plates
will be done by the vacuum unit on the passenger side the carburetor
as the engine runs and the venturi air velocity increases the vacuum signal
in the venturis the vacuum secondary assembly will automatically start to
open when needed to supply the proper amount of fuel
Holley has accessory kits available that contain different springs with
various tensions
that can be used to tune this system to your engines exact requirements
one more component to review is the choke assembly
with a manual setup you need to hook up a cable that will control the
position of the choke plate we saw
earlier on the main body the carburetor with an electric choke setup the
position of the choke plate
is controlled automatically when the engine is started
and then returns by itself to its closed position when the engines off
that will cover most to the details for a Holley 4150 vacuum secondary
carburetor
let's see what's up next