Bolting on a new carburetor, especially a Holley, is a
great feeling. The motor immediately sounds healther,
throttle response becomes crisper, and of course power
and fuel economy improve. However just like other mechanical
parts, a carb needs periodic rebuilding to keep it metering
efficiently. Holley carbs, if properly tuned and maintained,
will virtually last forever. Our 650cfm double pumper
had served us well for over 4 years, and had put up with
hundreds of jet changes, before it started running a little
rough.We had noticed some part throttle popping, rough
idle, and even inconsistant ET's at the track, so we decided
it was time for an overhaul. In this article we'll show
you how easy it is to rebuild your Holley, and we'll also
give you some tips on tuning it for maximum performance.
Holley "Trick Kits" and "Renew" kits
are the easiest way to revive your carb. They come with
everything you'll need, including gaskets, power valves,
and other hard and soft parts. We picked up the 37-485
kit for rebuiding most Holley 4150 and 4160 carburetors.
Be sure to get the specific kit for your carb. All Holley
carbs come with the part number stamped on the choke tower,
so use this to order the correct kit.
When it comes to Holleys, 4150
and 4160 refer to the two most popular models of Holley
carbs. A 4150 has both primary and secondary metering
blocks, allowing you to change jetting for both the primary
and secondary circuits. The 4160 models utilize a metering
plate in the secondary , which has fixed, non-adjustable,
metering orifices. All 4160 models have vacuum actuated
secondary throttle plates. The 4150 models come in either
dual mechanical throttle plates or with vacuum secondaries.
All 4150's have dual accelerator pumps, and dual fuel
inlets hence the term "double pumper." The difference
between vacuum and mechanical secondaries is simply in
how the carb actuates the secondary metering system. In
vacuum actuated carbs, the secondaries open up only when
the vacuum across the primary venturis reaches a certain
level. A diaphragm housing on the side of the carb contains
a changeable spring which controls precisely when the
secondaries open. On a mechanical secondary carb the secondaries
open when the throttle lever reaches a certain position.
The simplest way to tell which type you have is to look
down the throat of the carb, with the engine off, and
pull back on the throttle lever. If the secondaries open,
its a mechanical secondary. )
The beauty of Holley's is their simple and universal design.Not
only are they a snap to tune, but as we learned, they
are just as easy to rebuild. Follow
along as we rebuild and tune our double pumper.
Before you start, be certain to have containers
to hold and organize all the small parts which will
result from the disassembly. Spray can caps work
great for this purpose. A tuning tip: Notice how
we've spliced in a small section of rubber hose
into our cheapo chrome fuel line. This makes jet
changes very simple; instead of having to disconnect
the fuel line at the bowl fittings, the rubber hose
will flex enough to get the bowl off and allow you
to access the metering block to change the jets.
Start by placing the base gasket on the throttle
base. Do not use any sealer or adhesive on any of
the gaskets. Be sure all the passage holes line
by draining all the fuel out of the carb. Remove
just one of the lower bowl screws and let the fuel
drain out. Again, a plastic spray can top works
great to catch the fuel. We always carry one with
us to the track to avoid spilling fuel all over
Screw the base to the carb body. Use a dab of red
locktite on the eight screws to keep them from vibrating
loose and falling into the motor! Do not overtighten
the screws as the carb body will strip easily.
the fuel bowls and take out the floats, and needle/seat
assembly. Remove the metering blocks, and unscrewthe
power valve in the primary metering block. Some
Dominator style Holley's will have a powervalve
in the secondary metering block as well.
Replace the fuel bowl umbrella check valves with
the new ones provided. Be carefull not to tear them
note of the powervalve that came out. The powervalve
will be stamped with the vacuum rating, though its
difficult to tell in this photo, the numbers on
the right are a 6 on top, and a 5 below, identifying
it as opening at 6.5 inches-Hg.
17. The kit comes with new
accelerator pump diaphragms and springs, so swap
those out too. If you have a vacuum secondary carb,
the kit comes with a new secondary diaphragm, seal,
the accelerator pumps, and carefully turn the
carb over; two tiny needles will slide out of
the accelerator pump bores. Don't loose these!
18. Swap out the old needles
and seats for the new ones. The new ones comes with
Viton tips which seal better. Be certain to replace
the small gasket under the adjuster screw or you'll
get fuel squirting all over the engine.
All that remains is the carb body and throttle base
plate. Eight screws in the base plate will separate
it from the carb body.
With the needles and seats in, re-attach the floats.
Setting Holley float levels is somewhat tricky,
especially with the engine running. We recommend
setting it now. Adjust the screw so that the top
of the float is level with the bottom of the sight
hole. Note that we have replaced the stock hollow
brass floats with Nitrophyl floats. This is not
neccesary, but chances are if your carb is as old
as ours, the brass has corroded and may have pin
hole leaks, or the brazing has separated. Upgrading
now is cheap insurance against a hard to find fuel
Tuning tip: As you may have noticed, our carb does
not have a choke. Milling or cutting the choke horn
off is a popular and worthwhile modification which
will increase and smooth out airflow into the carb.
This is the perfect time to do it, since you'll
be cleaning the carb thoroughly afterwards. The
metal is very soft, and a die grinder, dremel tool,
or even a small hacksaw can get most of the choke
horn off. We have not had any problems, even on
cold mornings, running without a choke.
Replace the powervalve with the new one which comes
in the kit. Note that the kit only comes with the
stock 6.5" power valve. For most engines this
works well. Tuning tip: The power valve functions
to provided added fuel during temporary low vacuum
conditions. This helps eliminate flat spots during
part throttle, low vacuum conditions. The easiest
way to determine which power valve you need is to
take your engine vacuum reading at idle. Generally
a power valve half this reading will work well.
Another tuning tip: Most of the thottle bores have
casting ridges that disrupt airflow. Using a light
sanding roll will smooth these out.
Idle adjustment screws are one of the first things
to wear on a carb because they are so frequently
adjusted. When replacing these be sure to fish out
the small cork gasket, it serves to seal the screw
and keep it from loosening and changing the adjustment.
Use a light file across the carb body to clean up
any burrs across the metering block surface.
22. Eventhough the blue non-stick
gaskets are good, we recommend spraying them with
Pam or similar non-stick spray. Our last set lasted
several years, and never stuck or tore on us!
Most carbs will not need any further disassembly
of the base plate. Inspect the base plate for cracks,
and use a straight edge to check squareness of the
mounting surface. If it is anything but dead flat,
you should replace it. Check the linkage for smooth
operation. If there is any binding, or play in the
throttle shafts, or they leaked fuel, you should
either replace them with a Holley throttle shaft
Replace the fuel bowl vent whistles. They are held
in place with tiny self tapping screws, so hopefully
you didn't lose them.
With the carb completely disassembled, and all grinding
completed, start the cleaning. This is the most
important part of the rebuild because it is the
varnish and grime in the passages that causes a
poor running carb. We found that the carb body was
too big to fit into the carb dip, so we used spray
solvent to clean it. We let the metering blocks
and bowls soak in the dip over night. A word of
caution: Most carb dips and solvents will corrode
aluminum, so use only spray cleaner to clean the
With the metering block gasket oriented correctly,
place it on the carb body, and then attach the fuel
12. Use an air compressor
to blow out all the passages. If you dont have a
compressor at home, you can use compressed air in
a can, available at most computer and office supply
stores. We found the straw tip works great to get
in to all the metering passages and blow out any
dirt or gunk.
We like using these Moroso nylon bowl screw washers
instead of the paper gaskets. They dont leak, or
break apart, and again are great if you make alot
of jet changes.
13. With the carb thoroughly
cleaned, we can start reassembly. The Trick Kit
comes with a variety of gaskets for different carbs.
So be sure to carefully compare your old gaskets
to the new ones. The nice thing about the Holley
kit is that you get the blue non-stick gaskets.
If you make frequent jet changes, these gaskets
hold up great, and don't flake apart clogging up
26. Last but not least
screw down the shooters.Be sure to drop in the
discharge needles, point facing down. Note that
their are two gaskets, one below the shooter,
and inside (under the head of the screw.) Also
shown here are the two types of shooters, one
with nozzle extenstions, the other without. They
both perform the same function, but the ones with
the nozzles are said to be more accurate...will
you gain a tenth? Probably not.