CARBURETOR INSPECTION AND INSTALLATION
1. Inspect boxes and remove carburetors.
Carefully check for any damage.
2. The windows in the float bowls allow you to
check fuel levels and the condition of the castings.
The fuel level will slowly lower at an equal rate in
all float bowls over time due to evaporation. Note
that fuel will siphon through the idle circuits if
the Idle Air Inlets are accidentally blocked.
3. The general cleaning that was necessary with
the Webers is not needed with the PMO carbs.
4. Care must be taken in tightening the carbs
down evenly as it is possible to cinch the throttle
shafts. Tighten the manifold nuts down in three or
four stages, ending up with about eight to ten
pounds of torque.
5. PMO carbs come with either AN fittings or hose
barbs. Check all plugs on the carbs for any seepage.
6. With engine running, inspect fuel level. The
14 mm float height setting should bring the fuel
level to the middle or lower part of the sandblasted
dot with engine running. Check float adjusting
PMO CARBURETOR ADJUSTMENT (CONDENSED)
1. Adjust side-to-side balance at idle and at
3000 rpm. Read linkage adjusting instructions. If
you canít get balance at both rpm ranges, it means
the linkage geometry is off.
2. Adjust Idle Stop Screws (#154) evenly using
the STE to approximately 1200 rpm. You want both
sides of the engine pulling the same amount. Adjust
Mixture Screws (#162). Use STE to set idle down to
850-950 rpm after you adjust Mixture Screws. For an
engine to respond to Mixture Screws, the carbs must
be clean, the ignition must be working well, and the
engine must be in generally good shape.
3. Adjust Air Screws (#157) at around 850-950 rpm
using the STE. You want both sides of the engine
pulling equally. Adjust each barrel to the barrel
drawing the most vacuum. Initial setting is closed
all the way around. After adjusting Air Screws,
adjust Mixture Screws again. NOTE Some engines
will run rich at idle and or cruise. In these cases,
itís worthwhile to set the Air Screws one or two
turns open for initial setting.
4. Find the plastic cc measuring vial so you can
test the Injection Quantity. Seal off the small end
with a lighted match and put notches at .6cc and
.8cc. Hold the measuring vial under the pump jet and
measure the squirt as you work the throttle arm.
Each action should give you from .6cc to .8cc. You
can adjust the quantity by turning the nut on the
adjustable pump rod (#143). Shortening the stroke
normally gives less injection quantity once the
slack is removed at the diaphragm rivet. The
adjusting nut is 7/32.
A WORD ON EXHAUST
We recommend the early type exhaust on all
engines. The SSI heat exchangers are the determined
by whether a muffler is used or not. With a muffler,
1 1/2" o.d. is good through 3.0 litres and 1
5/8" o.d. for larger engines. With megaphones
the header size can be increased on engines above
2.4. For the street, the stock dual inlet muffler is
best. Good results can be obtained with the Dansk
copy or the steelpack Bursch. The stock muffler can
be modified for bigger engines.
PMO CARBURETOR ADJUSTMENT (UNAB.)
Itís being taken for granted that you have had
some experience working with Porsche engines, at
least at the tune-up level. PMO carbs are easy to
adjust when compared to other Porsche induction
systems. It takes a certain feel which usually can
Before attempting to adjust the carbs, the engine
must have correct cam and ignition timing along with
a careful valve adjustment. The points, plugs,
distributor cap, ignition wires, air cleaner
elements, etc., should be in like-new condition. A
C.D. ignition unit is mandatory. The distributor
must be mechanical advance or modified for straight
mechanical advance. The timing should be set 5-8į
at idle and between 35-38į at 5000 rpm. 32į is
about right for an engine above 9.5-1 compression
ratio. The compression differential between the
cylinders must not exceed 15 psi and should be above
150 psi all around. PMO makes a Pressure Control
Unit for use with the stock injection fuel pump. If
you change the pump, use a quality, rotary fuel pump
like the Holley Red Pump. If you use a regulator,
use the Holley regulator. NEVER use the little
chrome job with a pointer. It doesnít work and
also restricts flow too much.
Front, Back, Left, Right,
etc., are defined from your position as you sit in
the driverís seat.
The Throttle Body is the main part of the
carb. What it doesnít contain is attached to it.
The Mixture Screws thread into the base of
each throttle bore. They have springs to maintain a
setting and there are three of them in each
carburetor. Be careful as you turn these in as they
seat in the throttle body. Turning a mixture screw
in leans the mixture and turning it out does the
The Idle Screws are sometimes called the
idle stop screws. There is one on the back of each
carb and it determines the minimum throttle setting
through its action on the throttle arm of the carb.
When these screws are turned all the way out, the
throttle plates rest against the throttle bores.
The Air Screws are similar to the mixture
screws in that they operate on the needle valve
principle. There are also three of these in each
carb and they are located close to the mixture
screws. They have an 8 mm lock nut on them to
maintain their setting. As they are adjusted out
they let air slip around the throttle plates to
equalize the volume of mixture entering each
cylinder at idle. The basic setting is closed. Great
care is to be taken since these screws also seat in
the throttle body. Weíve recently discovered
that opening up the initial setting one or two turns
leans out idle and cruise without leaning out
Side-to-Side Balance means equal
vacuum between the two sides of the engine through
adjusting the Idle Screws and throttle linkage.
PMO carbs need the larger "BK" version.
TOOLS needed to adjust the carbs are
Stubby screw drivers to adjust the mixture and
STE SYNCHROMETER for adjusting vacuum at the air
screws (available at PMO). Two are needed to set the
Two 8 mm combination wrenches plus a 7 mm one.
A millimeter ruler for setting float height and
A small cc measuring vial; useful for measuring
the accelerator pump injection quantity.
These instructions start with the engine
thoroughly cleaned and the manifolds installed with
new gaskets. The linkage should be clean and high
temp grease added to the ball sockets. An 8 mm
open-end wrench can be used as a ball joint
separator. New gaskets are used under the
carburetors when they are installed. The air cleaner
base is the part of the air cleaner that is clamped
to the top of the carburetor by the air horns. A new
gasket should go between the air cleaner base and
the carburetor. When air cleaner bases are
installed, the Idle Air Inlets must not be
covered. Check diagram. If the Idle Air Inlets
are accidentally covered and the engine floods,
remove spark plugs, disable CD and turn engine over
before trying to start engine.
Next, the linkage should be hooked up except for
the short ball jointed carburetor rods. If you are
converting to PMOs from Zeniths, the linkage arm
connected to the crossbar on the right side
sometimes has to be bent further to the right to
allow the short ball jointed carb rod to travel in a
totally vertical plane when viewing the mechanism
from the rear of the vehicle. Find the Idle Screw at
the rear of each carb and turn the screw all the way
out so that the throttle plates rest against the
throttle bore walls. Now turn the screw in until it
just touches the throttle arm and then turn it 1/4
turn more. Repeat with the other carb. (Once engine
is running, youíll adjust idle speed to 850-950
rpm on most engines.) Now adjust the short
ball-jointed rods so that they can be snapped onto
the throttle arm ball without disturbing the
throttle setting. This should give the carbs
side-to-side balance. To fine tune the side-to-side
balance, study the linkage geometry instructions and
Oil the throttle shafts and radial return
springs; grease accelerator pump linkage (henceforth
every 3,000 miles). Check for smooth operation. Take
the play out of the linkage by shortening the long
ball-jointed rod that connects the pivot point on
the front of the left manifold to the crossbar
making sure that the throttle closes all the way.
Next, have someone get inside the car and floor the
gas pedal while you are looking into the carbs to
make sure the throttle plates rise to a complete
vertical position. You may have to adjust the
throttle stop behind the pedal. Be sure throttle
plates donít go past the vertical position. Full
throttle stops at the pedal, not at the carbs.
If full open stops at the carbs, both the linkage
and the carburetor throttle arms will be over
stressed. The PMO carbs give you the choice of AN
fittings or screw-in fuel filters.
You now have all your tools together and are
ready to adjust the carbs. The initial adjustment on
the Mixture Screws is two turns out from the seats.
The Idle Screws are 1/4 turn in after touching the
throttle arm on each carb. The Air Screws are
closed; 8 mm nuts are lightly locked with the screws
being held stationary. When adjusting the RPM, use
the tach in the dash. Tach-dwell meters have been
known to knock out CD units in Porsches. Warm the
engine to at least 140 degrees. Unlock the rod from
the right carb and evenly turn the idle screws in
until the tach reads 1200 rpm. Check the