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THESE INSTRUCTIONS COMPLETELY BEFORE YOU
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Graphic Diagrams: 1
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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
instructions.
PMO CARBURETOR ADJUSTMENT (CONDENSED)
1. Adjust side-to-side balance at idle and at
3000 rpm. Read linkage adjusting instructions. If
you can’t 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,
it’s 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 best
choice on engines 1975 or later. If you go to
headers for racing, the size is 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 Anza
copy or the steelpack Bursch. The stock muffler can
be modified for bigger engines.
PMO CARBURETOR ADJUSTMENT (UNAB.)
It’s 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
be learned.
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 5-8°
at idle and between 35-38° at 5000 rpm. 32° 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 doesn’t work and
also restricts flow too much.
DEFINITIONS
Front, Back, Left, Right,
etc., are defined from your position as you sit in
the driver’s seat.
The Throttle Body is the main part of the
carb. What it doesn’t 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
reverse.
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. We’ve recently discovered
that opening up the initial setting one or two turns
leans out idle and cruise without leaning out
transition.
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
air screws.
STE SYNCHROMETER for adjusting vacuum at the air
screws (available at PMO). Two are needed to set the
side-to-side balance.
Two 8 mm combination wrenches plus a 7 mm one.
A millimeter ruler for setting float height and
drop.
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, you’ll 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
diagrams.
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 don’t 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 side-to-side
balance of the carbs with the STE to make sure both
sides of the engine are pulling equal vacuum. Adjust
an Idle Screw if need be. Pick a barrel and turn the
Mixture Screw in until the engine speed drops and
then slowly out again until it runs smoothly.
Remember to let the engine speed stabilize after
each increment of screw turn. If you found that the
engine runs smoothly at less than the initial 2 turn
setting, then you can turn all the other Mixture
Screws a half turn in and start over. If you need 2
1/2 turns to make the engine run well, then do it. 3
1/2 turns out or more usually calls for bigger idle
jets.
If the engine doesn’t respond when you turn one
of the Mixture Screws in and out, it means that the
cylinder is not getting ignition or it is not
getting idle mixture. If the ignition and
compression on the cylinder check out, then an idle
passage is plugged. Remove the Mixture Screw and
Idle Jet and blow air back and forth to clear the
obstruction. Remove Idle Jets from holders and
inspect for obstruction. DON’T put compressed air
through the float vent. If this doesn’t work, the
idle passages have to be cleaned out with spray
Gum-out and possibly checked with a duct gauge. NEVER
dunk PMO carbs in carb cleaner. It will contaminate
the ball bearings. Put rubber lube on o-rings when
replacing idle jets. Be sure to leave idle air
inlets uncovered. Check diagram.
After you have the idle mixture initially
adjusted, you can bring out your STE and adjust the
Air Screws. Find the barrel which is pulling the
most vacuum, then adjust the other cylinders to an
equal vacuum using the Air Screws. The cylinder
which is pulling the most vacuum you leave alone.
Unlock the nuts before adjusting the Air Screws.
Tighten them by hand as you go along; when you’re
finished you can lightly tighten them using the 8 mm
wrench while holding the screw stationary with the
stubby screwdriver. Remember that the Air Screws
can be used to lean out idle or cruise by adjusting
them one or two turns out initially. The advantage
to this method is that it doesn’t effect
transition like closing the Mixture Screws does.
Now readjust the Mixture Screws until you
get the correct adjustment. Usually you’ll turn
the Mixture Screw in until the engine slows down and
then out again slowly until it runs smoothly and
then perhaps another 1/4 - 1/2 turn. You want the
maximum speed with the least fuel. It takes
practice. Sometimes as you turn out the Mixture
Screw it speeds up the engine enough to advance the
timing and that immediately adds 300 or
400 rpm. When you turn the Idle Screws back a hair
to correct this, the weights flop back and this is
what slows the engine. It’s best to get your
initial adjustment around 1200 rpm and hope the
distributor doesn’t intercede. Check the
side-to-side balance each time you adjust the Idle
Screws. The mixtures must be consistent from
cylinder to cylinder.
When you are finished adjusting the Mixture
Screws, back off the Idle Screws evenly to
850-950 rpm and check balance. Adjust the rod that
attaches to the right carb throttle
arm unless it will snap on without moving the
throttle. It’s imperative to check the
balance at 3000 rpm also. Before you put the air
cleaner tops on, take a look at the front of the
engine in the area of the pressure sender and
thermostat. If it’s wet with oil put a new o-ring
on the thermostat before your clutch gets wet.
You check the Injection Quantity with a narrow cc
measuring vial. Attach a wire around it and hold it
down to catch the gas as it squirts out of the
accelerator pump jet. .5 - .6cc for one complete
throttle action is about right. The injection
quantity can be varied by adjusting the length of
the stroke using the 7/32 nut on the threaded shaft.
The ignition is turned off during this operation.
Start engine frequently to burn out gas.
If you spend a day tuning your carbs you’ve
gained experience and patience. You have also
changed the shape of your knees. Treat the needle
seats with respect because they can’t be replaced.
For city and moderate speed highway driving we
recommend hot plugs NGK BP 5ES for compression
ratios to 9.1; NGK BP 6ES for ratios above 9.1 (Gap
plugs .028 - .030). Add a quality C.D.I. unit such
as the MSD 6A or the Crane HI6 if your car is not
equipped with a high energy ignition. Always check
for water in the fuel system. Protect your
investment! With today’s gasoline, heat insulators
must be installed under the manifolds. A variety of
these insulators are available from PMO.
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