CARBURETOR INSPECTION AND
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
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
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
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
A WORD ON EXHAUST
We recommend the early type
exhaust on all engines. The SSI heat exchangers are
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
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 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 5-8į 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 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. Weíve recently discovered
that opening up the initial setting one or two turns
leans out idle and cruise without leaning out
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
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
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 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 donít go past the vertical
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.