Carburetion & Injection

PMO Triple Throat Carburetors

Graphic Diagram Instruction Pages: Carb Drawing - Linkage Geometry - Venturis/Jets/Floats/Tubes - Linkage - Gasket


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.


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.


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 Dansk copy or the steelpack Bursch. The stock muffler can be modified for bigger engines.


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 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, 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 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 doesnít 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. DONíT put compressed air through the float vent. If this doesnít 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 youíre 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 doesnít effect transition like closing the Mixture Screws does.

Now readjust the Mixture Screws until you get the correct adjustment. Usually youíll 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. Itís best to get your initial adjustment around 1200 rpm and hope the distributor doesnít 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. Itís 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 itís 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 youíve gained experience and patience. You have also changed the shape of your knees. Treat the needle seats with respect because they canít 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 todayís gasoline, heat insulators must be installed under the manifolds. A variety of these insulators are available from PMO.