Carburetion & Injection

Tech Bulletins

1. Idle Air Jet Passageways                  Printer Friendly Version

These get blocked off frequently by incorrect installation of air cleaner gaskets and/or air cleaner bases. See diagram in instructions. The problem was corrected in late 2002 by manufacturing air cleaner bases which only go on one way and air cleaner gaskets with extra holes. These items are available from PMO.

2. Float Needle Valves

We started out with viton float needle valves which have a somewhat blunt viton tip. Historically, these have been the choice for off-road cars.

We changed to the Weber float needle valves when we found quality control problems with the viton valves. After that, we went to the double-ball type. Both didn’t work as well as the viton ones for holding fuel level. Consequently, we went back to viton, but we now individually inspect every valve.

As a footnote, not even the viton valves will hold the fuel level in some cases where solid motor and transmission mounts are used. For race cars, we recommend the Clubsport mounts which can be stiffened by modification. WEVO recently developed urethane motor mounts which should be the best of both worlds.

PMO has different thickness float needle valve gaskets available to set the float height. Ideally, the tang remains parallel to the rest of the metal stamping with the float set at the correct height.  The float tang should be at 90 degrees to the float needle valve when adjustment is completed.  The gaskets are available in .010, .020, .030, .040 and .060 thicknesses.

3. Reducing Injection Quantity

We have found that race engines often need varying levels of injection quantity. Coming out of a turn, too much injection quantity can actually cause bogging. PMO has different float bowl check valves to adjust the injection quantity up or down. Rob King at S-Car-Go has cut the injection quantity down to almost nothing with excellent results. The normally installed float bowl check valves have a .5 mm side hole for bleed back. The larger the bleed back hole the less injection quantity. The valves come in increments of .1 mm up to 1 mm. We have 1.5 mm valves for people who want to almost eliminate injection quantity but still start the engine.

4. More on Idle Air Jets

We made these out of brass for the most part. In humid climates they tend to corrode because of the interaction of the brass, the aluminum throttle body and the vegetable material top cover gaskets. We’re changing to stainless steel on the idle air jets and looking for a durable paper for the top cover gaskets. If you have a problem with corrosion on the brass idle air jets, this can be removed by soaking in vinegar. Spray the top cover gaskets with WD40 before putting top covers back on, or check with us about stainless steel idle air jets.

5. Solid Motor and Transmission Mounts

I've noticed a proliferation of advertisements for solid motor-transmission mounts for Porsche 911 race cars. I'm sure they sell well because they are CNC machined out of solid aluminum and they look "cool". They are also considered "hardcore". My opinion is that any practical advantage they possess is vastly outweighed by a myriad of disadvantages.

The basic problem with solid mounts in internal combustion reciprocating engines is that they introduce destructive harmonics into the body of the race car and also back into the engine. Everyone has heard the true stories of cracked sheet metal, loosened nuts and bolts, sheared-off exhaust systems and broken engine cases.

The excessive vibration often sticks the float needle valves in carburetors causing flooding and other times aerates the fuel in the float bowls causing a lean condition.

The intense vibration also can knock out components in EFI systems leading to the common full-lean-at-full-tilt-equals-fried-engine syndrome. The additional tooth- rattling vibration and noise increases driver fatigue and the possibility of a crash. And if there is a hard crash, the solid mounts increase the likelihood of catastrophic damage to the engine and transmission.

Given these well known facts, why would anyone use solid mounts? It is because they stop the engine-transaxle unit from twisting, thereby improving shifting. This condition is substantially improved by using the Porsche sport mounts and is totally cured by using a WEVO shifter along with the sport mounts. Frank Eibell in Florida has made and installed anti-torque bars on the transaxle to cure this problem. Jim Patrick in Arizona makes urethane dampened 914-6 conversion mounts. WEVO urethane engine-transmission mounts are available now.

An interesting theory is that solid mounts actually decrease horsepower by forcing the engine to absorb vibrations that would normally be absorbed by cushion mounts. For every power pulse in an internal combustion reciprocating engine there is a reactive pulse responding to inertia, operating in a counterrotational plane. Containing this reactive pulse with solid mounts diminishes the power pulse by adding secondary vibrations. Greg Edmunds utilized this theory in designing motor mounts for his .4cc x 3.75 HP model airplane engines. By trial and error, Greg was able to tune out the unwanted vibrations by adjusting the durometer of the rubber in his composite motor mounts. His engines put out more horsepower and the airplanes flew faster with the cushion mounts!  Full size aircraft never have solid motor mounts

6. Cinched Throttle Shafts

This has happened when the carbs are not tightened down evenly. Start from the middle, work your way to the ends. Start off with 2-3 pounds of torque, then 5-6 pounds, finish at 8-10. This requires going around three times, but it’s worth it. Practice by clamping a bolt in a vice and tightening down a nut using a torque wrench then feeling the torque with your open-end wrench. Always use heat insulators under the manifolds.

We have had trouble with the white or light grey gaskets being of variable thickness. If you have these gaskets let us know and we’ll send others at no cost.

7. Racing Fuel

Always use it when you’re at the track. Pump gas has extremely high vapor pressure and can percolate in the carbs causing flooding and possibly fires. Street engines 2.7 and above need a front oil cooler for longevity. Remote oil coolers are needed in all race engines. Pump gas has also been known to vapor lock inside of fuel lines knocking out fuel injection systems.

8. Bearing Arm Modification

In late 2002, we started notching out the right bearing arm to give clearance at the right carb accelerator pump. There is a diagram in the instructions to show you where to file out a notch. This diagram is on the Setting Linkage Geometry page in the carburetor instructions. Alternately, if you send us your old bearing arm, we’ll send you a modified one at no cost.

9. Screw In Fuel Filters

We got rid of them. If you have these and want to convert to inline filters, contact us and we’ll send you 1/8 pipe x 5/16 hose fittings at no charge. If you return the fuel filter adaptors to us, we’ll send you complete all-in-one fittings at no charge. On race cars, we always recommend the AN fittings with braided fuel line. We manufacture these fittings also.

10. Mallory Competition Pump

For hot street and race cars that are NOT using the original fuel injection fuel pump (along with our Pressure Control Unit) we recommend the Mallory Competition Pump. The part number is 4110. We still recommend using our Pressure Control Unit along with the Mallory pump since there are many advantages in utilizing a recirculating fuel system. We have a new, billet PCU that can be fitted with either hose barbs or AN -6 fittings.

11. Loose Fittings

To prevent the large 17mm fuel inlets or fuel inlet plugs from coming loose (especially in race cars) we recommend Loctiting them in.  We instituted this change in July, 2006.  The Loctiting is especially important for cars running the non-recommended solid motor mounts in which many fasteners and connections eventually come loose. 

12. Ignition Update

911 SC Distributors, and most other 911 distributors, should be recurved. MSD ignition components are recommended. The MSD part numbers are listed below (also in Update 17).

For ignition questions, contact Barry at IAE, 313-532-5350. Barry is the premier rebuilder and recurver of 911 Bosch distributors.

Rick Clewett (310-406-8788) puts together an excellent crank fire ignition system for all model years.

For DME-Motronic engines ('84-'98), Barry can modify the existing distributor(s) to function like 911 SC distributors and fire MSD 6AL ignition boxes (#6425). The MSD Blaster high vibration ignition coils (#8222) are mandatory. The 90 degree coil boot and terminal are also needed (#3331)

13. Carburetor Flooding

There are several things that can cause the fuel to rise in the float bowls above the proper level and cause flooding:

     -Defective Floats.  The floats are a fabricated product and sometimes the limiting tang on them is too long and catches on the webbing that supports the float pin stanchions.  If that is the case, then the tang must be shortened and the float drop readjusted.  Always check for this when installing new floats.  Also compare new floats with old ones to be sure the pontoons are in proper relation to the hinge so the floats don't rub in the float chambers.

    -Defective Float Needle Valves.  One problem is the cross drilled hole hitting the needle seat.  Another problem is the needle seat not having a chamfer, which will cause the viton point to catch instead of centering in the aperture.  Always check for these two defects when installing new float needle valves.

    -Solid Motor Mounts and Transmission Mounts.  The increased vibration can stick the float needle valves.  See Tech Bulletin #6 (above).
    -Volatile Fuel.  The pump gas available has high vapor pressure (up to 15 lbs./sq. inch on the Reid Scale) and will often percolate even with the insulators plates in place.  Air conditioning exasperates the problem by adding convection heat to the engine compartment.   Some percolation is usually not a problem.  A hidden kill switch to the fuel pump to allow the float chambers to empty is sometimes used on large street engines (also a good anti-theft device).  Always use racing fuel on the track since it is of higher quality and the vapor pressure is controlled (see Tech Bulletin #7, above).  Also, not a bad idea for high performance street engines.

14. Top Cover Gasket Update


For a while we used a black, rubber filled, gasket that was more corrosion resistant than the vegetable material ones that are generally used.  We discovered that the black ones swelled and would sometimes interfere with the floats.  Please let us know if you have the black gaskets and we'll send you the original ones at no cost.  At the same time it would be good to check to see if you have the current, Viton (red rubber) tipped needle valves.


15. Small Motor Jetting (revised July 2010)

The latest feedback on jetting 40mm PMO Carbs on smaller street engines (2.0 – 2.4)  is as follows: 32 venturies, 125 main jets, 180 air correctors, 60 idles with 130 idle airs, and F11 emulsion tubes.  We’ve also closed off the fourth transition port.  The fourth transition port (the top one) would often cause an overly rich condition on cruise because it overlapped the main circuit at the light throttle position.  We are offering a free plug kit to close the fourth transition ports on 40mm PMO Carbs and a free jet and venturi exchange. The latest production run of 40mm PMO Carbs (2010) have three transition ports as the modification works on larger engines, also. A lot of the new information has been from shops using the new chassis dyno’s that give air/fuel in real time.  The portable units, like the AFX Powerdex carried by NGK, have also become much more sophisticated and are useful for fine tuning under actual driving conditions. 

16. Venturis, Jets, Floats, Emulsion Tubes (revised April 2011)

Venturis, Jets, Floats, Emulsion Tubes Link

17. Ignition Update II

The capacitors in all Capacitive Discharge Ignition units wear out not only by use; they also wear out by age. If the stock Bosch CDI Unit is more than 20 years old we recommend replacing it with MSD components.

The MSD Blaster Hi Vibration Coil (MSD-8222) must be used with the MSD Digital Box (MSD-6425) along with the 90 Degree Boot and Terminal (MSD-3331).

The Bosch Blue Coil will ruin an MSD CDI box because of different impedance.

18. One Carb Running Lean

Steve Rowe at Rowe Performance related a problem with one carb running lean even though it had the same jetting as the opposite carburetor. Turns out that the problem wasn’t with the lean running carb, it was with the other carb that was actually running rich. The reason why it was running rich was the ceiling surface at the bottom of the emulsion tube cavity was corroded. This allowed gas to slip by the circumference of the main jet, causing that carburetor to run rich.

The only cure for this condition is preventative maintenance: the engine should be started and run at least every couple of weeks and the carbs disassembled and cleaned out at the first indication of water. Water is heavier than gasoline and will show up if the float bowls are drained into a container by removing one of the 10mm hex plugs at the bottom of each float bowl.

19. Binding Throttle Shafts

There have been two instances, on well worn PMO carbs, that one of the radial springs on the ends of the castings, operating on either the throttle arm or hook washer, has worn down the casting to the extent that the spring has bound between the throttle arm or hook washer and the casting.

There is no cure for this except to remove the spring. All the radial springs should be examined for this type of wear. We can supply another throttle arm – for the hook washer end - plus another spring tab and throttle arm. Care must be taken when removing the hook washer – it must be held by pliers – so as to not twist the throttle shaft.

We have addressed this problem in later years by extending the casting and also machining the outside diameter of the bearing cage so the spring does not tighten all the way. But it is wise to inspect the carbs from to time; and keep the springs greased. A good tool for applying the grease is a disposable acid tinning brush, trimmed down to make it stiffer.

20. Exhaust Header Lesson

I go back to a lesson I learned almost 35 years ago when we first started supplying 46 Weber kits for Porsche 911 SC’s.

There was one engine builder/shop that did almost everything correct:
-Good Mahle 98’s (designed for carbs/MFI)
-Early 911 S cams
-1 3/4” headers and a stock muffler

Result: the car would not get out of its own way. The shop put on stock early heater boxes (less than 1 1/2” O.D.) and the car ran gangbusters.

Everything I learned in the intervening 35 years has reinforced the original lesson that headers must be down sized for street use with a muffler.

Further info:
-Headers sized for fuel injection can be larger because fuel injection doesn’t need a low negative pressure signal.
-Headers can be smaller than the exhaust port and still work well. No matter what size the exhaust port is, it’s better to run the right size headers for the engine.
-Modified factory Porsche mufflers (including the “Sport” muffler) will cause a flat spot as will most, if not all, aftermarket mufflers.
-The future holds the possibility of automotive electronic mufflers being developed that will be able to cancel sound without restriction. This increases the likelihood that headers sized for racecars can eventually be used on the street.

Recommended header size:

                      Street                    Race
2.0 - 2.8          1 ½”                      1 ½” - 1 5/8”
3.0                 1 ½”                      1 5/8”- 1 ¾”
3.2                 1 ½” - 1 5/8”           1 5/8”- 1 ¾”
3.6 - 3.8         1 5/8”                     1 ¾” - 1 7/8”

21. 4th Transition Port Issue

Regarding Carbs sent March 16 through May 6, 2014:

Transition leanness in the 2,000 - 3,000 RPM range. This may be caused by not enough transition port mixture in this area.

Note that we used to have four transition ports but the top one sometimes would give too much overlap on the main circuit at steady cruise so we eliminated the fourth transition port. This helped cars on cruise in the 3,000+ RPM range but seems to have hurt at the 2,000 - 3,000 RPM range in transition. The 50mm carbs still have the original 4 hole pattern.

We put the fourth hole back in, but smaller. The fourth hole was .043 and the hole we are putting back is .032 for 40mm carbs and .035 for 46mm carbs.
(.032 dia. is a #67 drill or .8mm        .035 dia. is a #65 drill or .9mm)

We have also made a different pump cam - “the hatchet”- which gives a quicker accelerator squirt. The new pump cams are now standard equipment on PMO carbs. These are available at no cost.

If you are going to open up the fourth transition port hole in your carbs note that there is a drill dimple at that location. A new 4th transition port can be carefully placed using a jet drill.
To check whether the carbs have three or four transition ports you have to remove a transition port cover. The transition port covers are the 11mm diameter brass slotted plugs situated directly above the mixture screws. The (6) mixture screws are the ones with the springs, cup washers and o-rings.

There is a drill dimple at the #4 spot even if there is no 4th hole. This location would be the highest of the pattern, “highest” meaning toward the top of the carb. Also note that the transition port covers have a gasket and it’s almost impossible to get the gasket properly placed unless the carb is on its side.

The exception to all this is the smaller 2 to 2.4 liter street engines. If the carbs have four transition ports then those engines are better off with three transition ports. We have a free transition port plug kit and we are now recommending the 3rd hole, from the bottom, be plugged.

And to make matters even more complicated, we’ve now gone to a modified 3 transition port pattern with a much larger 3rd hole in our 40 and 46mm carbs.

22. 964 Camshafts with Carburetors?

The rule of thumb is that camshafts designed for CIS or EFI will work with carbs but are not a good choice if you are building a high performance carbureted engine. The early CIS cams (2.7) were pretty weak and the 911 SC (3.0) cams were definitely a step up and the 964 and 993 Super Sport cams continue in the right direction. But if you want an engine producing a lot of horsepower then you have to go with a cam designed for carbs or MFI.

Recently I had a
customer with a 3.8 RS long block that he wanted to set up with 50mm PMO carbs. I heard “3.8 RSR” (or maybe he said “3.8 RSR”) instead of “3.8 RS”. After dynoing the engine it was apparent that it was an EFI long block, not a punched out 3.2 or 3.6 with real RSR cams. The venturies, and jetting, had to be downsized from 45mm to 41mm to get flat fuel and torque curves.

It’s the same case if a club racer, who has a 3 liter 911SC or a 3.2 Carrera, 964 or 993, and wants to put on carburetors and go racing but doesn’t want to buy new pistons to allow, say, modified ‘S’ cams. The bottom line is that unless early style performance cams are installed with new, high compression pistons, the final result will be disappointing if substantial horsepower increase is the main goal.

The subject of pistons, cams and exhaust is a complicated one and we’d recommend viewing the Web Cam and Elgin Cams websites along with a discussion with your engine builder.

23. Pressure Control Unit (How it Works)

The pressure control unit (PCU) works by adjusting flow back to the tank by way of the needle valve on the return connection on the PCU.

No matter how high the pressure of a fuel pump, it is only pressure at a given output.  As the flow is released, so is the pressure, which never builds beyond what is needed by the carbs. But the flow back to the tank must not be restricted by a kinked line or defective connections.

The needle valve doesn’t close all the way, protecting the gage. The PCU is assembled with the needle valve open all the way.  As the needle is closed, with engine running, the pressure builds to the carburetors.

3 to 3.5 lbs/sq” to the carbs ideal.  The pressure will build slightly as the fuel warms up.  4 lbs. Is fine since that is in the range of Weber’s callout.

There are many advantages of returning excess flow back to the tank to control fuel pressure:

-More even pressure to all four float chambers, especially with crossover line
-Saves the float needle valves
-Keeps the fuel cooler and cleaner
-The pump is idling so less voltage drawn and less wear
-(All aircraft have a return line)

If a new fuel pump needs to be purchased, we recommend a Mallory 4110 and place it as low as possible in the “smugglers box” in the front of the car.

24. Ignition Update III

Barry at IAE: “If you still have the Bosch 3 or 6 pin CD box and need a replacement coil, note that the Bosch (silver) coils from Brazil are defective and have a short life (about 500 miles). These coils are sold through the Porsche dealer network and Bosch distributors worldwide. There is no stock available of the original coils that were made in Germany. The silver coils are Bosch’s mistake, and this mistake has infiltrated to Bosch coil stocks all over the world during the last few years. I recommend another coil, from Germany, available from Hans Aichinger, (Ph 310 375-1533)”.

More from Barry: “Keep in mind that almost all the Bosch CD units are decades old. The life of the charge capacitor in the unit can vary from 10 to 20 years. The Bosch CD rebuilds done here in the US are often very poor, the charge capacitor is the only part replaced (not a real rebuild). If you are concerned about retaining your car’s originality, you can regain original Bosch CD ignition performance by contacting Hans Aichinger about a true rebuild.

The definitive test of an ignition system is an Automotive Oscilloscope reading of the spark plug voltage.
It should be 30,000 volts. Even though the car still starts, that does not confirm adequate spark plug voltage”.

If your main concern is improving your ignition system, consider installing an MSD. (17. Ignition Update II)

25. Piston/Valve Clearance

Recently a customer bought a complete PMO 50 Carb Kit for what he described as an engine equipped with 3.8 RSR pistons/cylinders and RSR cams. We assumed early MFI RSR and jetted the carbs for that configuration and it over carbureted the engine. It turns out that the P/C’s and cams were later 993 RSR (EFI) and are significantly different from the earlier MFI RSR P/C’s and cams.

The same customer told Dema Elgin that he had RSR P/C’s and Dema, also thinking MFI RSR, sent the customer some racing MFI cams close to original RSR. The first thought in this situation is valve to piston clearance. It’s always good to check valve to piston clearance (with clay) before an engine is assembled, but, on this assembled engine, there is 2.5 mm at TDC. Dema Elgin recommends Two Additional Tests: Exhaust valve to piston clearance (at least .080" or 2mm at 8 degrees BTDC); and Intake valve to piston clearance (at least .040” or 1mm at 8 degrees ATDC).

26. Rocker Arms

The bean counters struck again! The rocker arms, sold by the dealers, are now made in China. Pauter Machine makes quality, forged rockers, domestically. Steve, at Rennsport Systems has facilities to rebuild (rebush, ream and regrind) used rockers, or new, Chinese ones. Dema can do the same.

27. To Find Real Gasoline

Go to Pure-gas.org

The additive xp3 (www.xp3.com) is beneficial in all engines as it stabilizes fuel and aids combustion. Monty, at Redmond European has used it for years and recommends it for all car and truck engines. Xp3 is especially recommended for engines running gasohol.

A Racing Fuel Comparison Chart is available at smithtex.com/racing/fuelcomp.html

F&L SP-1 Racing Fuel is a good fuel for racing and for higher compression Porsche and other street engines.

28. Possible Spring/Casting Wear on Older PMO Carburetors (and Remedy)

PMO carburetors, because of the ball bearing throttle design, will not wear out, unlike Webers.  But there is a correctable issue with some of the older PMO’s as described below:

On some of the older, unmaintained, PMO carburetors there is a possible wear condition on the casting caused by the radial springs on the ends of the carbs.  The rotational movement of the springs, hooked onto the throttle arm or hook washer, can wear into the casting and affect free throttle movement. (A similar wear pattern can be seen on Porsche MFI housings.)

Unhooking the linkage and removing the throttle return springs (extension springs) and trying the throttle will give you an indication of a possible problem. 

The wear can occur on the outer length of the casting which we call the bearing boss or cage. The radial spring can wear down the outside diameter of the casting and may eventually wear  a groove between the casting and the throttle arm.  In extreme cases, worn components have interfered with the throttle operation.  The older PMO carbs with the as-cast bearing cages are the most likely candidates for this to occur.  We started circle interpolating the bearing cage down to .900” diameter to loosen the spring and this has prevented this condition, to the best of our knowledge.  The outside-machined bearing cages are now smooth as opposed to as-cast.

To eliminate the point of wear, the bearing cage is reduced in length by .100”.  The removed aluminum is replaced with a precision brass washer available from PMO.  The last coils of the spring will then ride on the rotating brass washer, instead of the aluminum casting, thereby eliminating the problem. The ball bearing, Bellville spring, and inner retaining ring assembly remains in place and the felt washers are re-oiled and reused.

We’ve sent machining fixtures out to Porsche and carburetor repair shops and individuals we deem to be competent machinists.  The process is relatively simple using our fixture, but care must be exercised to do the machining correctly.  It’s hard to add material!

If a carburetor, or a pair of carburetors, exhibits a binding condition, they should be taken to one of above described facilities for inspection and repair.  The modification can also be done as preventative maintenance. There will be a fee for this.  PMO will support the cost of the modification if a carb is actually binding.

Shops that currently have the machining fixtures: 

Automobile Associates Canton, CT (860) 693-0278
Auto Edge Racing Mahtomedi, MN (651) 777-6924
JB Racing Tavares,FL              (352) 343-8900
Jerry Woods Campbell, CA (408) 369-9607
John Truman Eng Thompson, OH (440) 298-3858
Partsklassik Flagstaff, AZ  (928) 255-2824
Rennsport Systems Portland, OR (503) 244-0990
Targa Engineering Jupiter, FL (561) 801-3166
Francis Tuthill Porsche Oxfordshire, UK (0) 441295-750514
Redtek Brackley, UK (0) 1280-841911