PMO
 Carburetion & Injection

Technical Bulletins          

1. Idle Air Jet Passageways                 

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.

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 should be recurved.  MSD ignition boxes are recommended.  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 (#6420). The MSD Blaster high vibration ignition coils (#8222) are mandatory.

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.