SU Fuel Pump. The Series3 pump is just two 6-cyl pumps mounted back-to-back on an aluminum adapter. I bought two new 6-cyl pumps for $ 55 each and turned them into a single V12 pump.

The cam won't cause you to change the carb, but it may cause you to change the needle, which will control the richness of the mixture at various RPM's. The taper of the needle is what matters. An SU needle is measured at a number of points (about 16) along its length, and it tapers from base to tip. The diameter of the needle at each point will determine the amount of fuel delivered as the piston rises to that point. As the engine load and RPM increase, the piston rises, pulling the needle further and further out of the jet. A narrow taper will deliver more fuel, i.e. be richer, at high RPM than a broad one.

The needle you require will be determined by the breathing characteristics of the engine....cam changes, valve grind, porting and polishing all can affect the required taper. The proper way to set taper is to use a dynamometer to measure piston height at various engine loads, while measuring mixture with a gas analyzer. The taper can then be honed in to be correct at every piston height.

Clearly, few of us are able to do this job by the book. But there shouldn't be that much variation, unless you are radically altering the engine. Most dealers stock 'standard' and 'rich' needles, and one or the other will work. If you buy a set of high lift cams, then you generally get a correct set of needles.

1. Do you have the correct needles? XKE needles are marked "UO" if memory serves. There is a richer needle marked "UM" sometimes used.

2. Is the airflow equal through all the carbs? Check with a Unisyn.

3. Is there air leakage through the throttle bushings? Spray some carb cleaner on each bushing. If the engine speed changes at all, you have a bad bushing.

4. Do you have oil in the dampers? Try a heavier oil.

5. Wasn't there a different cam used on the Stromberg cars? If so, this would mean a non-standard needle might be required.

I've been meaning to write this up for some time, ever since I did the SU Performance Tuning 101 a few months ago. This one is more like Basic SU Adjustment for Happy Driving.

The trick to tuning SU carbs is to understand that there are two things you need to get right: the airflow, and the fuel mixture. While they are interconnected, they are also independent, and need to be measured and adjusted independently.

You will probably need to arrange to buy or borrow a Unisyn flow meter. The Unisyn is the usual gauge for getting the airflow balanced between the two carbs. This costs about $20 and is simple to use. It consists of an adjustable opening (same size circumference, but with a disc on a threaded rod that you can screw tighter or looser) that you use to set the level of a little float that rises or falls in a glass tube at the side of the gauge.

For the fuel mixture, I have become sold on a device called the Gunson ColorTune (maybe ColourTune, as it's a British co.). This is a spark plug with a crystal pressure- and heat-resistant window in it that lets you see into the combustion chamber while the motor is running. The color of the flame indicates the mixture richness. It costs about $40, and while its not absolutely essential, it makes life so much easier that it's worth the cost.

If you don't have a Gunson, I've included the standard directions here for determining correct mixture (step 4 of the Adjusting Mixture procedure).

To tune SU carbs, first locate the following components:

• Throttle linkage nuts. These are the things that connect the throttle linkage (the bar connected to your foot through whatever means your car uses, cables or rods) to the carburetors' throttle levers.
• Throttle stop screws. These set the idle speed for each carb, and are located typically behind the dashpot, on the same side of the carb to which the throttle linkage connects.
• Mixture adjusting nut. This is the lower of the two nuts at the very bottom of the carburetor. Later SU carburetors of the HIF type have integral float chambers, on which the mixture is adjusted by turning a crew. You'll need to experiment (and I explain how) to see which way makes this richer and which way makes it leaner.
• Lifting pins. These are little wobbly metal pins under the dashpot. When you push up on the pin, it raises the piston in the dashpot. Find these; they're crucial if you don't have a ColorTune. If you don't have or can't find them, you can raise the piston with a flat-bladed screwdriver pushed down the throat of the carb and twisted to lift it.
• The bridge. This is the part inside the carburetor, where the gas jet opens into the airstream. You'll see a needle inside the jet, and the jet itself should be a few fractions of an inch down from the bridge itself. The jet is the brass tube that sits in the center of the bridge, with a tapered needle poking down into it.
• The choke linkage nuts. Comparable to the throttle linkage nuts (and usually the same size), but on the linkage that goes between the choke cable and the mixture adjustment mechanism. They make sure that both carbs are enriched when you pull on the choke.

1. Start with the engine warmed up to operating temperature and perform your standard ignition tune-up (points gap, timing, spark plug gap, new condenser, etc.) first. If you've got a timing light and a dwell meter, you can verify all that stuff independent of the way the car is running. When it's warm, shut the motor off and remove the air filters.
2. Begin by balancing the airflow. To do this, first loosen the throttle linkage nuts. Leave them connected, just loosen them half a turn or so.
3. Back out the throttle stop screws till you can see that they are just touching the throttle stop. Then open each carburetor (that is, lower the throttle stop screw) 1-1/2 turns of the throttle stop screw and start the engine. It will probably idle at about 2000 RPM; don't worry.
4. Put the Unisyn over either carb and adjust the orifice in the Unisyn till the little float at the side rests at the middle of its graduated tube. (Pre-diagnostics: if the idle drops and the car wants to die when you slap on the Unisyn, the carb is too rich; if the idle soars upwards, it's too lean.) Hold the Unisyn over the carb for only long enough to see the level of the float, then remove it.
5. Place the Unisyn on each carburetor in turn to check its flow, adjusting the throttle stop screws until both carburetors register the same position on the graduated tube of the Unisyn. (The float will probably move either up or down in the tube, which is why you want to center it in Step 4.) When both carburetors flow the same amount of air, tighten the throttle linkage nuts, adjusting for the amount of free-play between the linkage and the throttle stops that your manual calls for (probably about 0.006"). Your goal should be to achieve the lowest possible idle with both carbs balanced and the engine running smoothly. (Note that the idle speed will very probably rise as you get the mixture correct.)

If you've taken more than five minutes to do this, rev the engine to over 2500 RPM (assuming the idle isn't already that high) for thirty seconds or so to clear the spark plugs. Then adjust the mixture.

Note: in the following procedure, one "flat" is the basic increment of adjustment, and refers to 1/6 of a turn of the mixture adjusting nut. This corresponds to the flat faces on the nut.

I'm going to give instructions for SUs with the separate float chambers. If you have the HIF integral-float carbs, you'll have to look in a manual to see whether you turn the mixture screw to the right or the left to make it richer or leaner; I've done that once but I can't remember. Alternatively, you can -- with the motor shut off -- peer down the throat of the carb and turn the mixture screw while watching the top of the jet. Remember that moving the top of the jet up will lean out that carb, while moving the top of the jet down will richen it.

1. Shut the car off and loosen the choke linkage nuts.
2. Adjust the mixture nuts (screws) fully lean. For separate float-chamber cars, this means raising the mixture nut all the way up against the bottom of the carb (or rather, against the spring). For HIF carbs, you can try turning the screw while looking down the throat to see which way the jet is moving. In either case, the idea is to zero out the jet: raise it all the way up in the bridge.
3. Now drop the jet an equal amount -- two full turns for HS-type carbs, two full turns (I believe) for HIFs. Then start the car. Note: In the following step, you might want to consider adjusting the carburetors one-half a flat too lean, as the mixture will be enriched when you put the air filters (which restrict air flow) on at the end of the tuning process.
4. Raise the lifting pin (or use a screwdriver if you don't have the pins) so that the piston rises no more than 1/16". Listen to the engine's exhaust note and compare it to the following conditions:

• If the exhaust note rises and stays high till you drop the piston, this carburetor is adjusted too rich. Turn the mixture nut one flat (one-sixth of a turn) up, moving the jet toward the bridge, then repeat Step 4.
• If the exhaust note falls and the car sounds as though it is going to stall, this carburetor is adjusted too lean. Turn the mixture nut one flat (one-sixth of a turn) down, moving the jet away from the bridge, then repeat Step 4.
• If the exhaust note rises briefly and then settles back down to something like the original RPM level, this carburetor is set correctly. When you have achieved this setting for both carburetors, continue with Step 5.

1. Tighten the choke linkage nuts so that the choke cable will pull an equal amount on both mixture nuts when you pull the knob.
2. At this time, I find I usually have to adjust the idle again because getting the fuel mixture right usually changes the idle speed. Since you know you have the throttles synchronized, I normally just adjust the idle without loosening the throttle linkage. The easiest way is to screw one of the screws out till it doesn't' even touch the throttle stop, then use the other to get the idle speed right. When that's done, you can screw the other stop screw down till it just touches the stop on that carb and you're set.
3. Replace the air filters and go for a test drive!

SU carburetors are most fuel-efficient when slightly lean, and provide the most power when they are slightly rich. You can use this knowledge to provide a certain amount of tuning for the kind of driving you do. If you learn to read spark plugs, you can get a basic idea of what your engine's condition is and make fine adjustments to the mixture nuts accordingly.

If you have a ColorTune, you simply install it in place of one of the plugs, then adjust the carburetor that feeds that cylinder (the front carburetor for 1 & 2, the rear for 3 & 4). The ColorTune will let you see the color of the flame. White flashes mean too lean; yellow flame means too rich. Blue (like a Bunsen burner) is correct, and blue with a faint orangish tinge is the best for power.

You can also modify your car's throttle response characteristics slightly by adjusting the viscosity of the oil in the dashpot damper. SUs are set up so that a thicker oil will resist the piston's attempt to rise in the dashpot for just long enough that the engine's increased load (when the throttle is opened) will pull more fuel across the bridge; this enriches the mixture and temporarily bumps power up to help the engine achieve higher speed more readily.

If you modify your engine, you will probably need to modify your needles, as it is the needle profile that determines the mixture curve for different air-fuel loads.

If you experience uneven idle, hunting, or an idle that changes (rises or falls) as the engine's temperature climbs or drops, you probably have vacuum leaks. The most serious fault on most old SUs is wear in the throttle shaft area. To test for this, spray some carburetor cleaner on the outside of the throttle shaft; carburetor cleaner is non-combustible, and if the engine speed drops, it means some of this is getting into the air stream from outside the carburetor. You may also have leaks from the manifolds, from tubing such as the vacuum advance line to the distributor (if fitted), or from other places; the carb cleaner trick works well for locating those leaks as well.

Other problems that SU carbs experience involve dirt in the dashpot and occasionally in the float chamber. The dashpot is a precision piece of machining that involves very close tolerances so that the piston doesn't stick or bind when it rises and falls. A little grit between the piston and the dashpot can make the car jerk and sputter. Take the dashpot off, wipe the insides down with carb cleaner and a lint-free, clean rag, then reinstall it, getting the screws down tight. Also, don't swap the pistons between dashpots; they're matched to one another so that the clearance between the piston and the wall of the dashpot makes a tight seal but permits easy rising and falling.

Dirt in the float bowl basically shuts off that carburetor (or can make it flood open, depending on whether the dirt is wedging the valve open or closed). You can try rapping on the float bowl with the handle of a screwdriver, but your best bet is to take the cover off, clean out the valve fittings, and reinstall everything, with a new fuel filter for good measure.

Some older SU models also have adjustable floats, in which you need to set the float height (which basically equals the fuel level in the float chamber) by bending a brass rod. These carburetors were replaced in the mid-1960s with carburetors that had fixed, plastic floats, which are basically trouble-free unless abused. The stop at the back of the floats can break if they are installed badly, and the brass pin that holds them in place can wear an oval hole in the float pivot. New floats are fairly inexpensive and aren't a bad idea if you're doing a rebuild.

Grose-Jets are very popular with some people and a big pain for others. It appears -- and this is just conjecture -- that Grose-Jets work best in cars with adjustable floats, as they are longer than the stock SU

float valves. The standard failure for Grose-Jets is to flood the carburetor. I have never had problems with the stock SU float valves or floats.

Are the carbs set too lean? Look at the spark-plug color. The tip will be Gray or tan if you are running too lean. Too lean means that the correct ratio of air and gas is out of wack and you are getting too much air with the gas. It can make you run hot. Too rich means the opposite with too much gas in ratio with the air and the tips will usually be black with oily deposits.

If gasoline is gushing out of the orifice between the carb and the air cleaner, the most likely reason is a stuck float. Replace with a new float valve and check the float for buoyancy. Might as well get new shaft seals while you have them off, too. The kit doesn't include them.

Carb Insulator Blocks-Stromberg 175cd SCII You need both gaskets with the insulator in between. The gasket without the cut-out goes against the plenum. The one with the cut-out goes against the carb. The lower right is the correct placement.

At one time Haynes had an entire manual on SU carbs (as well as one on Zenith Strombergs, for the truly challenged) in addition there is (or was) a Haynes Manual devoted to the E-Type. I also recall Moss Motors in Goleta, California had a video available called "tuning your SUs" or the like. I don't know if the books are still available, although I suspect that Moss may still have some of the videos in stock, with their tremendous MG and TR following.

Haynes Publication #299"SU Carburetors. Also pretty good write-up in the maintenance section of the "operating handbook" (owners manual), at least in the one for the 69 model year.

There are two ways you can bend copper pipe at sharp angles.

1.- Take the length of pipe you need and add approx. 6". close one end of the pipe with lead solder. Now fill the pipe with fine sand and tamp in place. Sand must be very dry. Then close the opposite end with solder. Use or make a small bending jig and heat the area you wish to bend the copper pipe and slowly bend the pipe. Do not overheat the pipe to cause the silver solder to loosen at the ends. After the pipe is bent, cut the ends and drain the sand.

2.- The second method is to insert a heavy duty coil spring into the copper pipe. The coil spring should be slightly smaller than the I.D. of the pipe. (smooth fit)

I would try doing this first with soft copper tubing. It makes life happier.

When removing fuel pump hoses, undo the clamp, then split the old hose with a razor blade. This eliminates the chance of damaging the fuel pump. When ready to install the new rubber hoses, use silicone lubricant on the hose ends to make the job easier. These techniques also apply to radiator and heater hoses.

However, besides acting as an octane booster tetraethyl lead acts as a shock absorber between the exhaust valves and valve seats. The concern we enthusiasts have is for the potential of excessive valve seat recession or pound-in because of being run without leaded fuels. The reason that lead is so important to these older cars with "soft" seats is that the lead acts as a lubricant between the valves and the valve seats, cushioning the valve each time it seats to prevent exhaust valves and/or seats from recessing or pounding-in. In older Porsches, with their softer seats and valves there is some cause for concern. The newer cars, from 1977/78 on were designed so they could run on unleaded gasoline. In 1978, when Porsche started to use catalytic converters to meet the emissions standards requiring the use of unleaded gasoline, they changed the valve seat material to an sintered iron material that they call Pluko. Just how hard are "soft" seats and how hard do they have to be to prevent recession or pound-in. Really soft seats are cast iron seats with a hardness of from Rockwell 12 to 25. Seats with a Rockwell hardness of 45 to 50 are probably hard enough in most cases to provide for satisfactory protection.

It is also interesting to note that when lead was introduced to gasoline in 1923 there was very similar concern about the effect on the engine's valves and valve seats because of the addition of lead to gasoline as there is today with the removal of lead from the gasoline. At the time the engine designers felt that the lead caused a serious service problem with the spark plugs and exhaust valve damage caused by the corrosive effect of lead oxide. The designers forged ahead and used lead as an additive because of leads superiority over all other antiknock additives of that era. With higher-octane gasoline the engine designers could use higher compression ratios to achieve major gains in both power and fuel economy. During this period of automotive history the designs and materials used in high-compression engines made tremendous improvements over a very short period of time. These included special alloy exhaust valves, and seats and sodium cooled valves all to combat valve and valve seat erosion, caused not by the removal of lead, but the addition of lead to the gasoline. With the introduction of lead to gasoline the octane number was increased from about 50 to today's 90+ for high octane unleaded gasolines making possible a boost in compression ratios from 4 to 1 up to the 9+ to 1 used by all of our modern Porsche engines. Because of advanced catalytic cracking methods and other octane boosting additives the gasoline companies find it easier to make high octane unleaded gasolines today than they did sixty years ago so the removal of lead from our gasoline hasn't caused much problems.

Engines that are run on unleaded gasoline burn cleaner and will run much longer between tune-ups than cars that are run on leaded gasolines. All modern normally aspirated Porsches (4, 6 and 8 cylinders) have a recommended service interval of 15,000 miles including the oil change intervals and the turbocharged cars have a recommended oil change interval of 7,500 miles.

Bosch has 30,000 and 50,000 mile spark plugs and the unleaded fuels are largely responsible for these extended service intervals. It also looks like engines that have been run on unleaded fuels will last longer than those run on leaded fuels, it is not uncommon for 911 SC engines to run for 175,000 to 200,000 miles without requiring any major maintenance.

Another concern for the enthusiasts should be the use of alcohols as a gasoline octane booster or Gasohol itself. A number of auto manufactures recommend that you do not use Gasohol, and Porsche says that if you have any running problems, fuel economy, stalling or problems with vapor lock or hot star problems you should switch back to a gasoline which does not use alcohol as an additive.

Gasoline, alcohol and water have no desire to be together. Water and gasoline will not mix and stay mixed. If moisture gets into a fuel tank contaminating gasoline, the moisture will eventually end up in the bottom of the tank. On the other hand, if a gasoline with alcohol or Gasohol is used the alcohol will hold some of the water, loosen up dirt,scale, rust, etc. and carry this debris to the injectors or carburetors causing contamination of the fuel induction system. If you have enough moisture in the tank you can get a condition called phase separation where the alcohol combines with the water in the tank and settles to the bottom of the tank. This phase separation can cause the engine to run very poorly if a large dose of this water and alcohol mixture is ingested by the injection system or carburetor.

Alcohols increase vapor pressure and increase the probability of fuels boiling or vapor lock. The vapor lock depends on ambient temperatures, engine temperatures and the temperature of the fuel. When vapor lock occurs it can be impossible to start the car until the temperature goes down and the fuel vaporizes less readily. Fuel pumps, fuel injection systems and carburetors are designed to handle fuel in the liquid phase not a vapor phase, and therefore the problem. Carbureted engines seem to have more problems with this high RVP problem than the more modern injected cars do so this is probably even more a problem for the older Porsches than it is for more modern fuel injected cars.

The major oil companies in the states west of the Rockies do not use alcohols as an octane booster. Some of the independents use it extensively. In the San Francisco Area, Beacon gasoline usually contains ethanol. They have started just recently posting it on the pumps. In the Midwest there is lots of Gasohol. Some Gasohols contain ethanol, some contain methanol, and others contain mixtures. Ten percent is the maximum that is permitted by law. Ethanol is less corrosive than methanol, but methanol is cheaper and contains more energy.

We had originally thought that the oil companies using alcohol would be required by state law to post that fact on their pumps. But, only about 45 states require posting. The others don't care if it is posted or not although some oil companies are posting in some of the states where it is not required. Oil companies using alcohol vary from area to area. It is most common in the Midwest and frequently found along the East Coast from New Jersey to Georgia. You should try to avoid use of any fuels that are blends of gasoline and alcohol if at all possible, and that is also true of any additives that include alcohol as a major ingredient.

Because of the problems caused by mixing alcohol with gasoline there are now companies producing additives to protect cars from the use fuels with alcohol in them. If you live in a state that uses alcohol

in the gasoline you should look into one of these additives, "Alcohol Protector" by Gold Eagle Co. 4400 So. Kildare, Chicago, IL 60632. Fuels containing alcohol have the ability to absorb moisture from the

air. Once the alcohol absorbs water to its maximum, about 1%, a phenomenon known as phase separation can occur. The risk is that the water and alcohol mixture can get trapped in some fuel injection systems and cause corrosion or gum things up.

Here in the US the gasoline companies have started to use MTBE instead of alcohol in gasoline. The reason that the gasoline companies are blending Methyl Tertiary Butyl Ether (MTBE) is to help control exhaust emissions from vehicles during the winter months when air is more dense and it takes cars longer to reach their operating temperatures. The use of oxygenated gasolines help to improve air quality by reducing carbon monoxide (CO) emissions which has been a serious problem in the winter. I am told that this formulation is required in Colorado, Arizona and some parts of Nevada from November through March. In some metropolitan areas such as Los Angles and New York these reformulated gasolines are also used all year around to help reduce carbon monoxide emissions.

MTBE was originally used as a high octane-blending component. As such, it was mostly used in blends of unleaded premium gasolines to raise the fuels octane. MTBE is just one of many oxygenates used in formulating gasoline. For the past decade or so the gasoline companies have been using alcohols as oxygenates to boost the octane in gasolines as the use of lead was reduced.

MTBE acts more like hydrocarbon gasoline blending stock than any of the alcohols and is very soluble in gasoline. Also, unlike alcohol, It will not separate from gasoline when water gets into the gasoline,. As far as oxygenates go, Methyl Tertiary Butyl Ether is probably the best one to add to gasoline and is becoming more common all the time even though it is is more expensive because it works well.

Alcohols produce about half of the energy that gasoline does so when you run an engine on alcohol you use about twice as much fuel to get the the required energy. The reason that an engine burning MTBE gets about 3% worse fuel economy is that like the alcohols it has less energy. The end result is that you have the same energy you just have to burn more fuel to get it. But be thankful that we are not yet required to burn 100% alcohol because you would have to burn almost twice as much to get the same energy as you are presently getting with your MTBE blended gasolines. A 944 would have to have a forty gallon fuel tank to maintain the same cruising range that it has now on gasoline.

Because these oxygenated fuels produce less energy there can be some problem with using them in older cars because they will have the effect of leaning out the mixture. This lean mixture could cause poor running

and detonation and if adjustments are not made to richen the mixture it could also lead to damaged pistons, valves, or cylinder heads. Modern cars with oxygen sensors and closed loop control systems will not have any problems because they will adjust the mixture to maintain a very low level of oxygen in the exhaust. The older cars can be adjusted or rejetted to run a little on the rich side to improve driveability and eliminate any of these long term problem.

We need to be much more concerned about the use of alcohols in gasolines for any purpose whether as a water dispersant, as a gasoline octane booster or as Gasohol itself. A number of auto manufactures recommend that you do not use Gasohol, and Porsche has said that if you have any running problems, fuel economy, stalling or problems with vapor lock or hot star problems you should switch back to a gasoline which does not use alcohol as an additive.

The united states uses a different method of rating octane that is used in Europe which causes some confusion. We use the average method which is the RON + MON divided by two and is referred to as the CLC method (US Cost of Living Council) or AKI (antiknock index). The older Porsche manuals for cars like your 1969 911S would have referred only to the RON method of rating fuels. The owners manuals since 1977 have referred to it as CLC and in the late 80's as both CLC and AKI.

Regular fuels in Europe have octane ratings ranging from 91 to 98 RON (Research Octane Number) which corresponds to 87 to 93 CLC (average method). As an example the 1977 911S/Turbo Carrera owners manual says that the 911S engine requires 91 RON octane or 87 CLC octane so that they can quite readily run on unleaded regular. The same manual says that the Turbo Carrera for racing or sustained high speed driving requires 96 RON octane or 92 CLC octane.

The testing method that the RON is averaged with to derive our CLC numbers is the Motor Octane (MON) which is a much more severe testing method. The motor method tests the engine at a higher speed than the research method. Both of these tests relate the knocking characteristics of gasoline against standards to establish the octane numbers. RON is considered a better test of anti-knock characteristics for engines operating at full throttle at low engine speeds. The MON test is considered to be a better indicator of anti-knock characteristics at full throttle high engine speed, and part throttle low and high engine speeds. The CLC method attempts to create a more meaningful rating by averaging the two. The CLC octane rating will usually be four to five points lower than the RON rating.