This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.
Well adjusted carburetors are key to good engine performance. If your carbs are unbalanced, then different cylinders in your engine will receive different amounts of fuel and air, and the all important air/fuel mixture ratio will be off. This will result in less horsepower, a poor idle, and higher emissions. It's a wise idea to check the balance of the carburetors each time that you perform a major tune-up on your engine. The most popular system for use on the 911 are the Weber triple-throat carburetors. The remainder of this project will discuss the procedure used for balancing a 911 engine equipped with Weber carburetors.
If your carburetors are poorly balanced, then you will have a multitude of problems with your engine. The engine will have a tendency to spit and backfire through the exhaust. Unbalanced carbs may also give the engine an uneven idle, or make the car very difficult to keep at idle without stalling. It's important to make sure that your engine is in good overall health (good compression), and that your idle jets are clean as well. Dirt in the idle jets, poorly adjusted valves or a burnt exhaust valve can also result in the same symptoms that are associated with unbalanced carburetors.
The first step in adjusting your carburetors is to make sure that the rest of your car is tuned and performing properly. Make sure that your valves are properly adjusted (Pelican Technical Article: Valve Adjustment), and the timing and dwell are set properly (Pelican Technical Article: "Setting the Timing, Dwell, and Idle Speed"). Make sure that your spark plug wires are in good condition, your plugs are new, the carburetor floats are properly adjusted (Pelican Technical Article: Carburetor Rebuild), and the idle jets are clean. Let the car warm up to its normal operating temperature before starting to adjust the carburetors. Once the car is warm, begin by adjusting the idle to about 1000 to 1100 RPM. You want to set the idle to be a little bit higher so that the car will not die out when you are making adjustments. Adjust the engine to this value by carefully turning the idle adjustment screws located on the ends of the carburetors.
Now, remove the air cleaner from the top of the carburetors. The stock air cleaner is simply clipped onto the top of the engine, and can easily be removed. If you have an aftermarket filter assembly, you can remove these by simply unbolting the air cleaner housing on each carburetor. Now, disconnect one of the two small drop links on the throttle linkages so that there is no connection between either of the two carburetors.
Once you are sure that the timing is set correctly, and the idle speed is set to your working range of 1000-1100 RPM, it's time to synchronize each of the carburetor throats. A special synchronization tool (sometimes called a Unisyn or synchrometer) is used to measure the air flow being sucked into the manifold by each carburetor throat. The goal of the synchronization task is to make sure that the amount of air flow into each throat is the same for all six cylinders.
Start with cylinder number one, located on the rear left side of the engine. Place the synchronizer over the top of the carburetor stack and adjust the tall glass stack to a vertical position. Turn the adjusting disc on the synchronizer until the small indicator bubble inside the glass rises about half-way up the height of the synchronizer. This basically calibrates the synchronizer tool for cylinder one.
Now, place the Unisyn on one of the throats of the right bank of cylinders. Adjust the idle adjustment screw located at the end of the carburetor until the ball in the Unisyn is at about the same height as it was on cylinder one. The idle speed should either rise or drop off a bit from the level that you had it set at.
Starting with cylinder number one, adjust the mixture adjustment screw at the base of the carburetor. On each Weber carburetor, there are three mixture adjustment screws that have a spring wrapped around the inside to prevent them from turning. Turn this screw in until the idle begins to drop. Then back off the screw about an 1/8th of a turn or until the idle comes back to its previous level. The goal is to turn in all the mixture adjustment screws right to the point where they begin to affect the idle. Repeat this procedure for each cylinder on each side of the car. If your mixture adjustment screws are turned in too far, then you will be starving your engine of air. This will result in a popping-type backfire out of the exhaust. When all the mixture adjustment screws have been set, then readjust the idle adjustment screws located on each end of the carburetor until the idle is adjusted back to 1000-1100 RPM. Use the Unisyn to make sure that both banks of carburetors are drawing the same amount of air while the car is at 1000-1100 RPM.
Next, take the Unisyn and measure the level of air flow that each cylinder is drawing through the carburetor. Make a note of the level of the highest cylinder: you will adjust all the other carburetors to level of this cylinder. There are three air correction screws that are located at the base of the carburetors and have a small nut that locks them down to prevent them from rotating. Adjust the air correction screws so that the level of each cylinder as shown by the Unisyn is equal to the reading on the highest-level cylinder. Adjusting the carburetors in this fashion will give the car a strong idle and make it run very even. If for some reason, you cannot adjust all the cylinders to the reading on the highest cylinder, then adjust that one down slightly to the point where all the cylinders can be synchronized together. After adjusting all the air correction screws, reset the idle back to the 1000-1100 RPM range, using the Unisyn to verify that both sides are evenly balanced.
Once all the cylinders are set and synchronized, it's time to adjust your linkage. The accelerator linkage must open each carburetor by the same amount at the same time. In other words, both the left and right carburetor linkage must be synchronized with each other as the throttle is depressed.
First, carefully inspect your throttle linkage. All of the links and ball joints should be tight and have almost no slop or backlash. If there is significant wear on these joints, then they should be replaced, as you will find it very difficult to adjust the linkage and balance the carburetors.
To adjust the linkage, you will need to change the length of the linkage arms by rotating the ball joints at the ends of the arms. The linkage has two drop links, whose length controls when the throttle bodies are opened and closed. If the one carburetor opens earlier than the other carburetor, then you need to decrease the length of the throttle linkage drop link until they both open at the same time. The drop links are threaded with both a left and right-handed thread (similar to the tie rods), so you can get a very fine adjustment simply by rotating the drop link in either direction. Tighten up the retaining nuts on each end of the drop link when you have finished adjusting the linkage.
In addition to checking both the idle mixture and the linkage, you should also check the adjustment of the accelerator pumps. These pumps are designed to inject extra fuel into the carburetor throats under acceleration. There are two of them, and they are located in the center of each carburetor. They directly affect the performance of the 911 up to about 30 mph. This pump jet squirts out a stream of fuel when the throttle is opened from being completely closed. It may be easier to perform this test if you remove the velocity stacks on top of the carburetors.
To check the adjustment of the accelerator pumps, place a small vial under the pump jet located inside the carburetor throat. You will have to fashion a small wire to hold the vial, as space is pretty tight inside the carburetor throat. Pump the throttle completely from closed to open twice, and measure the amount of fuel in the vial. For cold weather, the factory recommends 0.55-0.65 cc of fuel. For warmer weather, it's recommended to have 0.40-0.50 cc of fuel. Adjust the length of the small linkage attached to the accelerator pump on the carb to increase or decrease this amount.
Early Weber carburetors don't have adjustable accelerator pumps. A poorly adjusted accelerator pump will tend to make the car hesitate under acceleration. This symptom is also experienced when the diaphragm on the carburetor is cracked or leaking. It's very often the case on 911s that have been sitting for many years for the rubber diaphragm to break. Replacement of the diaphragm during a routine carburetor rebuild will solve this problem (see Pelican Technical Article: Carburetor Rebuild).
When finished, go drive the car and check the performance of the engine. Often it is the case where several attempts will be required to properly tune and balance the carbs. When you have completely the entire procedure, it's a wise idea to go back and check all the measurements one more time. If you find that you are having difficulty balancing or tuning the carburetors, it may be that they need to be rebuilt. Vacuum leaks around seals and worn throttle bodies will make the carburetors almost impossible to properly tune. If a carburetor throat doesn't seem to respond to any changes in the idle mixture screw, then this is a clear indication that the idle jets may be clogged, or your carburetor may need rebuilding. See Project 28 for more details on rebuilding the Weber brand of carburetors.
Another problem common to carburetors is the clogging of the idle jets. These clogged jets will cause the engine to run rough. The solution is to simply pull them out of the top of the carburetors and clean them out with a little bit of carburetor cleaner and compressed air. You should make sure that your jets are clean before attempting to tune and balance the carburetors. Refer to Project 28 for more details. Approximately 60% of the time when you are driving, the idle jets will be what is primarily supplying fuel to the car. Dirt in the idle jets will make the car run sluggish, as if you have lost power in a cylinder.
Another common problem with Weber carburetors is that they sometimes spit out the top of the velocity stacks. This is usually caused by a design defect in the early specifications of carburetors used on early 911s. The spitting is caused by the idle jets being too small. The cure for this problem is to install larger idle jets. In the old days, you could check the jetting of your carburetors by taking a close look at the spark plugs. Unfortunately, you can't read spark plugs as well with today's unleaded gas formulations. In the older days, a black sooty plug meant that your car was running rich. A nice brown plug meant that the mixture was set properly. With today's unleaded fuel combined with lots of additives, the plugs will often look black and sooty, even if the engine is running well.
Clean fuel is especially vital to good working carburetors. It's a smart idea to add an extra fuel filter right before each carburetor just to remove an excess dirt that may clog the idle jets. Unlike high-pressure fuel injection systems, carburetors don't get cleaned by the pressurized fuel flowing in and out of the small passages. It's also very important to make sure that the air filter is clean when you reinstall it.
It's important to note that Weber carburetors have no choke or starting circuits. You must use your foot on the accelerator pedal to pump the accelerator pumps in the carburetors when first warming up the engine. When starting the car warm, take the pedal and floor it, then turn the key. Don't pump the accelerator pedal when the car is warm, as you will probably flood the engine. Weber carburetors have a tendency to boil over the fuel when the car is warm, and dump it into the combustion chamber, flooding the engine. Holding down the pedal is the correct procedure because chances are that there is already a lot of fuel in the combustion chamber. When the car kicks over, simply release your foot from the pedal and bring the car down to idle.
If after going through the entire adjustment procedure, the engine still doesn't perform properly, you may have some problems that are caused by other factors. If your engine is not in good health and has a compression leak, then you may get backfiring through the carburetors. If the small passages within the carburetors are clogged, then they might need to be rebuilt. If turning either the idle mixture screws or the air correction screws doesn't affect the idle of the engine, then you may have to rebuild the carburetors.
There are many different opinions on how to best tune carburetors, and the procedure described here is simply one of many. Just about every book I've read has a slightly different procedure for tuning the carburetors. In the end, practice and increased familiarity with how the tuning of the entire system affects engine performance will enable you to eventually tune the carbs 'by feel.'
The Unisyn synchronizer is a very useful tool for balancing your carburetors. Make sure that it completely covers the top of your carburetor or velocity stack when taking measurements. Once you have the synchronizer set for a particular cylinder, don't change the diaphragm setting until you are finished checking all the other cylinders. Also, avoid looking directly down the stacks of the carburetors while the car is running. If a backfire occurs it can come up the stack and hit you in the face.
It's also very important to make sure that your linkage is properly adjusted. The small droplinks (shown by the yellow arrow) that are connected to the throttle and control the butterfly valves need to be appropriately adjusted. Make sure that they are opening the valves at the same time. The white arrow shows the idle adjustment screws that are located on the end of each carburetor.
This photo clearly indicates the adjustment points for the Weber carburetor. The white arrows indicate the idle adjustment screws. The green arrows point to the idle mixture adjustment screws. The yellow arrows indicate the air correction screws, and the red arrow points to the accelerator pump adjustment rod. The location of the idle jets which must be kept clean are indicated by the blue arrows.