By Allen Bishop
In Vol. 2, Issue 7 (August 1995), I outlined some of the essential factors that affect proper carburetion. What I neglected to mention is the most fundamental and obvious function of the carburetion system: to deliver the correct mixture of air and fuel over a wide range of engine speeds and loads. Weber carburetors combine simplicity with an unparalleled ability to fulfill the above requirements on hundreds of different engines. They became obsolescent when the requirements for fuel mixture ratios and delivery could only be managed by computer-controller systems. A Weber carburetor is essentially a "slave" device within its range of adjustment, while the current fuel injection systems are information-gathering devices that deliver fuel to the engine based upon what they are programmed for as the optimum amount for best power and emission levels for thousands of rpm/load, air density and temperature conditions.
The adjustment process for Weber carburetors is necessary only when they have been removed from the engine for disassembly and cleaning, or whenever the engine exhibits erratic running. Remember, the ignition system must be performing correctly prior to any adjustments to the carburetors, and the engine itself must be in reasonably sound internal condition. The following is an outline of the adjustment process.
Since the Ferrari engines use multiple carburetors operating synchronously, the first consideration is that all three, or six carburetors must be adjusted to flow the same amount of air to each cylinder. The most commonly used device to measure the airflow is the "Synchrometer," a flowmeter of very simple design having a speedometer-like needle that moves across a scale. This meter is placed successively into each carburetor intake directly, or with the aid of an adapter. The process is as follows:
Ensure that all linkages are correctly assembled and with all connections tight, make sure that when the throttle pedal is floored that all throttle plates are wide open. Do not make this test by any means other than depressing the throttle pedal "to the metal." Use a flashlight to look into each carb; have a helper hold the pedal down. Do not use a brick or any other artificial means to verify full throttle position. Activate the electric fuel pump and allow it to fill the carburetor float bowls. If the carbs are newly overhauled or the Ferrari has been standing idle for some time, gently tap the carburetors with the handle of your golden screwdriver. This will unstick a bottomed float/needle. Watch for fuel leaks! If a leak erupts, either have your helper cut the ignition, or remove the ground strap from the battery immediately. Have the clamping bolt loose for this purpose, but always maintain a snug fit on the battery post. None of the fuel delivery connections need to be brutally tightened to effect a seal. Remember, the threads do not create the sealing effect. A word here about the washers on the fuel manifold rail itself. Ferrari seems to have used red fiber washers on the three-carb setups; copper ones on the six-carb cars. I prefer the copper washers, but in either case the washers must be an exact fit. Parker Hall at Kilimanjaro Designs has an excellent fiber washer replacement kit for three-carb engines, while metric supply companies can provide replacement copper washers. All sealing surfaces and flanges must be smooth and parallel. Fiber washers, new or old, may seep fuel slightly for a short time until they swell. Again, this applies only to cars with new washers, or ones that have been out of use for some time.
Carefully watch for flooding! If a float(s) is not sealing, the carburetor barrels will begin to fill up with raw gasoline. Cut off the fuel pump instantly. If the throttle adjustment screws are fully backed off and the plates are fitting correctly to the barrels, the overflow will actually hold--for a while. First get the fuel out of the barrels by drawing it off with a kitchen baster or battery filler. The gasoline may be replaced in the tank. If the fuel runs down past the throttle plates into the cylinders, the spark plug(s) related to those cylinders will have to be removed. With the ignition system disabled, place paper towels over the plug ports and have your now long-suffering helper crank the engine. This will blow out the accumulated fuel from the affected cylinder(s). Allow the paper towels to dry out, then dispose of them properly.
Close the idle mixture screws on all the carbs, and open them 1/2 turn. When closing the screws, it is important to seat them gently, do NOT tighten, then back them out.
Start the engine and stabilize the rpms to about 1800. Your assistant may be gone by now, but if he or she has been patient, have the helper hold the throttle steady from the drivers seat. Otherwise, you will have to mechanically block the linkage to keep the engine running steadily. It is neither necessary nor desirable to warm up the engine prior to beginning the synchronization of the carburetors. Remember--they are OUT of synch at this point, and the actual air/fuel mixture is not correct, either. The longer you run the engine in this condition, the greater the chances of plug fouling.
Three-carb setups: With the engine rpm steady, run the Synchrometer over all six intakes and note how much variance there is in the flowmeters needle reading. With the engine at about 1800 rpm, the reading should be quite high, but not peaked out at the limit. With a 10mm wrench, slightly loosen the throttle bellcrank levers attached to the main rod on the valve cover ONE at a time, and GENTLY tap the bellcrank lever in either one direction or the other. You will find that the carburetor flow rates respond instantly and with pronounced effect on the engines running. Match the readings on all three right-hand intakes, this is called the "control side" of the carburetors. Now take readings on the left intakes; they should be very close to the right side in readings. If not, the coupling sector gears may be meshed improperly, or the small external clamp that locks the left-hand throttle shaft to its sector gear may be loose. Either a screwdriver or an 8mm nut driver is used to tighten/loosen this clamp. Allow the engine to idle. Avoid "winging" the throttles as much as possible. Be sure to tighten the 6mm bolts on the bellcrank levers. Now you can proceed with the final adjustment. First check the synchronization of the right side intakes. Be very careful with the adjustment of the throttle stop screws, the airflow on all three carburetors can be matched. Carry out this operation on the right side of intakes first. Slight free play in the linkage will permit minor adjustment to each carburetor without disturbing the settings on the other two. Next check the flow on the left three intakes. In order to match them to the right side and to each other, it is necessary to loosen the small clamps on the end of each throttle shaft. This frees the left bank throttles from the coupling gears beneath the cover plate. Adjust each throttle individually, keeping in mind that these three now are also free of any return spring action. Therefore, GENTLY tap the throttle plate you are about to adjust shut with a long screwdriver. I have seen all too frequent evidence of overzealous tapping action on these throttle plates; it looks like a small woodpecker has been at work down there. Gradually adjust the throttle plates open with the screws until all three are in synch, again one at a time. Tighten the clamps--easy--do not kill em, they hold very effectively. An excellent illustrated outline to the above procedures is described in the 250 GT owners manual, later edition for the GTE. The language is so atypically concise that I suspect it was written by the late Mike Parkes who was not only a fast driver, but an engineer as well.
Finally, it is necessary to adjust the mixture. By now, the engine will be, shall we say, warmish, and you may actually want to shut down and take a break at this point. I neglected to state earlier that an electric fan placed in front of the cars grille will do wonders to prevent overheating. Throughout the adjustment process, it is a good idea to frequently check the water temperature gauge and shut down if it starts to pass over the normal mark. Once stopped, the engine will remain at operating temperature for up to 1/2 hour in temperate weather. Furthermore, the carbs will now "soak" engine heat for a while. This will make them slightly more responsive to mixture adjustments for a short while after restart, but with air and gasoline flowing through them again, they soon return to what is their normal operating temperature.
With engine running, make a final check of the synchronization. Correct mixture adjustment requires experience, along with concentration and a good ear. Working with only one screw at a time, slowly and carefully turn the mixture screw inwards a very small amount, then stop and LISTEN. Some engines are more responsive to the slightest changes than others. The key factor is to not become impatient or frustrated with this process. Make very small changes, stop and listen. If you have any experience with microscopes, imagine studying a slide specimen at 1000x with the fine focus mechanism a few thousandths of an inch above the slide. Always try to adjust the mixture towards the lean side--screw in--but do not let the engine stall. A person observing you should be almost unable to detect the movements of your hand/screwdriver. Today, the most precise method of mixture adjustment is with an exhaust gas analyzer. I doubt if the most practiced hand could match the results a correctly calibrated analyzer can deliver.
In my final installment, I will delve into the intricacies of dialing in a set of six Webers along with some final thoughts and advice. Stay tuned.
About the author: Allen Bishop owns Galloway Enterprises, a full service shop specializing in Ferrari maintenance and repair. Located in the Pacific Palisades, he has been working on Ferraris for more than 20 years. If you have any questions, feel free to call him at: 310-454-1904.
For a complete look at Sempre Ferrari, you may want to check out the
rest of the articles from
Volume 5, Issue 1 - January 1998