Tech Tip - Carburetion
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 co
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