Tech Tip - Carburetion
By: Allen Bishop
One of the first, if not the first, bits of Ferrari lore that I ever learned was the admonition to keep your gold-plated screwdrivers away from the carburetors. The origin of this worthy piece of advice was said in a fit of annoyance by the late Luigi Chinetti, who was exasperated by American Ferrari owners who could not seem to resist the temptation to tamper with the Webers.In the late 40's and early 50's, Ferraris were thin on the ground, particularly in the USA, and little technical information was forthcoming on them from the Factory or anywhere else for that matter. Many of our mechanics were certainly familiar with a set of three deuces (Stromberg 97s) on a hopped-up Ford or Merc Flathead V-8, but those were almost always set up to operated progressively, i.e., the engine ran off the center carb at low revs, the two end carbs "coming in" only when the "loud pedal" was floored. Hey, I grew up in street rod territory, and I remember how in awe I was when I realized that a "street" Ferrari's carburetors all opened at the same time! Unlike the guys with the gold-plated screwdrivers, I was scared stiff of the thought of messing with Webers. In fact, the first set of Webers I ever rebuilt was for a 250 Mille Miglia - the fabulous but problematical 36 IF4C four-barrels.
I would like to outline some of the checks and procedures necessary to tune and maintain the carburetors on Ferraris built prior to the Bosch fuel injection era. For a few moments, put down your gold-plated tools. No, I'm not going into the theory of carburetion, but there are some very critical functional checks that often are ignored, but are basic to correct operation.
First, the engine's ignition system must be in completely correct order. This is another topic altogether which I won't discuss in any further detail here, but it is primary to correct carburetor setting.
Next, all the linkage and cables from the pedal to the carburetors themselves must be correctly adjusted. Primary to this are the levers that push and pull the throttles open and shut. They must be exactly the same length or strange things will happen. The throttle return spring must have its tension set via the adjustable bellcrank to a reasonable level. These springs are an excellent example of Ferrari overkill; excessive tension can and will lead to needless and disruptive wear on the carburetors. These springs will pull a barn door shut!
Regarding the throttle pedal itself, there are two questions to be answered. First, when floored are the throttle plates wide open? Second, is the pedal's position essentially comfortable for your driving style? If you participate in track events, can you safely engage in heel-toe braking maneuvers? The pedal position is variable within certain limits and should be considered if your right ankle bothers you after a drive. Also, if your car is equipped with a throttle cable make sure it is in good working order, not ready to snap at 100 MPH.
Air filters have to come off for any but the most minor work and their condition should be verified. Dirty filters restrict air flow and will disrupt the air/fuel ratio at all RPM ranges.
Relative to the above; it was sort of chic to run road cars sans air filter a number of years ago, the carbs being fitted with Weber aftermarket "velocity stacks." These have to rate as one of the most useless decorations ever put on a V-12. Not only do they flow air poorly, but also they require a change in jetting which, of course, was never done. Furthermore, in the event of a blowback, they can allow a nice fire to start as well as under the best of circumstances allow all the dried paint and debris on the underside of the hood to drop right in for a friendly clogging. With the emphasis on originality over the past years, these little pieces of junk have largely vanished. The above remarks do not apply to those engines which were Factory fitted with sta
For a complete look at Sempre Ferrari, you may want to check out the rest of the articles from Volume 2, Issue 7 - August 1995