The mixing of fuel and air to get gasoline engines to run has been handled in a few different ways over the years. From the earliest carburetors, to the latest, most sophisticated fuel injection systems, their main purpose is to mix fuel with air, in precise amounts as required by the engine, for it to run properly.

Over the years, as technology marches on, some of the older techniques and the nuances of “classic” car repair and maintenance seems to fall to the wayside. Carburetor tuning seems to fit this category.

In the old days, before constant injection systems (CIS), electronic fuel injectors, digital motor electronics (DME), and complete engine management systems, there were carburetors. From the primitive carburetors used in the early 1900’s to the latest PMO Weber replacements, their job is simply to mix fuel and air.

Our Porsche cars through the years have used a few different types of fuel systems. If you lift the engine cover on an earlier 911 you’ll find either a pair of Solex carburetors or a set of Webers. The latter became very popular due to their ease of tuning. Jet replacement on a Weber can be done without major disassembly or removing the carburetor from the engine. Almost all the internal parts of a Weber can be changed to allow the carburetor to be used for number of different size engines. The same triple throat Weber that is used on a stock 2-liter Porsche engine can be used on a 3.2 liter engine just by changing some of its internals.

If you look down the throat of a, triple down draft Weber, as found on some of our older Porsche cars, the actual internal bore of the carburetor (called the choke) can be changed. This is really its biggest feature, which is why these carburetors lend themselves to so many different engine displacements. The bigger the engine the more air/fuel the engine will require to run. The carburetor has to be able to flow enough CFM (cubic feet per minute) of air. If you install a smaller choke into the carb, you in turn, decrease the CFM of air that will flow through the carb, making this same carb, with other jet changes, good for an engine of a lesser displacement.

Another fantastic feature is there ability to be easily tuned for a particular application; the ease of changing either the choke, main jet, idle jet, air correction jet, or the emulsion tubes and the availability of the needed parts. In the “old days” we would run the car at wide open throttle down the highway for a mile or two then shut off the engine, pull over to the side and remove the spark plugs to check their insulators for the proper, light tan, color. If the center insulator looked too dark, or maybe even black, the main jets would be changed for smaller ones that wouldn’t let as much fuel through. On the other hand, if the color were too light or white, larger main jets would be required. The idling characteristics could be changed be carefully picking the proper idler jets.

Many of our Porsches from the late 70’s and through the early 80’s came through with the “power robbing” CIS fuel system. A lot of owners of these cars have had them converted to carburetors for better throttle response and to yield more horsepower. There is something about the sound of a naturally aspirated engine that is music to every car enthusiast’s ear. Tuning these carburetors has become fun, as well as being part of my job, for both me, and my engineer consultant friend Tim Trapani. Bruce Anderson does a great job describing different jet sizes for particular applications in both his 911 performance books. This book seems to have become a bible for both the “do-it- yourselfer” and the professional. I highly recommend it for any Porschephile. Keep the shiny side up. See you at the track.