Tuning a carburetor to supply an engine with the correct jetting-air/fuel mixture has always been a job that almost impossible for most hot rod owners and tuners. In the past, most performance engine tuners looked at the spark plug, the exhaust port and the first 6 inches of the header for proper color and then make a guess at what jet size change is needed. One of the disadvantages of this method is that the header and spark plug can only indicate what the mixture was at the exact rpm and load condition the plug check was done at, so you were mainly just tuning by trial and error.
Now a new, more scientific-modern method of checking the air/fuel mixture is the use of an infrared exhaust gas analyzer and/or an extended range oxygen sensor in the exhaust system; now the fuel mixture can be read at any rpm and load condition you wish to see. The content of the engine's exhaust can be read to indicate what the air/fuel mixture is at any rpm or load and how efficiently the engine is burning the fuel.
The proper tuning of any engine can make the difference between a great running engine and an engine that always sounds and runs like it needs a tune-up. For most hot rodders, one of the biggest mysteries is how do you jet the engine in order to obtain the correct air/fuel ratio necessary for your performance engine, to not only supply drivable horsepower when you want to go fast, but also supply the engine with the correct air/fuel mixture for when you are driving in heavy traffic or while cruising down the highway.
If the air/fuel mixture is too rich for the engine while you are running at cruise speeds, the engine may tend to load up and foul the spark plugs, while if the air/fuel mixture is too lean the engine may misfire at idle and at light loads or tend to run hot or overheat. Having a air/fuel mixture correct for all driving conditions will allow you to get all the horsepower out of the engine while getting as many miles as possible from a tank of fuel without overheating or causing any engine damage from having too lean of an air/fuel mixture.
The new advances in exhaust gas analysis technology and extended range oxygen sensor technology has made it possible to read and/or record what the air/fuel mixture actually is under almost any driving condition. In the past exhaust gas analyzers have tended to be large and expensive, but the new units on the market are not only compact and portable, but also affordable.
The performance and replacement carburetors sold today have a generic tune-up or jetting unless the carburetor is built for a specific engine package and fuel. A carburetor not built and tuned for a specific engine, exhaust system, and fuel should supply an air/fuel mixture rich enough for a variety of engines (but this is not always the case). If the carburetor is supplying too lean of an air/fuel mixture, the engine will run sluggish, overheat or the lean mixture could cause engine damage. If the carburetor is supplying an air/fuel mixture that is too rich, the engine may tend to load up, foul the spark plugs, run sluggish and lack power.
Correct carburetor selection can make the job of fine tuning the air/fuel mixture a easier job, my favorite performance replacement carburetors are: for a mild engine a Quadrajet, Edelbrock Thunder or Performer 650 cfm or smaller, on a high performance engine I prefer the Mighty Demon carburetors from Barry Grant Inc. or the Holley 4150 HP, on a supercharged engine the blower carburetors available from Barry Grant have supplied us with outstanding results.
The fuel you use (pump or race), the air density (i.e. altitude, barometric pressure, air temperature, humidity), compression ratio, camshaft, exhaust system, ignition timing curve, engine condition, fuel pressure, air flow thru the air cleaner etc will all effect the carburetor tune-up needed to get the correct fuel mixture for your engine.
The first order of business is to get the correct ignition advance curve for the engine and fuel being used, then the fuel pressure must be checked to be sure it has the proper system pressure at all engine load conditions. If the fuel pressure drops below the proper pressure, the carburetors air/fuel mixture will go lean and engine damage may follow. Once the ignition advance curve has been confirmed to be correct, many of the problems that we see can be traced to the fuel mixture being incorrect for the engine's needs.