Tasty tips for carburetor tuning
So why should we care about a low-tech item that no one uses anymore? Well, one reason is that there are a lot of cars running today that still use this old technology. Most collectible and resto vehicles rely on a carburetor to deliver the proper amount of fuel into the engine. And since these cars are older, the carburetors likely have problems.
Another reason that a vehicle may use a carburetor is for performance gains. It is true that fuel injection systems are very precise and can be modified for power increases, but often the cost involved in these modifications can be high. For this reason, the performance carburetor is still a very popular option in the sports racing world. So most people restoring an older car, or making performance modifications, will want to know a few tips and tricks for this old-school component.
The example we chose is a 1972 Porsche 914. In factory form these vehicles come with a very efficient fuel injection system. There are many options for performance gains with this system, but most of these modifications are much more expensive than a good set of 40 millimeter Weber carburetors.
The Weber brand is one of the most adjustable carburetors available today but because of this, they are often the most mis-adjusted carburetors. Keep in mind that the information we're providing applies to almost all carburetors, not merely Webers.
The basic function of a carburetor is fairly simple. Air drawn into the engine through the carburetor mixes with fuel. It is the carburetor's job to mix the fuel and air in the correct amounts. Too much fuel, and the car will be sluggish, smelly and use excess fuel. Not enough air will cause the engine to "ping" or detonate the fuel prematurely in the combustion chamber, possibly damaging the engine. The engine tuner must therefore find the fine line between too much and too little fuel. This line can be narrow and elusive if you dive into a carburetor without any knowledge of the internal workings.
One rule that will save time and frustration is to have all other systems of the engine correctly set up before attempting to make adjustments to the carburetor. These would certainly include the ignition system, valvetrain and overall engine condition. The carburetor relies on these other systems to work properly. Trying to fix an ignition problem by adjusting the carburetor is a losing battle.
Passages & Circuits
Carburetors have a number of passages, jets and orifices to meter the precise amounts of fuel and air into the engine. There are separate passages for engine idle, part throttle, full throttle and acceleration.
Each system is referred to as a "circuit." Since all cars need to transition smoothly from one type of operation to another, then more than one circuit needs to be working at a time. For example, to move a car away from a stoplight, three circuits are used. As you wait for the light to change, the idle circuit alone will be functioning. When the light turns green, you will apply your foot to the throttle, and the carburetor will transfer from idle to part throttle and, since you are accelerating, the acceleration circuit will be used as well.
A problem exists in our chosen demonstration vehicle. The engine has been left completely stock at the moment, so all of the internal components were designed for the fuel injection system. The engineers at Weber, however, have designed their carburetors for performance engines with high power camshafts and ignition systems. The ignition system is easy enough to modify, but changing the fuel injection camshaft involves a complete engine disassembly. To avoid this, we will have to determine which circuits in the carburetor are being affected by this and then make the necessary adjustments.
When the engine is started and idling in neutral, there's no problem. The idle can be adjusted to a normal condition, and everything is fine. When the engine is revved up in neutral, things are OK, but not perfect—throttle response is fair but not great.
When the car is driving, there is a huge lack of power when the throttle is depressed past about half way. The engine coughs and sputters unless it receives full throttle or less throttle. So far we have determined that it is not the load on the engine or the rpm, but the actual position of the throttle.
A quick test to see what will improve the situation is this: Either increase or decrease the fuel mixture of the idle with the idle mixture adjustment screws. Make sure that if there is more than one of these screws, they are all in about the same position and that you adjust them all in the same increments.
In the case of our Porsche, a rich idle with excess fuel improves the lack of power at part throttle. Why is this? Well, remember that more than one circuit of the carburetor operates at one time. As the engine moves from idle to part throttle, it starts to have a lack of fuel. Since the idle circuit is still working, and we have it running with excess fuel, it will to a small degree mask the problem of not enough fuel in the part-throttle circuit.
More driving of the car points to the same conclusion. When under part throttle—and under load—the car starts to "ping" or detonate. This is the destructive sound of the fuel being forcibly ignited before the spark plug fires. Since we know that the ignition timing was set correctly beforehand, this points to a lack of fuel. The mixture is too lean, and gets too hot without enough fuel to cool things down.
If you encounter a lean-out condition, back off the throttle! It is very easy to damage the engine in a matter of just a few seconds. Another interesting point is that when this Porsche is given full throttle, the pinging stops. This tells us that the full throttle circuit in the carburetor is working perfectly.
How is this situation solved? Weber carburetors have an easily removable emulsion tube. Here, fuel and air are pre-mixed before entering the main airflow through the carburetor. This tube can be thought of as a drinking straw with a hole in it—bad for drinking soda, but great for mixing fuel with air. The fuel is drawn in from a jet in the bottom of the tube (main jet), and air in through a jet in the top (air bleed jet).
A number of holes in the tube itself determine how the fuel and air are mixed. We know that the main jet and air bleed jet do not need to be changed, because at full throttle the car runs great. Since these jets are working fine at full throttle, they will handle anything less than that with no problem. This means that our adjustment will come from changing the emulsion tube itself. At this point it would be wise to consult a parts guide for your particular brand of carburetor, as often the part number has little to do with the function of the part. The new emulsion tub is slightly larger in diameter, and has fewer holes. This keeps too much air from mixing into the fuel before it enters the main air stream. Since there is less resistance from the air, the fuel moves a little faster through the passageways.
If you are restoring an older car, make sure that you check all of the air-bleed jets, main jets and emulsion tubes for dirt, or old varnished-on fuel. All of these holes are very small, and just a thin film of old, dried up gas, or a speck of dirt, can restrict a significant amount of fuel or air and will make a car run very badly. Keep in mind that even "professional" carburetor rebuilders often overlook these small orifices.
A test drive reveals that now the little Porsche behaves perfectly. There is no hesitation, no lack of power at any load, and no detonation. The change of a small part has made a big difference. As always, care and patience are critical, and sometimes the trial-and-error method is the only way to really sort out a problem.
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