Carburetors oftentimes get a bad rap. At their core, these air and fuel mixing devices are simple. All it takes to turn something scary-seeming into a part of your car you can confidently diagnose and repair is a bit of basic understanding. This week in the garage, I’ll talk you through some basic carburetor terms to build that foundation.
The float bowl is far and away one of the most straightforward components on a carb, so we will start here. The float bowl holds small amounts of fuel. That’s it. Told you it was simple.
Needle and seat
The needle and seat are the door and door frame that allow the fuel into the float bowl. The needle has a tapered point that matches the cut that is threaded into the float bowl. When mated together, they create a seal. The key to remember here is that seal is only good for a certain pressure, often 4-6 pounds. Standard mechanical pumps push fuel with about that force, but if upgraded to an electric fuel pump you might need to look into a regulator to ensure you do not overwhelm the needle and seat.
The needle and seat don’t just allow fuel into the carb randomly. It is the float that controls that valve, and most vintage carb floats are thin brass pieces soldered together. Old-school carbs sometimes feature cork floats, which sidestep the corrosion problem that can put pinholes in brass with a tendency to degrade that can make the cork literally fall apart. If a float sinks, or otherwise won’t float, a carburetor will flood because the needle and seat will not close and keep the carb at the proper level.
If you thought the float bowl was simple, a jet is even more basic: A precision-sized hole for fuel to be drawn into the airflow of the carb. The size of the hole determines the amount of fuel that can be pulled through it. A couple things to note about jets, though: they clog and they are easily damaged. Use only the properly-sized, well-fitting screwdriver to remove or install a jet to prevent stripping it. Also, when cleaning jets, do not stick anything harder than brass through the orifice as that can change the size of the jet and cause tuning headaches.
Idle speed and mixture
These two terms get lumped together for this discussion, mainly because folks will use one to Band-Aid the improper adjustment of the other. Idle mixture determines the amount of fuel that can be pulled through the idle circuit. The idle speed effectively determines the amount of air entering the engine at idle by making small adjustments to the opening of the throttle.
Often, a poorly adjusted idle mixture will be covered up by raising the idle speed. This will cause issues when the idle speed adjustment reaches the point where the throttle is being opened enough that the engine is attempting to idle on the main circuit and all the fuel that comes with that, rather than the small amount from the idle circuit as is proper.
The last item to touch on is one that not all carbs include. As carburetors evolved over time, engineers looked for a “no-compromise” solution to solve the problem of large and immediate throttle inputs and their associated stumbling. (Large and fast throttle openings cause a sudden increase in airflow cannot pull in the proper amount of fuel quickly enough, leading to bogging down or hesitation.) The fix is an accelerator pump—a simple piece that squirts a bit of raw fuel into the airflow when the throttle is quickly opened. It’s not a particularly elegant approach, but it works. Some carbs have adjustable accelerator pumps that let tuners dial in the raw fuel squirt to their liking.
Okay, so maybe some carbs are more involved than other, but I promise they are not the mysterious black boxes that some folks make them out to be. Taking one apart with a service manual and a curious mind will teach you a lot.