Regardless of the carb platform,...
Regardless of the carb platform, all that have fuel bowls have vent tubes. The pressure differential between the air in the venturi and the ambient air pressure pushing down on the fuel through the vent tubes is what sets fuel flow in motion from the bowls to the boosters. Covering the vent tubes while a motor is running will kill it immediately.
Carb Sizing
A quick Google search will find several formulas that recommend a specific carb size based on factors such as engine displacement and rpm range. While they're better than nothing, they offer ballpark estimates at best. "If you have cylinder heads that move a lot of air and a cam that takes advantage of that airflow, you need a much larger carburetor than those charts recommend,"explains Judson. "However, I fyou have factory heads that aren't very good, you're better off with a carb that's smaller than the charts recommend."
Perhaps the best way to dial in carburetor sizing is through trial and error with a vacuum gauge. While cylinder heads are flowed at 28 inches of water, carburetors are flowed at 1.5 inches of Hg, which equates to roughly 20 inches of water. Consequently, if a motor never pulls 1.5 inches of vacuum, then the carburetor never flows at its peak potential. According to Judson, on a typical street/ strip motor, it's ideal to shoot for 1.5 inches of vacuum at Wot.
Start with a carb on the small end of the spectrum-such as a 650-cfm unit-hook up a vacuum gauge to the intake manifold, and run the car up to peak rpm. "If it pulls, say, 2.5 inches, you know that the carb is too restrictive,and you need to step up in size," Judson explains. "Let's say you ran the same test with an 800-cfm carb and only pulled 1 inch of vacuum at WOT. Then you might have picked up some horsepower up top at the expense of low- and midrange power, in which case you should size back down to somewhere in between." Furthermore, a carb that is too large won't show any kind of reading on the gauge, which means it isn't presenting any restriction in flow whatsoever -most likely resulting in poor low-rpm metering. It's better to err on the small side than the large side when it comes to carb sizing, but shooting for 1.5 inches of vacuum at WOT will typically yield the best compromise between peak horsepower and driveability.
Jet Sizing
Understanding how jets regulate the air/fuel ratio is rather simple. Larger jets (with bigger numbers) richen the mixture, while smaller jets lean it out. However, jet sizes don't necessarily correlate to a specific diameter. For instance, a 40 Holley jet has a .040-inch diameter, but a 70 Holley jet has a 0.073-inch diameter. That's because all jets are numbered based on what they flow, not their drill size diameter., Just remember: The bigger the jet, the greater the flow.
The needle-and-seat assembly...
The needle-and-seat assembly and float control the fuel level in the bowl much like a toilet. Fuel enters the bowl through the seat, which gradually raises the pivoted float arm. As fuel level rises, the float arm pushes up on the needle, which slides into the seat and seals off the fuel passage when the bowl reaches maximum capacity.
Judson's Six Tips For Carb Nirvana
1 "Everyone wants to put bigger jets in a carb because they have a hot rod and think more fuel equates to more power. The exact opposite is true. Lean is mean. The average carb is engineered on the rich side to save you, so fine tuning usually involves removing fuel from a motor, not adding it."
2 "Bigger isn't better. Don't just go out and install a bigger accelerator pump, a bigger shooter, and a bigger needleand- seat assembly. Carb manufacturers already know how much fuel a carb flows, and the factory setup is fine for 99 percent of motors out there."
3 "People think carb gurus can get more power out of a motor because they make the carb flow more air, but that's not true. Where they get the power is in flattening the fuel curve from minimum to maximum rpm."
4 "Never start out by decreasing the jet size, or you'll risk burning a motor up. First, up-jet and make sure the motor loses power just to be safe, then startreducing jet sizes. As a rule of thumb, go up one jet size, and if you lose power, down-jet by two jet sizes."
5 "If you're running at the dragstrip, gauge your losses or gains based on trap speed, not e.t."
6 "Carbs work great as long as the fuel bowls are full. If your fuel system can't keep up, nothing you do to the carburetor means a thing, so it's paramount to have a good fuel system."