High-g turns can sometimes cause the fuel in the float bowls to slosh away from the second
In other words, chassis adjustments can make a major difference in lap times when you're trying to find the optimum setup during practice, while an engine-tuning change-unless something is really wrong-generally won't result in a major difference in lap times. The biggest reason to make something like a carb tuning change during practice is if the carburetor isn't functioning properly, or there's a driveability issue. Since practice time is valuable, concentrate on getting the best-handling race car possible and don't allow the little things to distract your focus.
Of course, if your race car is significantly off the lap times the leaders are producing, you're going to have to figure out the source of the trouble. Is it a chassis problem, or is the engine down on power? That's a debate between engine builders and chassis specialists that's raged for decades, and it likely won't be going away any time soon.
One of the best ways to determine if your car's slower times are a result of a problem with the engine or chassis is with the stopwatch. Start comparing times between your driver and the fastest cars on the track. But don't time complete laps. Instead, start clocking segments. First, compare your driver's times to the fastest cars just through Turns 1 and 2. Then do the same thing through Turns 3 and 4. Finally, compare your driver to the leaders through the straights. Pick a point to begin timing where most cars have straightened out and the drivers are just getting to full throttle, and stop at the point where most drivers are just getting out of the throttle.
If the biggest difference between your driver and the fastest cars' times is in the turns, then you should begin looking for a chassis problem. After all, every engine produces the same amount of power when the carburetor's throttle blades are closed. There's probably some problem hindering your driver from getting through the turns cleanly. But if the biggest loss of time came on the straights, then you may have an engine that's down on power. Just make sure to always look at the big picture. If handling is so poor on turn exit that your driver is picking up the throttle later than the competition, then his straightaway speeds are obviously going to be lower too.
Dorton also cautions that there are a few ways that a poorly adjusted carburetor can cause your car to be slow through the turns-particularly a turn exit. Fuel sloshing in the carburetor can cause the engine to stumble or bog a bit when the driver picks up the throttle, especially on high-grip tracks where there is strong side-loading.
You may also want to keep an eye on your float levels if you notice a stumble or hesitation through the turns. If you have the float level too low, high-g turns can sometimes cause the left-side secondaries to become uncovered. This is especially true on the rear of the carburetor where fuel can not only be pushed to the right side of the float bowl, but also the rear as the driver accelerates out of the turn. In situations like this, you may have to raise the float level a bit.
On the front of the carburetor, you may also have to deal with spillover if you're able to get the forward bite maximized for very hard accelerations. If the float level in the front is too high, the fuel can actually spill over into the vent tubes. This time around you may need to lower the float level.