Dirt Late Model driver Scott...
Dirt Late Model driver Scott Bloomquist doesn't typically use a setup that causes the left-front corner to raise up in the air, but engine builder Vic Hill-who builds and tunes Bloomquist's race engines-says properly setting the carburetor's float levels is still critical.
Too often racers will receive an engine package from their engine builder that has been dyno tuned for maximum power and then are afraid to touch it on the racetrack for fear of messing it up. But top teams understand that conditions in the engine dyno room are carefully controlled to help any engine produce peak power. Fuel flow is steady; plenty of fresh, clean air is fed to the carburetor and every possible variable-including humidity-is monitored and accounted for. That level of control just isn't possible on the racetrack and sometimes-often actually-adjustments have to be made in compromise.
This is especially true with the carburetor. The best setup on level ground where nothing changes isn't always going to work best on a carburetor inside the racecar where everything from the weather, to track conditions, to the setup can affect your carburetor settings. And one primary area for that is the carburetor floats.
Hill makes a quick float adjustment...
Hill makes a quick float adjustment after Bloomquist completed hot laps. Bloomquist said the car had a slight stumble in the middle of the turn so Hill suspected the left-side jet at the back of the carburetor was getting uncovered and raised the rear float slightly.
A carburetor's floats are designed to control the amount of fuel held in the bowls that is available to be pulled through the metering blocks. The carburetor jets should always be covered, but you don't want too much fuel in the bowls so that it sloshes out and into the vent tubes. If the floats are set too low, the fuel reserve in the bowls will be too small and increase the opportunity for the jets to become uncovered and the engine to run lean. If the floats are set too high, excessive fuel can slosh out of the bowls and into the vent tubes causing the engine to stumble.
On the classic Holley carburetor, the accepted method for setting the float level is to run the engine until everything is warm, and with the engine at idle, unscrew the plugs in the sight windows for the float bowls. Raise the float just until fuel begins to escape from the window, then lower the float back down a bit and lock everything in place.
When adjusting the float levels,...
When adjusting the float levels, the first step is to remove the plug from the sight window. In many custom-built racing carburetors this plug is replaced with a clear window, making this step unnecessary. For these photos we used an engine on a stand, but the process is the same in the racecar.
But in racing, especially with Dirt Late Models, that method must be adapted for conditions. We spoke with engine builder and racer Vic Hill of Vic Hill Racing Engines in Tennessee, and he gave us a few pointers.
"Often, you will have to raise or lower the floats depending on track conditions," he explains. "If the track is rough and you are really bouncing around, the fuel in the bowl can slosh around so much that it will leave a jet uncovered and the engine will stumble and blubber. That also depends on the amount of fuel pressure you are running.
Use a large screwdriver to...
Use a large screwdriver to loosen the lock screw. Next, use a 5/8-inch wrench to change the float level. Turning it counterclockwise raises the float level. Make adjustments a quarter-turn of the nut or less each time before rechecking. For a baseline, you want to set the float so that the fuel in the bowl sits just below the sight window.
"The same thing can happen if you run a setup that has the car sitting down on the right-rear tire in the turns and the left-front wheel is way up in the air. In those conditions a lot of guys will run those crooked carb spacers, but I prefer to stay away from them if I can. A lot of times you can avoid it by being careful with your float levels. When the car gets like that, sometimes the angle of the chassis and the G-forces will cause the left-side jet on the back of the carburetor to become uncovered and you will need to raise the float level on the rear bowl.
"With the same setup you may also have to lower the float level in the front bowl to keep fuel from sloshing over the bowl and into the vent tubes. Another instance when you may need to lower the floats is on a dry-slick track where you are on and off the throttle a lot. Then the car is jumping around causing the fuel to slosh forward and back and can get into the vent tubes."