On the kart: the Walbro/B&S slide carburetor bolted to the intake (the only one permitted, part 555658), with the black slide inside it under the knurled cap, the float bowl underneath, the green 555729 air filter on its mouth, and the pulse line from the engine side cover running to the diaphragm fuel pump that keeps the bowl full.
The principle in one sentence
Air moving faster has lower pressure, so air rushing through a narrow throat sucks fuel up out of the bowl into the stream all by itself.
The one equation
pressure drop goes as (air speed)^2Squeeze the same airflow through half the opening and it moves twice as fast; twice the speed is four times the pressure drop, and four times the suction on the fuel. That is Bernoulli's principle in plain words: in a moving fluid, speed up means pressure down, because the energy has to come from somewhere. The narrow throat is the venturi.
Math: inversely proportional, and squares. "Speed is inversely proportional to opening" means speed = constant / opening: halve the opening and the speed doubles. "Goes as the square" means the number is multiplied by itself: 2^2 = 4, 3^2 = 9. Put them together: halve the opening -> speed x2 -> pressure drop x4. On a calculator, the square is the x^2 key, or just multiply the number by itself.
Why it works
Think of the air in the intake as a crowd walking through a doorway. To get the same number of people per second through a narrower door, they have to walk faster. Fast-moving air has used up some of its "push" (pressure) to get moving, so the pressure in the throat is lower than in the bowl, which sits at normal air pressure through its vent. The fuel in the bowl is pushed up the jet by that difference. More airflow, more suction, more fuel: the carb meters itself.
On our kart, in numbers
The slide is the power cap. The slide is a barrel that drops into the venturi; the throttle cable lifts it. How far it can lift sets the biggest opening the engine can breathe through, and that sets peak airflow, and airflow is power. The class ladder in the Briggs rule set is literally a set of slides: a .310 restrictor for Kid Kart, red (555733) and green (555740) slides for the juniors, yellow (555741) in Briggs' own chart for Senior, and at CCKRA the black slide for Junior and Senior (that's our slide; owner-inspected 8/10). Tech checks the slide cutaway with a no-go tool (.075 in) and warns that another .010 in of opening is worth maybe 0.1 hp and a DQ.
How much air. The engine displaces 206 cc and fills once every two revolutions (mechanism 7). At 6100 rpm that is 206 x 6100 / 2 = 628 liters a minute, about 10 liters a second, if it filled completely (typical; real filling is less). All of it through one venturi.
Air density and mixture. The jets are fixed (stock idle and main jet, the rule says), so the fuel flow for a given suction is fixed. On 9/19 the air went from 84 F to about 70 F; lesson 11 worked out that's 2.6% denser air. Same slide opening, 2.6% more air molecules per second, same fuel: the mixture goes 2.6% leaner at night. A kart with jetting freedom would rejet; an LO206 can't. Briggs allows one adjustment: bending the float tab to change float height, which changes how far the fuel has to be lifted to the jet.
The pulse line. The float bowl is refilled by a diaphragm pump that runs on pressure pulses from the crankcase, taken from the oil-fill fitting on the side cover (the rule says where). No pulse, no pump, no fuel: the "zip-tie the pulse line" item on the checklist is a fuel-supply item, not a tidiness item. And the pump must sit below the control plate, not vertical, for the same reason a water pump needs to be primed.
Two things to notice
- The carb doesn't know what gear you're in. It responds to airflow. Airflow depends on rpm and slide opening. Full throttle at 3000 rpm and full throttle at 6000 rpm are two different carbs, as far as the fuel is concerned. That's why the same jet can be right on the straight and slightly off out of a hairpin.
- The air filter is part of the venturi. A dirty filter is a smaller opening before the venturi: less airflow at every slide position, and (because the restriction is upstream of the throat) more suction per unit of airflow, so richer. The fresh-filter-every- trip rule is a mixture rule.
Problems
1. Halve the door. If a restrictor plate cut the venturi's open area in half at full throttle, by what factor would the air speed in the throat rise, and by what factor would the suction on the jet rise? Would the engine run richer or leaner at that point, and what would that do to power?
2. Night jetting. Air at 84 F is 2.6% less dense than air at 70 F. If the daytime mixture was right, is the night mixture rich or lean, and by what percent? If Briggs let you change the main jet, which direction (bigger or smaller hole) would fix it?
3. Fuel per lap. At 6100 rpm the engine breathes about 628 L of air a minute. A gasoline engine burns roughly 1 gram of fuel per 14.7 grams of air (typical). Air weighs about 1.2 g per liter. How many grams of fuel per minute at full throttle, and how many per 58 s lap if the throttle were open the whole lap? (Real laps are less; lesson 9's throttle trace says how much less.)
Go look: pull the air filter (fresh one back on after) and look down the carb throat with the throttle closed and open: that's the slide moving, and the small tube you can see at the bottom of the throat is the jet's nozzle. The brass barb on the side cover with the hose to the pump is the pulse fitting.