1983 Yamaha 30EN Outboards Service Manual, Page 41Get this manual
2 Flows of fuel and air (At low speeds) In the low speed range (idling and troll ing), the throttle valve is almost closedWhen the crankcase pressure lowers, airfuel mixtures are sprayed into the main bore through the pilot outlet and bypassThe air sucked through the pilot air jet is mi xed with the fuel sucked through the pilot jet, and the resultant mixture is metered by the pilot screw and sprayed into the main bore
When the throttle valve opens and passes the bypass hole (or holes), fuel streams into the main boreThere are two bypass holes, and fuel is sprayed through one or two bypass holes depending on the throttle opening