hello to all i'm new here. i'm having a question regarding my CBR carburetor. kindly check the attached image below.



So in this type of carburetor, we can see a big hole in front of the throttle valve ( number 1 ) and another three little holes just under the throttle valve ( number 2 ) . in our manual they have mentioned that those three tiny little holes as "bypass holes". they advice us to align the butterfly valves to those bypass holes to bench sync the carb. most people say that they connect to the idle circuit. but they are almost covered with the butterfly valve.

so my question is, can someone explain me how it works and also the purpose of the bigger hole ( number 1 ) ? when bench syncing do we need to sync as the butterfly valve covers all these threes holes or just slightly open ? coz if the holes are covered how it helps to maintain the idle ? thank you

 

OK, so hole number 1. that's the Idle port. Basically that is where Fuel and Air flow into the engine when it's idling. The Pilot screw adjusts the amount of fuel that is mixed with air that is flowing through the "Slow Air Jet #1" here, marked item 4 in the picture below.



In your photo above, circle 2, the small holes are called Bypass Ports. When the Throttle Plate (butterfly) is closed, only one of those tiny holes is exposed to the downstream side. This allows air to flow into the upstream holes (the two not yet exposed) and out through the first hole. This slightly leans out the air fuel mixture. Once you start to move the throttle ever so slightly, the second hole is exposed on the down stream side of the carburetor. This changes the roll of the ports and then starts to allow fuel to flow out of these small ports. As the Throttle Plate is opened further it helps smooth the transition between the Slow Speed and the Mid-range systems.

I hope this helped. It was a good question. Not many people really care to know how it really works.

 

This is not the same carburetor design, but the concept is the same.

 

thank you very much IDODirt. this is very informative & helpful. thx again for your explanation.