Posted by Chris Case [107.194.26.164] on Wednesday, September 25, 2013 at 15:48:52 :
In Reply to: Works when you stomp on the gas. Period. posted by Chris Case [107.194.26.164] on Wednesday, September 25, 2013 at 15:25:09 :
There are five pathway to get gas into a motor via carbs. Understanding them will allow you to better troubleshoot carburetors. .
1) Main jet. It controls fuel flow during mid to full range operation. More air flowing through the venturi sucks more gas. Hence the "vacuum controlled computer".
2) Enrichment circuit. Often a spring operated needle valve that is sucked downward by vacuum. The needle restricts the opening in a jet. Less vacuum, at full throttle, lets the spring raise the needle, allowing extra gas to flow around the main jet. Adjustments can be by spring pressure, jet or needle size.
3) Accelerator pump, gives a squirt of gas when the pedal in nailed. Stomping on the pedal allows a gulp of air that the other fuel pathways can't respond to fast enough. So the squirt of gas makes up for the shot of air.
4 & 5) Idle and transfer circuits. These allow a small amount of gas to flow around the butterfly throttle. At idle completely around. But as the throttle valve opens, again the vacuum operated computer is a bit slow, so there is a Transfer slot or hole just above the idle port that allows more fuel to flow out of the idle circuit as the butterfly passes the port. The transfer port is above the idle adjust needle, so the idle needle has no effect on the transfer system.
Add a bowl of gas, with a float valve just like a toilet.
And a Choke butterfly that can shut off incoming air and richen the mixture IMMENSELY for starting, and you have a carburetor.
Inside this carburetor is a restriction called a Venturi. It is shaped like an airplane wing rolled up from tip to body. Just as the wing shape makes lift (excess vacuum), the venturi makes greater vacuum too. This vacuum sucks the fuel through the main jet, and is the sole signal for our "vacuum operated computer".
Happy trouble shooting!
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