How to Adjust a Rochester 2 Barrel Carburetorby Don Bowman
Rochester 2-barrel carburetors have an adjustable float, a replaceable main metering jet, an accelerator pump and an adjustable idler metering screw for each barrel. They also have an adjustable electric choke control spring and a vacuum choke pull off and a throttle idle control screw. Before any of these can be adjusted properly the engine has to be in good running condition with all cylinders working properly and the timing adjusted to the recommended idle speed. There can be no vacuum leaks at the intake manifold.
Remove the air cleaner. Make sure the engine is cold for this procedure and the ignition key is off. Watch the top choke plate as the throttle is opened by hand. The choke should snap closed when the engine is cold and the throttle is opened. The choke has a spring located in the black circular hosing on the carburetor. When the engine is cold, the spring contracts and applies pressure to the choke to close. When the carburetor is left the way it was when the engine was shut off, the choke-- even with the contracted spring putting pressure on the choke to close--can not close with the throttle closed. As soon as the throttle is moved toward open, the tab on the throttle allows the choke to close. It will close all the way to increase the vacuum when starting to suck the gas into the engine for ignition. As soon as the engine starts, the vacuum choke pull-off is designed to pull the choke open just enough to allow enough air in to allow the engine to keep running. The electric choke spring begins to heat up and expand, and as it does, it begins to open the choke until it is all the way open.
Adjust the choke if, when the engine is cold and the throttle is moved, the choke does not close all the way. To adjust the choke, loosen the screws on the choke spring housing and rotate it until the choke is all the way closed.
Adjust the choke if it closes all the way when the engine is cold, but does not open all the way after the engine is started and warmed up. Adjust the housing until the choke just opens all the way. If the choke has no pressure to close and adjusting it still provided no tension, then replace the choke spring.
Check the vacuum pull-off and make sure that as the engine is started, the pull-off pulls the choke open a small amount. If it does not, check the vacuum hose for leaks. If there are no leaks, make sure there is vacuum to the vacuum pull-open. If there is vacuum, replace the pull-open.
Start the engine and let it warm up to normal operating temperature. Check the idle speed with the tachometer. Idle should be 800rpm. If not, adjust the screw on the throttle lever until the idle is 800rpm.
Adjust the idle mixture with the two idle mixture screws located in the front of the carburetor at the front bottom base of the carburetor. Attach the tachometer and using the small screwdriver turn the left idle mixture screw in slowly and watch the tachometer start to drop. As soon as it shows a decrease in rpm, stop and turn the screw out until it begins to decrease the rpm. Turn the screw back out until the highest rpm is achieved and then set the idle speed with the throttle screw. Do the other side the same way and then repeat from the beginning of Step 6 a second time on both sides.