Rochester B, BC & BV Idle System

Rochester B Carburetor

Idle Circuit
Fuel is fed from the float bowl through the main metering jet and into the main well assembly. Vacuum then pulls the fuel up through the crossbar, the idle air bleed, then down to the idle mixture discharge. Air is mixed with the fuel in the crossbar with air bleeds. These are very small holes so carefully check them for clogging.

Adjusting Idle Mixture
Watch a video about our premium add-on kit, which includes tips on adjusting the idle mixture.
There are a couple of ways to adjust the idle mixture.
Using a vacuum gauge.
  • Hook up the vacuum gauge to the intake manifold.
  • Bring the engine to operating temperature.
  • Adjust the idle screw to idle RPM as set by the manufacture.
  • Adjust the idle mixture screw until you get the best vacuum reading along with a steady needle.
By ear.
  • Bring the engine to operating temperature.
  • Turn the adjusting screw out 1/4 turn.
  • Start turning in the screw 1/4 turn at a time, wait a few seconds for the engine to catch up.
  • Turn the screw in until the RPM starts to drop.
  • Turn the screw counter clockwise 1/4 to 1/2 turn.
Part Throttle Idle Discharge
There is a gap between the idle and when the accelerator circuit takes over. This is handled by the secondary idle discharge. As the throttle valve opens off of idle, a second passage is exposed in the carburetor bore allowing additional air/fuel mixture to enter the manifold. This passage is easily clogged so when cleaning your carburetor be sure you blow compressed aire through this hole and follow the air up to the air bleeds.
Won’t Idle
carb will not idle after following step, by step rebuild. will run as long as im pouring gas into it.
Either a vacuum leak, lack of fuel, or electrical system. We aren’t addressing the electrical system here.
Vacuum leak around the mounting gasket, throttle body gasket, intake manifold, or any vacuum line leading from the carburetor. Spray carburetor cleaner around these places to see if the RPM smooths out.
Test the fuel pump. Should be around 4 – 4.5 lbs.
Look for any kinked lines. If using rubber fuel hose it may be collapsed inside causing a restriction.
Make sure the fuel filter is changed. There may be a filter inside the fuel inlet (on some models). Any inline filter should be changed.
Did you adjust the idle mixture screw?
Check the float to see if it drops enough to pull the needle out of the seat.
Maybe the needle was damaged when installing and is sticking closed.
Look at the idle passages in the illustration, then make sure each one is clear by blowing compressed air through them. You will need to determine the ending route of the passage so you can feel if any air is coming through. Simply blowing through a passage may not be enough to clear it. Ethanol residue leave hard deposits.
Rochester 1 Barrel Parts
Idle Discharge (can cause hesitation)
This picture illustrates where the idle mixture discharge hole and the idle discharge holes are located. At idle the idle mixture comes from the idle mixture hole. As you throttle up, there is a short lag between idle & the accelerator pump circuit.
Rochester B Idle
This is where the idle discharge comes in. As the throttle valve opens, it exposes the idle discharge holes, allowing additional air/fuel mixture to enter the intake. These small passages can get plugged which will cause a slight hesitation off idle.

Updated on 12/09/2020

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