Carburetor Systems

The Float Bowl• Fuel inlet system• Stores fuel for

immediate use• Maintains level

• Viton Needle, brass seat

• Dampening spring• Vapors vented to

carbon canister

Idle Circuit

• Low airflow @ idle, main system will not operate

• High vacuum under throttle plate draws fuel out idle ports

• Sometimes adjustable• Idle air bleed helps

atomize fuel

Transfer Circuit

• Adds fuel as throttle plate opens, overcomes “lag”

Main Metering Circuit

• Fuel supplied by “main jets”

• High venturi vacuum creates low pressure at nozzle tip

• Atmospheric pressure in float bowl pushes fuel out discharge nozzle

Power Circuit• Provides extra

fuel at WOT• Mechanical

power valve, maybe adjustable

• Sometimes combination with metering rods

• Flows through a circuit into main metering system

Accelerator Pump

• During sudden acceleration, mixture leans out

• Pumps a lot of fuel into venturi

Choke Circuit

• Very rich A/F needed on cold starts

• Closes off top of venturi to raise the vacuum level allowing greater fuel flow into manifold

• Opens as engine warms up, or as an electric coil heats up

Choke Coil Operation

Choke Pulloff

• Forces the choke open slightly just after engine starts.

• Position determined by – torque of coil– air velocity on blade– vacuum on piston– must adjust correctly!

Altitude Compensator

• Compensates for altitude• Adjusts an air bleed

Idle Speed Control

Throttle Kicker

• Electric motor or solenoid, increases idle

1. Dashpot, 2. Throttle kicker, 3. Throttle cable, 4. Idle adjustment