Note: as always, we’re assuming the engine is properly tuned and will only cover issues with the carburetor

The most common cause is the choke unloader or reloader being set improperly

Other causes

• automatic choke improperly adjusted or damaged
  • Accelerator pump stroke should be .31”, or 8mm. Adjust by bending accelerator pump connection link. Measurement is taken at boot end of accelerator lever.
• fast-idle speed incorrectly set
• accelerator pump stroke out of adjustment
• air cleaner hold-down nut too tight, binding choke plate
• carburetor to intake manifold nuts loose
• intake manifold to cylinder head mounting nuts loose

Could be caused by the Thermostatic Valve equipped on some vehicles.

As the valve opens, it allows outside air to flow directly into the intake manifold, leaning out the rich air fuel mixture. Valve starts to open when engine compartment temperature reaches 140 F, or 60 C and fully opens at 167 F, 75C.