Carb question

[Racincc85]

I was wondering why it is necessary to pump the gas pedal while starting a car that has a carb on it? I have always worked with fuel injected cars, so carburators have always been somewhat of a mystery to me.

[JDPascal]

Carburators have an accelerator pump that shoots a jet of raw fuel into the venturi in addition to the fuel going thru the main metering jets. It richens the mixture for accelerating at speed and for starting before you start to crank the engine.

Tapping the gas pedal also sets the automatic choke (if there is one) on a cold engine , so the fuel mixture is rich enough to start it.

If you continue to work the gas pedal, as you crank the engine you will likely flood the engine with too much fuel and it won't start. To fix the flooded condition, you hold the gas pedal to the floor while you crank the engine. If that doesn't fix it, let it sit for a while.

A fuel injection system accomplishes the same functions using all the sensors and a computer. It is usually harder to flood an injected engine.

JD

[curtis73]

Several reasons, but it all comes down to fuel delivery when the engine is cold. It takes a rich mixture to start an engine; which is automatic with EFI since the computer reads the air temp and coolant temp to adjust firing pulsewidth.

A carb has an accelerator pump which literally squirts fuel as you press the pedal. Pumping the pedal gives it a couple squirts to enrich the start up mixture.

The choke on a carb is opened by a thermostatic coil. Once its open, it lets the throttle close the whole way. Then when you shut it off and the coil cools, the choke rests against the throttle and can't choke. Moving the throttle (pumping the pedal) allows the choke to close and the fast idle cam to hold the blades open a touch.

The choke mechanism is also why you have to "kick down" the idle on carbed cars. Once its running on fast idle and the choke heats up, it can't drop down until you crack the throttle and let the choke move out of the way.

Edit... JD and I must have posted at the same time. So, I will just say, "ditto" to everything he said

[benchtest]

It does two things: 1) The primary reason is to set the choke. By opening the throttle it allows the high idle cam to move. This allows the choke door to close and it sets the high idle screw on the appropriate step of the high idle cam. 2) It sprays a bit of fuel into the intake to aid in starting the engine.

[Omega_5]

Aren't carbs wonderful things....
Cars should go back to carb.....jk..... but seriously...

[44Magnum]

All that gas in the flooded engine that you dump by holding pedal down while cranking ultimately goes into the crankcase and mixes with oil. Once engine gets hot the gas will evaporate out and the oil wil lbe OK.