View Full Version : Cold and hot idle speeds

11-01-2014, 06:06 AM
What are the correct cold idle and hot idle rpm speeds for my car?
I have a 260 V-8 with an automatic choke and an "instant-quick and liquid-smooth" Ford-O-Matic transmission.

The engine presently idles at about 575 - 600 rpm while in Drive and it does not change speed as it goes from cold to warm, meaning that both the cold and hot idle speeds are set to the same rpm. That is not a problem and is actually sort of nice, but it caused me to wonder if the automatic choke had stopped working.

I checked and saw that the carburetor flap valve stands vertical when the engine is warm, tilts down about 1/3 when the engine cools, and drops to 3/4 closed when the engine is cold and I tap the throttle to the floor. So, the automatic choke is functioning perfectly. :) As an aside, this is what I like about these old cars. You can actually see how things physically function, and you can literally touch them and move them back and forth with your fingers. They are so simple and yet they work so well.

What are the Ford-specified cold and warm idle speeds for my engine/transmission combination, and do those idle speeds apply to the transmission when it is in Drive or in Park?

11-01-2014, 09:34 AM
the 575-600 is correct or real close for a warm car in gear.
Fast idle should be around 1200 or so cold.
As the engine warms and the choke opens the idle should drop to the 575.

There should be a fast idle adjustment on the passenger side of the carb.
Adjust that screw in turns up the idle.

11-03-2014, 11:16 AM
Yeah - what Nathan said.

In essence the choke "flap valve" also rotates the fast-idle cam. It is all driven by a bi-metallic spring in the choke housing (the round black thing on the side of the carb). This part is adjusted to either rich or lean by roating it CW or CCW. Rich holds the choke on longer, lean releases the choke sooner.

The screw that rides upon the fast-idle cam determine the position of the lower butterfly, and can be adjusted to change idle speed when the choke is on.

Really, you have two idle screws. The one on the drivers side of the car is negated (not resting on anything) when the fast idle screw is riding on the cam. The cam has a sharp drop-off when the choke is fully open and can not effect normal temp idle speed when the choke is fully open, but you can have the fast idle screw set too low as to not increase idle speed when the choke is closed. The cam is also "stepped" so you usually have 2 or 3 levels of idle speed as the choke valve begins to open.

If the fast idle is set too low in choke closed mode you will be running very rich because you are not increasing the intake of air along with the increase of fuel. You run the risk of fouling things up. More fuel requires more air - hence, fast idle.

Usually there is a spec for this in the manual, and it's not usually the same hot versus cold. But fast idle is usually a couple hundred RPM higher - at least.

11-03-2014, 08:03 PM
Hello, Nathan and Roger.

Thanks for the well-timed explanation. It was 44 degrees this morning! That is unusually cool for this early in Fall. The "slow" fast idle seemed to be alright while the weather was warmer, but I noticed this morning that I had to take care to accelerate slowly for perhaps the first five minutes so as to avoid stalling the engine.

A few hundred more RPM on the fast idle would have come in handy this morning. I'll speed it up a bit this weekend.

Thin blooded Floridian that I am, I drove to work this morning with the windows rolled up and the vent doors closed and latched. Then I pulled out my heater and temperature knobs to knock down the chill. The heater quickly warmed up the interior without even having to turn on the fan. [thumb]

11-08-2014, 09:37 AM
It was 52 degrees this morning and I fiddled with my idle speeds.

Here was the initial choke position with the engine cold:

This is where it was after I tapped the gas pedal to the floor three time to release the choke. It closed a little bit more than it had been in the initial position. I sped the cold idle up from about 600 RPM to about 800 RPM while it is in drive and 1000 - 1100 rpm while in neutral. The car will now be less likely to stall in the morning while it is cold.

After starting the cold car I had to drive it about two miles before the automatic choke opened up. I would rather it open a little sooner so I moved the plastic cover on the choke a small bit clockwise in the "lean" direction. I'll drive it again tomorrow morning when the engine is cold to see if the choke opens any sooner now. This picture shows the choke plate standing vertical after the car had been driven for a few miles to warm the engine up. The automatic works! After the engine had warmed up I slowed the hot idle from 600 down to 500 RPM. That way it does not buck so much when putting it in gear and does not want to creep forward as much while in Drive.

So now the motor should idle in drive at 800 rpm while cold and idle at 500 rpm in drive after the engine warms up.

Update: I drove the car around in traffic this afternoon after adjusting the idle speeds. The 500 rpm idle is really nice. When stopped at a red light the car idles so smooth and quiet that it doesn't seem like it is running, but it still spins fast enough that the generator light from coming on. When I lift my foot off the brake the car just very slowly creeps forward, it is almost at a stop. Perfect. [thumb]

11-13-2014, 06:07 PM
Maybe the answer is obvious, but I can't see it.

What is the purpose of the rectangular heat shield that is in front of the automatic choke? What is it shielding against? There is not any hot thing in front of the choke.


Is it blocking the wind being thrown back from the radiator fan?

Why does the automatic choke need to be shielded from heat? Heat is what activates it.

11-14-2014, 06:33 PM
What is the purpose of the rectangular heat shield that is in front of the automatic choke? What is it shielding against? There is not any hot thing in front of the choke.

A friend of mine at work (a mechanical engineer with a 1967 Fairlane) explained that what I thought was a heat shield is really a cold shield. The automatic choke requires heat to operate. On a cold day (think zero degrees) the fan would blow low temperature air across the choke and never allow it to warm up. The shield prevents that stream of cold air from blowing directly across the automatic choke.

That makes sense.

11-14-2014, 09:26 PM
That's true. Some also had the heater hose strapped to the outside of the housing. The choke tube feeds hot air off the manifold too, and in fact there is a very small vacuum source into the choke housing to cause the engine vacuum to pull the hot air up and into the housing.

Sounds like you have it all running good now - and it's always good to know for yourself how you got it that way. [thumb]