On an Automatic car, do you adjust the idle screw with the car warm in Drive with someone inside holding the brakes on. Or In park? I always did it while car was in park. But just read that you should have the car in drive. I also read that when adjusting idle mixture screws, it should be in drive also. Not park. Which is the correct way????

 

Drive, then park. You want to adjust the idle so that it's smooth & within spec in gear and want it to change very little when moving into park.

 

you dont have someone with their foot on the brake while adjusting. Buy a pair of wheel chocks and leave the window open so you can reach inside.

 

Use the emergency brake to hold the car in position when in 'Drive' (if it works well). Use chocks or a [trusted] friend, if not.

 

Or do what I did. I did not have an assistant, no chocks I wanted to trust; ditto for the ebrake. I set the initial idle with the car in Park. Dropped it into drive and checked RPM. Tweaked it one more time and got it right.

 

I would not trust chocks. A friend is OK. If I have my idle mixture screws adjusted correctly, timing correct and carb. is working well, I just set mine to 800rpm in park. When I put it into drive it drops naturally to 650-700rpm. I also set the A/C solenoid the same way. A/C on, set idle to 850rpm in park then it will settle in at 650-700rpm in drive with the A/C on. Good luck with it, mike...

 

ditto, thats what 99% of the people will do.

 

Set it too the smoothes idle with out the thud when putting it in gear.

 

No one has answered the question about the idle mixture screws. I would be interested to read what the difference is in setting them on an auto vs a 4spd. What is the difference?

Thanks.

 

With an automatic you set the idle speed and idle mixture in drive.
With a stick you set both idle speed and idle mixture in neutral.
Detailed instructions are in the service manual.

 

Kinda hard to set the idle with the clutch out.

I know how to adjust the screws but my question was " Why not both in neutral? thats the concept I dont understand. What happens when tuning the mixture screws on an auto that is different then a 4spd.

 

Put it in park, adjust the idle screw to the spec. When dropped into drive, the idle should be correct. Unless engine mods have been made.
I would never have a vehicle in gear running without someone in the seat. That is just asking for something to happen.
The mixture screws I do with an out side tach instrament that is more accurate than the car tach. I adjust one to get the highest idle then the other the same way, then back to the first to get the highest idle. Then adjust the idle screw to get my best driving idle.

 

Use whatever you can to be safe. Did this 25 yrs. ago with a 2 door 70 Impala on a slight hill, in drive, with the parking brake on. Car stalled, parking brake didn't hold and the car rolled down the hill backwards. Not good.

 

I just received an e-mail flagging this post to me, asking if I could shed light on the original question.

It is correct as stated by several people that automnatic cars should have the idle mixture adjusted in "drive." Here's why:

Normally, we think of a carb as metering air/fuel based on the properties of the Bernoulli principle: As air flows through the venturi, a pressure drop occurs, and fuel is metered based on differential pressure between atmosphere and the venturi: Fuel is metered in direct proportion to the mass air flow going through the venturi. Thus, air/fuel mixture ratio stays constant, regardless of the size of the engine and other engine variables, since fuel is simply metered correctly to a given mass of air passing through the venturi.

At idle and light cruise, however, things change: Airflow through the venturi is so low that the venturi principle does not work. So 2 additional circuits are built into the carb: The idle and transition circuits. The idle circuit discharges its fuel at a point below the throttle plate, and does not rely on the Bernoulli principle at all: It discharges fuel purely as a function of manifold vacuum versus atmospheric pressure. I.e. manifold vacuum "sucks" fuel from the carb (in actuality, atmospheric pressure is "pushing" fuel into the low pressure manifold, but that's symmantics...). An engine pulling a lot of vacuum at idle will actually run richer at idle than an engine pulling poor vacuum (engine with a big cam) using the same carb with the same idle mixture screw setting. For this reason, when you drop an automatic car into "drive," the manifold vacuum will change, and the idle mixture leans out simply due to less diffential pressure between the manifold and atmosphere to "push" the fuel through the idle circuit. For this reason, adjust the idle mixture screws in "drive" to assure proper fuel mixture ratio. This will, of course, result in the mixture being slightly rich in neutral, but the "drive" mixture is generally of more importance.

Lars

 

Thank you Lars, by answering the OP's question in depth, you answered mine as to "why".