Lower idle speed in gear than in neutral.

[bla8310]

I am getting a lower idle speed when in gear than in neutral or park.
I have replaced the fuel lines, the fuel filter (however not the tank filter?)
Putting the car into park will cause a pretty good clunk because of the
idle speed increasing. I've been putting it into neutral, turning it off and
then putting it into park.
I have very little experience with Carbs so any suggestions on the basics
places to start would be appreciated. I have a full tank of gas, so if the
tank filter is part of the issue I may run it awhile to lower the level of gas before I attempt that replacement.

Thanks
Brad

[wbain5280]

Which model, what year, actual engine speeds please.

It's normal for the engine speed to decrease with an automatic when in gear. Idle speed must not be high but on spec, approx 700 - 800 rpm and not drop too low. You will also notice that the oil pressure will decrease to approx the '1' position when hot and increase to '3' after driving from stop. You must also follow the proper oil viscosity.

Year and model please, also add this info to your profile.

[bla8310]

I have my current vehicles listed on my bio. I will look again and try to figure out why its not showing on my signature.

The car in question is a 67 W110 230. My oil pressure gauge will vary
as you described. What do you recommend for oil viscosity? I am located
in the western suburbs of Chicago.

Like I mentioned: The idle when in qear will almost stall. It was worst before I replaced a couple of the flexible fuel lines and the fuel filter. It idled better but as I have put more miles on it, the idle is getting worst. However, as I mentioned, the idle when in neutral or park raises significantly. I don't know what speed the idle is at, not sure how to check.

Thanks
Brad

[bwostosh]

Sometimes the vacuum advance will allow the high idle,
Kind of like a feedback loop, high neutral idle increases vacuum, increasing advance, increasing vacuum, increasing idle, etc until the transmission load forces it down.

I've seen a 1500 rpm idle brought to 600 with a clunk.

Try disconnecting vacuum advance to test this.

[mercmad6.3]

If your car is an Automatic:
The carb will have a switch on it. This is connected to the trans. Check the fuses .
With the key on and the engine not running touch the acelerator pedal. You should hear a click under the car. Push ti right to the kick dwon positon and you should hear anthor click. if you do not hear the click as you touch the pedal or lift your foot off ,the fuse has blown or the solenoid on top of the trans has siezed or it's linkage is seizing.
When the car is in gear at idle and not moving the switch on the carb sends a signal to the trans and this allows a valve to open which cuts fluid pressure. This reduces strain on the trans and prevents the car overheating in traffic. The trans has a fluid coupler ,not a torque convertor and because of this there is very little slip .
If you keep your foot on the pedal the trans stays in gear and the idle will start to drop.