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Idle rpm drops substantially when putting car in gear

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Michael_

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1969 dodge charger r/t 440, 727, stock.

So when i go from Park to Drive my idle rpm drops from around 1100 to 400 or 1000 to 500 (cant give you an exact number, but basically in half).

The car has power brakes and the pedal also gets harder when the rpms drop.
During normal driving it works great but rolling around at idle the pedal gets pretty hard and need to press harder for the same braking power.

The car also vibrates/shakes a little when its running at 400-500rpm but runs completely fine while in Park or when actually driving (with higher rpms).
Engine has great power, water temperature is also great.

What could it be?
I guess i should start looking for vaccum leaks, carb idle mixture and temporarily bypass the brake booster itself but maybe this is a common problem so i can save myself some work?
 
Could be a lot of things. It sounds like the cam has been replaced with one too big if you have to idle it at 1100 rpm. Most stock engines idle around 650 in drive and 750 in neutral
 
toolmanmike said:
Could be a lot of things. It sounds like the cam has been replaced with one too big if you have to idle it at 1100 rpm. Most stock engines idle around 650 in drive and 750 in neutral
Click to expand...

Of course i cannot say for sure but i would be very surprised if the cam has been replaced since the car is a very original survivor.
But maybe the previous owner set the idle higher to "fix" a vaccum issue that just got worse?

Well 650 in drive is still 150-250rpm higher than what i have. That definitely seems to be the problem.
 
Last edited:
Possible diaphragm let got in the brake booster
At least that’s what my issue was when I experienced the same symptoms
 
Did it just start doing this?
 
WileE said:
And... what are your vacuum readings ?
Click to expand...

I dont know yet.

threewood said:
Did it just start doing this?
Click to expand...

The idle was like this since i bought it a couple years ago. i didn't knew thats too high.
But the brakes have not been like that so that's new and i think (makes sense) the idle also did not drop this much when putting it in drive.
 
Something to try is advance the distributor a few degrees which will increase idle. Then adjust the curb idle screw out to lower idle back down where you want it.

This has always worked for me in equalizing neutral/drive rpm variations.
 
Not enough initial timing is another possibility. But probably a combination of things - overly tight converter, vacuum leak, retarded timing. . .
 
To rule out the brake booster you can pull the vacuum line to it and cap it.
See if it improves.
 
Yes, sure sounds like a vacuum leak to me; at least that’s the first thing to check.
 
To check for vac leaks, you can spray some brake wash (flammable) around the base of the carb and intake, vac hoses etc....
 
Tune problem. Carb, vacuum leak, timing. Bad booster could do this.

What ignition do you have, perhaps a pertronix? What coil?

Nothing wrong with the torque converter.
 
R413 said:
Tune problem. Carb, vacuum leak, timing. Bad booster could do this.

What ignition do you have, perhaps a pertronix? What coil?

Nothing wrong with the torque converter.
Click to expand...
A TQ is very rarely the problem. Check all the easy stuff first before diving deep.
 
Cranky said:
To check for vac leaks, you can spray some brake wash (flammable) around the base of the carb and intake, vac hoses etc....
Click to expand...

Personally, I find this to be an untrustworthy method. It may, or may not identify the leak.

First confirm if there is a leak by placing your hand over the carb to slowly choke it out. Listen closely for a slight increase in rpm before it chokes out. If that happens, you have a leak.
 
BSB67 said:
Personally, I find this to be an untrustworthy method. It may, or may not identify the leak.

First confirm if there is a leak by placing your hand over the carb to slowly choke it out. Listen closely for a slight increase in rpm before it chokes out. If that happens, you have a leak.
Click to expand...
But.....if brake wash is used, it will usually tell ya pretty quick if there is an air leak....
 
the short coming is that you need to find the leak for this method to work. Intake leaks under the intake usually will not present with this method. That is because the leaks are almost always between the head and under the pan gasket, not at the intake. I have probably lived this five times.

Folks regularly claim they have no vacuum leak because they sprayed some organic aerosol around some suspect locations. This is simply not conclusive.

Hand over the carb works 100%. But you do need to be sensitive to the subtleties of the method.
 
Buy some tools. Start with a vacuum gauge and a timing light.
 
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