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Old 11-06-2020, 09:08 AM   #1
kens65fb
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When I apply the brakes my idle drops too much

My 350 SBC (mild cam, turbo 400) idles fine, at about 900 RPM.

When I apply the power brakes in PARK, the idle drops about 200 RPM.

When I shift the automatic trans into, DRIVE, or REVERSE, my idle drops an additional 150 RPM, or so, which I believe is probably normal.

It is at this point, the idle, is down to about 550 RPM, and the idle becomes rough, barely running, but does not stall.

I know that is not electrical, as I have completely removed the ALL wiring from the vehicle.

The only electrical wiring left is the ignition switch, with 3 wires.

Any thoughts
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Old 11-06-2020, 09:26 AM   #2
Steeveedee
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Re: When I apply the brakes my idle drops too much

Your booster is likely leaking somewhere.
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Old 11-06-2020, 01:22 PM   #3
kens65fb
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Re: When I apply the brakes my idle drops too much

Anybody else ??

What is the best way to test the booster?
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Old 11-06-2020, 02:33 PM   #4
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Re: When I apply the brakes my idle drops too much

Mity-vac or plug it.
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Old 11-06-2020, 02:33 PM   #5
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Re: When I apply the brakes my idle drops too much

I'm assuming you have a vacuum power brake booster. Must have a vacuum leak.. either in the booster itself or possibly in the vac line or connection? Disconnect the vacuum line, plug it and see if it makes a difference in the idle.
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Old 11-06-2020, 04:20 PM   #6
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Re: When I apply the brakes my idle drops too much

I was also thinking vacuum leak. But could it be the torque converter?
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Old 11-06-2020, 04:24 PM   #7
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Re: When I apply the brakes my idle drops too much

Like the others have said, power brake booster.
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Old 11-07-2020, 08:29 AM   #8
kens65fb
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Re: When I apply the brakes my idle drops too much

O.K. s the latest.

I disconnected the vacuum line to the booster and plugged it, idle remained at 900 RPM.

When I applied the brakes, NO decrease in RPM, so, I'm guessing that the booster is a problem, but when I shifted into DRIVE, the RPM dropped to about 575 RPM. about where it was previously, with the booster connected.

I'm at a loss, someone mentioned torque converter, can that affect idle?
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Old 11-07-2020, 08:50 AM   #9
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Re: When I apply the brakes my idle drops too much

How are you setting idle? What screws are you using, the idle mixture at the carb base or the single one at the throttle?
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Old 11-07-2020, 09:00 AM   #10
kens65fb
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Re: When I apply the brakes my idle drops too much

Quote:
Originally Posted by The Rocknrod View Post
How are you setting idle? What screws are you using, the idle mixture at the carb base or the single one at the throttle?
It is a brand new Edelbrock 1906 carb with an electric choke, I am NOT a seasoned mechanic, but I adjusted the (2) screws at the base of the carb, with a vacuum gauge to get the best vacuum.

The screw on the right side seemed to do little, but the best vacuum I could get was 15-16 inches.

Then I set the idle screw to about 900 RPM
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Old 11-07-2020, 09:45 AM   #11
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Re: When I apply the brakes my idle drops too much

Set your idle when hot at 750 rpm.
Then check your initial timing.
If it’s below 10 degrees bump it up to 14 degrees. Plug off vac advance line doing that.
Check your idle is at 750 again.
Adjust your mix screws.
See how that works.

Is your vac advance hooked to ported or manifold vacuum port?
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Old 11-07-2020, 09:47 AM   #12
kens65fb
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Re: When I apply the brakes my idle drops too much

Quote:
Originally Posted by geezer#99 View Post
Set your idle when hot at 750 rpm.
Then check your initial timing.
If it’s below 10 degrees bump it up to 14 degrees. Plug off vac advance line doing that.
Check your idle is at 750 again.
Adjust your mix screws.
See how that works.

Is your vac advance hooked to ported or manifold vacuum port?
I don't have a timing light, I am using ported vacuum.
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Old 11-07-2020, 09:59 AM   #13
geezer#99
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Re: When I apply the brakes my idle drops too much

If you’re gonna get it running right you need a timing light.
A digital or dial back type works.

Or you could set the timing like I learned to 50 years ago.
By ear!
Advance it until it pings, back it off a touch. Lock it down, adjust idle down to a guesstimated 700 and test drive.
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Old 11-07-2020, 10:07 AM   #14
geezer#99
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Re: When I apply the brakes my idle drops too much

Two weeks ago I gifted one like this to a new mechanic in training.

https://www.summitracing.com/parts/s...iABEgLe8vD_BwE
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Old 11-27-2022, 01:17 AM   #15
guyryan100
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Re: When I apply the brakes my idle drops too much

Harbor Freight has a cheap one with dial back feature, worked great for me.
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