When I apply the brakes my idle drops too much - The 1947 - Present Chevrolet & GMC Truck Message Board Network

kens65fb · 11-06-2020, 09:08 AM

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

**Steeveedee** · 11-06-2020, 09:26 AM

Your booster is likely leaking somewhere.

kens65fb · 11-06-2020, 01:22 PM

Anybody else ??

What is the best way to test the booster?

**The Rocknrod** · 11-06-2020, 02:33 PM

Mity-vac or plug it.

jchav62 · 11-06-2020, 02:33 PM

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.

forestb · 11-06-2020, 04:20 PM

I was also thinking vacuum leak. But could it be the torque converter?

**straight6chevyguy** · 11-06-2020, 04:24 PM

Like the others have said, power brake booster.

kens65fb · 11-07-2020, 08:29 AM

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?

**The Rocknrod** · 11-07-2020, 08:50 AM

How are you setting idle? What screws are you using, the idle mixture at the carb base or the single one at the throttle?

kens65fb · 11-07-2020, 09:00 AM

Quote:

Originally Posted by The Rocknrod

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

geezer#99 · 11-07-2020, 09:45 AM

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?

kens65fb · 11-07-2020, 09:47 AM

Quote:

Originally Posted by geezer#99

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.

geezer#99 · 11-07-2020, 09:59 AM

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.

geezer#99 · 11-07-2020, 10:07 AM

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

https://www.summitracing.com/parts/s...iABEgLe8vD_BwE

**guyryan100** · 11-27-2022, 01:17 AM

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

11-06-2020, 09:08 AM	#1
kens65fb Registered User Join Date: Dec 2018 Location: Dallas, PA Posts: 551	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

11-06-2020, 09:26 AM	#2
Steeveedee Who Changed This? Join Date: Jan 2013 Location: Simi Valley, CA Posts: 10,676	Re: When I apply the brakes my idle drops too much Your booster is likely leaking somewhere. __________________ ~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA

11-06-2020, 01:22 PM	#3
kens65fb Registered User Join Date: Dec 2018 Location: Dallas, PA Posts: 551	Re: When I apply the brakes my idle drops too much Anybody else ?? What is the best way to test the booster?

11-06-2020, 02:33 PM	#4
The Rocknrod Moderator Join Date: Aug 2011 Location: China Spring, TX Posts: 7,280	Re: When I apply the brakes my idle drops too much Mity-vac or plug it. __________________ Master Chief, US Navy, Retired 1964 C10 Step-side Short Bed - In Progress 2009 2500HD Duramax LTZ 1948 GMC Panel 1956 Big Window Stepside 1953 5 Window Chevy Stepside 1932 Coupe New Seat Foam for Bench Seats?

11-06-2020, 02:33 PM	#5
jchav62 1962 C-10 Join Date: Apr 2005 Location: Las Vegas, New Mexico Posts: 4,978	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. __________________ Joseph Chavez "See the USA in a Chevrolet!!" ......................_____ .........________//__{\_____ ,,,,,,,/__(O)___//___/__(O)_/ 1962 C10 Shortbed Fleetside 327/th350 GV overdrive 1974 C10 Longbed Fleetside 350/th350 "The Trash Truck" 1999 Chevy 2500 OBS, Longbed 5.7/4L80E 4x4 2015 GMC Sierra SLE 1500 4x4

11-06-2020, 04:20 PM	#6
forestb Registered User Join Date: Jan 2009 Location: Burbank, CA Posts: 3,619	Re: When I apply the brakes my idle drops too much I was also thinking vacuum leak. But could it be the torque converter?

11-06-2020, 04:24 PM	#7
straight6chevyguy Senior Member Join Date: Dec 2018 Location: yamhill OR Posts: 909	Re: When I apply the brakes my idle drops too much Like the others have said, power brake booster. __________________ 1965 chevy c20 292 4speed http://67-72chevytrucks.com/vboard/s...d.php?t=777601 1976 chevy c10 250 4speed http://www.67-72chevytrucks.com/vboa...d.php?t=787037 1978 chevy c20 350 4speed 1998 chevy c1500 4.3L 5speed 2002 Chevy s10 4.3L auto 4x4 Plus a Mercury & some Fords... To many projects!

11-07-2020, 08:29 AM	#8
kens65fb Registered User Join Date: Dec 2018 Location: Dallas, PA Posts: 551	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?

11-07-2020, 08:50 AM	#9
The Rocknrod Moderator Join Date: Aug 2011 Location: China Spring, TX Posts: 7,280	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? __________________ Master Chief, US Navy, Retired 1964 C10 Step-side Short Bed - In Progress 2009 2500HD Duramax LTZ 1948 GMC Panel 1956 Big Window Stepside 1953 5 Window Chevy Stepside 1932 Coupe New Seat Foam for Bench Seats?

11-07-2020, 09:45 AM	#11
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,731	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?

11-07-2020, 09:59 AM	#13
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,731	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.

11-07-2020, 10:07 AM	#14
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,731	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

11-27-2022, 01:17 AM	#15
guyryan100 Registered User Join Date: Sep 2001 Location: Cumming, GA Posts: 483	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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