Idle lowers when braking / coming to a stop

[zdude1967]

When stopping at a red light the idle lowers on my round top SU carb 260z.It takes anywhere from 3 to 15 seconds to increase by itself back up to 800/ 900 RPM. It lowers to about 500 RPM and as mentioned increases slowly back to regular idle speed. If I pull away from the light before the idle increases the car stammers. What is causing this, vacuum leak, vacuum advance issue or something else. How can I fix this. Some additional information the connectors on my carbon canister are broken so they are not connected very well- would this cause it....Thoughts , suggestions???

[Patcon]

Does it do it if you are parked and press the brakes?

My suspicion would be blown brake booster,

 

[dutchzcarguy]

8 hours ago, Patcon said:

My suspicion would be blown brake booster,

You would also hear a hissing noise in the footwell near your brake pedal.. push the pedal with your hand and listen with your ears near it..

13 hours ago, zdude1967 said:

Some additional information the connectors on my carbon canister are broken

Bad connections are always not good... get rid of them..

[zdude1967]

I will try the brake booster while parked tomorrow and get back to you. What do the connectors to the carbon canister do- as I mentioned they are kinda jacked up.

[dutchzcarguy]

9 hours ago, zdude1967 said:

What do the connectors to the carbon canister do-

As european cars don't have that junk on board  , i can't help you!  

[Dave WM]

19 hours ago, zdude1967 said:

I will try the brake booster while parked tomorrow and get back to you. What do the connectors to the carbon canister do- as I mentioned they are kinda jacked up.

the carbon canister is part of the emissions control system. It collects fuel vapors from the fuel tank, stores them in carbon, then when the engine is on, a ported vacuum signal opens the canister to vacuum into the intake manifold, thus burning the vapors that would have otherwise been dumped into the outside air. There is a filter element on the bottom of the canister that draws outside air into the carbon while sucking the fuel vapors. this outside air cleanses the carbon so it will be ready to accept excess fuel vapors that occur while the engine is off and the gasoline in the tank vaporizes.  You can check to make sure you have ported vacuum by feeling small vacuum hose and operating the throttle while the engine is running. the large line under the vacuum servo goes to the intake manifold, the other nipple on the canister goes to a rubber line that connects to a hard line that runs along side the fuel and brake lines to the rear of the car.

For the system to function as designed you should make sure the line back to the vapor canister in the rear quarter is not "stuffy" (plugged up). there will be some resistance to air flow as the rubber line that goes to the carbon canister has a restrictor and the there is a two way restrictor in the line near the rear of the car as well.

[zdude1967]

I got the Z out yesterday and have some updated info. At idle if you rev it the idle drops and takes awhile to recover as indicated above- so it does not have to be driving to do this. If you pump the brakes at a standstill the idle also drops not as severely- it recovers after a few seconds. I took the car out yesterday and it was colder out about 65 degrees - the last time I dialed in the cabs it was probably 90 ish. Anyway the idle issue was worse yesterday- when approaching a stoplight I put the car in neutral and rolled to the stoplight -it stalled, it also stalled a few other times when coming to a stop if I didn't work the accelerator pedal so it would not stall. . Don't know if the lower outside temp contributed to the idle drop being worse or if it is something else. As I mentioned one of my evap canister line connectors is broken so the line just hangs there connected to nothing. This connector traces back to the line from the distributor vacuum advance. It comes off the vacuum advance and breaks into two directions one going a few inches and connecting to a nipple on the front carburetor the other travels to the Evap canister.  BTW I have the Crane electronic ignition module and coil installed as well. Thoughts, suggestions, would replacing the brake booster help the idle dropping?

[HappyZ]

It definitely sounds like a vacuum leak.  I'm troubleshooting very similar symptoms, but mine appears to be the carb shafts.  Someone should be able to chime in, but I would guess you could eliminate the brake booster by plugging the booster hose to see if the problem goes away.

[Yarb]

Post is fairly old. @zdude1967Can you provide an update on your findings?

[Tweeds]

Just here to agree that it's probably the brake booster. HappyZ makes a good point - isolate the brake booster to see if it's the problem. I had a similar issue, except the car would often flat out stall when braking. Vice gripped the brake vacuum hose and no more stalling or bogging down, but those brakes more stiffer than all Hell! 

[HappyZ]

That's strange...It came up on my "Activity in Last 24 Hours". 

[zdude1967]

Thanks for the reply's. This post was from awhile ago. The issue was the brake booster hose had a leak it has since been repaired. 

 

Thanks Again everyone