'83SC - Strange Decel Valve Behavior

[Racer Chris]

First let me say that I have read quite a few of the CIS troubleshooting topics on this forum and didn't find any info directly related to our problem.

When we started working on the car to improve engine running (several issues found and resolved along the way) we discovered the decel vacuum line was removed and left unplugged, creating a vacuum leak.
We reconnected the line to the valve and it seemed to work, although rpm decay to idle was fairly slow.

After a short time operating the car, the decel valve seemed to be sticking open most of the time and not allow the idle to go below 2000rpm.
We removed the valve and cleaned out some crud but that didn't affect performance. The valve itself functions properly.

What we did find during our tests is that the vacuum signature off the port its connected to isn't what we expected.
At idle the vac is 15", then it goes up to 20" as the throttle is opened, before dropping as the rpm continues to increase.
On overrun the vacuum climbed to about 25" and then dropped back to 20".
At that level of vacuum in the line, the decel valve stays open and raises the rpm to about 2K.
That rpm is enough to keep the vacuum at 20" and the decel valve won't close.
So it seems that the circuit is self defeating, never allowing the decel valve to finish its cycle.

We've inspected the connections thoroughly, and to the best of our ability we determined that all the connections are where they belong on the throttle body.
What could cause the vacuum to increase as the throttle is partially opened?
It doesn't seem that it should be that way to operate the decel valve.
We expected a normal manifold vacuum signature at the decel valve connection, just like on the ignition timing retard line.

Any help is appreciated. Thanks.

[kodioneill]

Possibly a plugged Cat converter. Also check you distributor advance / retard many of these old distributors have a issue with a gummed up advance plate.

[Racer Chris]

No Cat. The exhaust has been backdated.
The distrbutor advance plate was gummed up but that was already taken care of before we reconnected the decel valve.

[john walker's workshop]

plug the hose and forget it. 99% of SCs work fine without it. you have to squeeze it in a vise to alter it's function.

[Racer Chris]

Originally Posted by john walker's workshop

plug the hose and forget it. 99% of SCs work fine without it. you have to squeeze it in a vise to alter it's function.

Thats what we've done.
Interesting that squeezing it will alter the function. Conversely, if it saw a backfire could that expand it enough to alter the function negatively?

I'm still interested in understanding the odd vacuum signature, which is independent of the condition of the valve.

[Jared at Pelican Parts]

John is right here. I had the same problem on my old SC. Plugged the line and all was well.

[jbandbj914]

Mine is plugged and all is well!

[Joe Bob]

Decel valve is an add on smog device.....give it the float test.....

[Scott R]

I won't advise against whats been recommended about removing the thing. But I will offer an alternative. I have what is probably a museum piece CIS system, it's completely rebuilt and runs like a top. I really do rely on that decel valve to keep the RPM's from that sharp drop that can happen.

Now that being said, I have a box of these things and about half of them leak past the diaphragm, the other half probably stick. I have two brand new valves and they work perfectly. One is the one I run currently. So I have to wonder if they have a limited life span and when they get weak or sticky people just unplug them?

[Vincent Hill]

Originally Posted by Scott R

I won't advise against whats been recommended about removing the thing. But I will offer an alternative. I have what is probably a museum piece CIS system, it's completely rebuilt and runs like a top. I really do rely on that decel valve to keep the RPM's from that sharp drop that can happen.

Now that being said, I have a box of these things and about half of them leak past the diaphragm, the other half probably stick. I have two brand new valves and they work perfectly. One is the one I run currently. So I have to wonder if they have a limited life span and when they get weak or sticky people just unplug them?

Scott, I like your thinking! I worked with mine for a WHile and finally plugged it and everything worked perfectly after. But I am like the guy that started the thread, "If it is on there I want it to work correctly. Then seeing what you wrote made sense which for me means that I can buy a new one and get it perfect and then it will go bad again for something I do not need or have use for.