Decel Valve...good or bad

[matt gineo]

I would like to buy a used Decel valve for my 76 2.7 CIS. The used one is $100 vs $400 for a new one and there are no new ones avail in the US right now. Is there any way to test if it's good or bad before installing it?

[tirwin]

Not a lot of time right now. Apologies for the rushed answer. The decel valve has a diaphragm in it. I'll bet there is a way to test by pulling a vacuum with a hand pump. You can get one from Harbor Freight. Now the question is what is "normal"?

Have you thought about just plugging it off and running without it? A lot of people, including myself, do just that.

[Bob Kontak]

Originally Posted by tirwin

Have you thought about just plugging it off and running without it? A lot of people, including myself, do just that.

+1 - me too.

[matt gineo]

Originally Posted by Bob Kontak

+1 - me too.

I would love to go without the decel valve but I'm not sure what the darn thing does other than being part of the CIS system. What does it do and is there a downside to running without it?

[Bob Kontak]

It bleeds air past/into the vacuum side of the throttle plate to allow your high rev deceleration to float down to 950 rpm's slower than "right now".

[tirwin]

From the CIS for Dummies thread:

The decel valve (called the "vacuum limiter" in the PET and also sometimes referred to as the "auxiliary air device") is the third component that can bypass air around the throttle. The purpose of the decel valve is to prevent an overly-rich condition when the throttle suddenly closes. The result is a slower throttle response. Some people do not like the slower throttle response so it is not uncommon to find the vac line connection disconnected and plugged, as well as the connection on the decel valve itself. The decel valve looks deceptively similar to the AAV. If the diaphragm in the decel valve leaks, it can cause a high idle.

I believe the history of the decel valve is that it was yet another component added to the CIS system to deal with the increasing smog requirements of the late '70s - early '80s. If you plug it off, you will get better throttle response and you might notice some exhaust burble when you take your foot off the gas. That's about it.

[matt gineo]

Originally Posted by tirwin

From the CIS for Dummies thread:

The decel valve (called the "vacuum limiter" in the PET and also sometimes referred to as the "auxiliary air device") is the third component that can bypass air around the throttle. The purpose of the decel valve is to prevent an overly-rich condition when the throttle suddenly closes. The result is a slower throttle response. Some people do not like the slower throttle response so it is not uncommon to find the vac line connection disconnected and plugged, as well as the connection on the decel valve itself. The decel valve looks deceptively similar to the AAV. If the diaphragm in the decel valve leaks, it can cause a high idle.

I believe the history of the decel valve is that it was yet another component added to the CIS system to deal with the increasing smog requirements of the late '70s - early '80s. If you plug it off, you will get better throttle response and you might notice some exhaust burble when you take your foot off the gas. That's about it.

Thanks Bob and Tirwin for the great input, you convinced me, I'm going to run without it!

[Bob Kontak]

Originally Posted by matt gineo

Thanks Bob and Tirwin for the great input, you convinced me, I'm going to run without it!

I am digging tirwin's input. (does that show my age)

I fully support the CIS for Dummies effort.

[matt gineo]

Originally Posted by bob kontak

i am digging tirwin's input. (does that show my age)

i fully support the cis for dummies effort.

+1

[tirwin]

Originally Posted by matt gineo

+1

Thanks, Bob and Matt!

Glad it helps. I'm working on more content as I can. Lord knows I struggled to get my brain wrapped around this stuff. I'm just trying to write things down in a way that makes sense to mere mortals like myself.

Bob, I am a child of the '70s so I dig it too!

[matt gineo]

Originally Posted by tirwin

thanks, bob and matt!

Glad it helps. I'm working on more content as i can. Lord knows i struggled to get my brain wrapped around this stuff. I'm just trying to write things down in a way that makes sense to mere mortals like myself.

Bob, i am a child of the '70s so i dig it too! :d

+1