Help me, I'm too stupid to figure this out, Decel Valve Options

[pbanders]

I have no idea of how this thing can do what it does. I understand WHAT it's supposed to do - limit the intake manifold vacuum during overrun conditions - I just can't see HOW it does it. I've never seen the inside of one.

The valve has three ports, "control" (the skinny one on the end), "manifold" (the fat one on the side), and "atmospheric" (the fat one on the other end). The control and manifold ports are both connected to the branch "tee" fitting on the intake manifold, so they see the same level of vacuum.

If I remove the decel valve and bench test it, when I pull a vacuum on the control port alone, at some vacuum level (which is adjustable), the valve opens and I can blow through the other ports. However, if I simulate what the valve sees in the engine - that is, the control port and the manifold port both at the same vacuum level, as I expected, the valve doesn't open, no matter what level of vacuum I pull. I expected this result, because there's no pressure differential across the ports to open the valve.

What am I missing here? Clearly, the valve works in a car. How?

[SirAndy]

What am I missing here? Clearly, the valve works in a car. How?

i'm not much help here, but i simply removed mine and plugged the hoses ...

i like the off-throttle response much better without the decel valve ...
(IMG:style_emoticons/default/bye1.gif) Andy

[pbanders]

What am I missing here? Clearly, the valve works in a car. How?

i'm not much help here, but i simply removed mine and plugged the hoses ...

i like the off-throttle response much better without the decel valve ...
(IMG:style_emoticons/default/bye1.gif) Andy

Yep, know all that - just want to know HOW this thing works, thanks.

[JoeSharp]

Brad: What I thought I learned from Paul Anders.
My vacume line is tee'd from the distrabutor line and the valve gets it's air from the air box and feeds it under the butterfly.
:PERMAGRIN: Joe

[Joe Bob]

Hi Brad...long time no talk....

A decel valve in MY old time memory....limits the speed revs going DOWN from WOT to idle or from freeway speed to onramp speed.....

[TravisNeff]

It sounds like the control (skinny) end would need to be above the throttle plate to work, no? How else can this valve work if there is no way it can tell if the throttle snaps shut??

[pbanders]

It sounds like the control (skinny) end would need to be above the throttle plate to work, no? How else can this valve work if there is no way it can tell if the throttle snaps shut??

Ah, someone who is beginning to understand my confusion. Actually, no, if the control line was connected above the throttle plate, it'd never see any significant vacuum under any throttle condition. When the throttle is WOT, the pressure in the air box is only a few mtorr below atmospheric.

The way the valve tells the throttle is snapped shut is that the manifold vacuum level abruptly increases. The decel valve is also known as a "vacuum limiter" - it limits the maximum vacuum in the manifold. And, I know that it does what it's designed to do, but I'll be damned if I can figure out HOW, given how it's designed and connected.

[pbanders]

Hi Brad...long time no talk....

A decel valve in MY old time memory....limits the speed revs going DOWN from WOT to idle or from freeway speed to onramp speed.....

Yup, that's what it does - it's HOW it does that, considering the way it's designed and connected, that I'm after.....

I figure someone like Krusty can explain it.....

[Joe Bob]

OK.....the valve is open when WOT or at sustained speeds...I'm thinking that the valve works when the revs go UP and then slowly closes due to it's internal limits on decel...?

[mightyohm]

Maybe there is a restriction in the control end, such that when the manifold is under vacuum, the control port vacuum internal to the valve lags, and holds the valve open slightly until the pressure across the valve equalizes?

You may not be seeing the correct behavior on the bench because you are not testing the valve dynamically.

[McMark]

I'll take a wild ass guess and say it has something to do with the pressure waves in the plenum. Pulling a vacuum on the decel valve doesn't really simulate real world conditions.

[mightyohm]

(IMG:style_emoticons/default/agree.gif) what he said

[pbanders]

I'll take a wild ass guess and say it has something to do with the pressure waves in the plenum. Pulling a vacuum on the decel valve doesn't really simulate real world conditions.

I agree that pulling the vac on the bench isn't the same situation. But how does it differ and why does that make the valve open?

As you can tell, this has bugged me for a while. I spent a couple of hours looking at Google and trying to find out an answer. Even poked around at the USPTO database. I'm sure the explanation is simple, I just don't see it.

[Joe Bob]

Try Google using....add on smog devices from the 70s.....or smog devices that didn't work and are NLA....or suckass smog devices....

There's a guy I argue with at symposiums from the Ca Air Resources Board that was instrumental in some of this stuff....he's older than dirt and really believed in this crud back in the day. He was behind the repeal of the rolling 30 year smog exemption in CA. I'm surprised that no one has ever done a drive by on his office....I'll look him up and PM ya....

[pbanders]

Maybe there is a restriction in the control end, such that when the manifold is under vacuum, the control port vacuum internal to the valve lags, and holds the valve open slightly until the pressure across the valve equalizes?

You may not be seeing the correct behavior on the bench because you are not testing the valve dynamically.

You mean like a metering port? That's an idea. I haven't seen any evidence of it, however. I'll check it out.

[pbanders]

Try Google using....add on smog devices from the 70s.....or smog devices that didn't work and are NLA....

I tried using about a million Google terms, including "explanations for dumb-shits" but all I got back was my photo.

Most everything I found told me what the valve did, which I know - not how.....

[TravisNeff]

Sounds like what Mark and Jeff are saying makes sense. Could it be as simple as the larger port will see waves faster than the smaller port, thus metering the valve back and forth over the atmosphere port?

We just need to find a junk one and cut it open.

[Matt Meyer]

FI Hose Diagram

I never had a decel valve.

But according to the above diagram the valve is purely a vacuum system. I thought it threw a switch to the FI brain.

The diagram shows the control (I think) attached to the reference (atmos) of the fuel pressure regulator.

[pbanders]

FI Hose Diagram

I never had a decel valve.

But according to the above diagram the valve is purely a vacuum system. I thought it threw a switch to the FI brain.

The diagram shows the control (I think) attached to the reference (atmos) of the fuel pressure regulator.

Yeah, there I go, always assuming D-Jetronic without saying so, sorry. I'm talking about the way it works in the D-Jetronic setup, not the L-Jet setup. You're right, it's totally different in the L-Jet, I have even less of an idea of what it's doing when I look at that diagram.

[pbanders]

Sounds like what Mark and Jeff are saying makes sense. Could it be as simple as the larger port will see waves faster than the smaller port, thus metering the valve back and forth over the atmosphere port?

We just need to find a junk one and cut it open.

Agree, that's what I want to do, I need to find a donor valve. I still don't think that the way it works is as complicated as being determined by pressure waves - I think it's probably really simple and I'm just missing it.

The other issue I have with the decel valve is: what's the right setting for it and what's the adjustment procedure? I know mine's been jiggered with (that's because I'm the jiggerer), and I'd like to set it back to the original setting. But the things have been NLA forever, so I don't have an OEM unit to compare mine to.

No wonder everyone rips it out. It's easier to do that.