So... While getting everything put together over the weekend, got to some collective head-scratching about how the decel valve actually does what it does. I know that it bleeds air back into the manifold on decel (high vacuum), to keep the revs from dropping too fast, but mechanically what is going on there? Because two of the three ports are plugged into the same port on the manifold.

-Josh2

Don't ask me to explain it, but this may help.

Hmm, then I'm confused. Based on that diagram DD's vac line drawing is in error. And I've been using that layout for as long as I've had the car.

Click to view attachment

If the pic Ferg posted is right, the middle port should be upstream of the throttle, and the two end ports should be tied together with the the manifold.

Or...am I reading this wrong?

-Josh2

I pulled the pick from Pbanders site...

http://members.rennlist.com/pbanders/DJetParts.htm

FYI I'll have my bone stock 2.0 at lunch this week, to take a look.

It adds extra air on deacceleration ... One for emissions control. Two to supposedly supress backfiring on deacceleration.

Engine will run without it.

That's all folks.

DDD's drawing is accurate. The little guy inside the unit just knows what to do and when to do it.....................................

The Cap'n

The center line/side port goes to the air cleaner. The control port and end port get the vacuum. The smaller control port controls when it opens to allow air in. Brad suggests 2 pounds over your idle vacuum. I just went through this with mine. I have it set to open fully at 18lbs. Works nice very nice now. If you have a vacuum gauge like a minivac, you can easily test it. blow into either the side or the end port, while increasing vacuum on the control port.

The Cap'n, mechanic to the CA blonds, speaks...

Rich

Actually...this from the factory 1973 S&T manual, where they were diagrammed... There are also factory testing instructions available.

My california L-jet 75 1.8 had this.

I removed it.

I get a nice popple during decel/lift in gear now.....

One more place for vacuum/air to leak from. Is/was it truly necessary?

As I understand, it helps smooth out the deceleration and air into the engine. The way I see it, if a team of engineers (I'm not an engineer) added it to the engine it should be left in the system.

I would like to know it's true function and reasoning to being there.

If it is to aid the crappy plenum/runner design then that means now that I deleted it I will have to get to work on megasquirting my 914 and making a custom plenum + larger t-body.


I mean once I eliminate that device and throw the balance off, I might as well go all the way.

I took it off, capped the small line, and used the big lines for the AC idle up solenoid.

Now I don't have to have the idle at 2K to keep it from dying with the AC on.


And for all of the left coasters.. NO, you cannot remove the AC in Texas.

i've heard it's real hard on your M.P.S. diaphram due to fast/high vacuum related to the fast engine decel. i would think it is o.k. under normal driving... at least thats my gamble until ii get it put on

edit: i mean"it's hard on m.p.s. NOT to have decel valve..."

My understanding this was for emissions control and euro spec cars did not have a decel valve. If I remember right.

That sounds like it could make sense. I know nothing of D-Jet map sensors however.... but I am willing to bet it is nothing like a modern sensor that can do vacuum to 45+ psi of positive pressure.

My L-Jet california car had one. But instead of a mechanical adjustment there was a vacuum port there.