Decel Valve and air plenum styles

[Paul914]

I'm putting a 72 1.7 back together, and converting it back to fuel inj from a single carb .. Unfortunately (or fortunately) my other two cars are 2.0's, so I dont have a working model to compare some connections to. So here are my questions about the vac connections .. first off, when going through my 1.7 injection stuff, I found two different style "air plenums" (air intake distributor) .. the first has only one hose connection by the temp sensor in the plenum - I've established that it is for the pressure sensor. The other plenum has two extra hose connections .. one for the decel valve and one that goes to the oil filler/neck (I think) ..

So my question is this: I've heard that the decel valve is purely an emmissions device - is that correct? Being from NJ (where they only test emmissions at idle), it shouldn't hinder passing inspection, no? What other effects (performance?) does removingthe decel valve have? Also - if I WERE to remove the decel valve, which air plenum should I use?

Right now, I have the decel valve installed and the second of the two plenums (above) installed .. but I would rather eliminate it and go the simple set up - less to go wrong, less places for vac leaks, etc .. But the biggest question I have is if I remove the decel valve and use the first plenum (with the 2 less hose connections), where do I connect the hose coming from the oil filler??

Sorry this was so long .. let me know if you can help!

-Paul

Oh yeah - one more thing .. right now, the rubber 3 way connector between the aux air valve and the air plenum (where the small 5mm line from the decel valve connects) is kind of beat up - thats what inspired possible removal of decel .. can I just plug the small decel valve line on a perm basis (and simply use a piece of 13mm hose between aux air valve and plenum instead)??? Jeez .. who'd of thought 2.0's were easier to deal with than 1.7's ..

[This message has been edited by Paul914.]

[This message has been edited by Paul914.]

[JP Noonan]

I have a 72 1.7L that came from a complete/original (but rusty) perts car. I put a mixture of 2.0L and 1.7L FI on the car to get it running. First off the oil filler tube (13mm or 12mm) goes to the air filter housing BEFORE the thottle body. The large port on the manafold after the TB is for the aux air vavle, if you have the oil breather hooked up behind the TB I suspect you have a very high idle. I removed the decel valve and bloked off the lines, with no problems. I still have a high (1200) idle, I think it is because the TB is too big for the engine. Unfortunately I don't have the plenum if front of me, but I think that both of those extra ports are for the decel vavle (if it isn't for the Aux Air).

The main difference between 1.7 and 2.0 FI is the TB, the intake tubes, the head temp sensor, the calibration of the pressure sensor, and I think the TB position sensor opens the opposite way.

The other thing that hooks up to the air filter housing in front of the TB is the flashback vavle crankcase vent system. A hose runs from each head to the flashback valve (it is bolted to the fan housing on the opposite side from the dizzy) then to the air filter housing.

Like I said I have worked with so many mixed up systems I don't remember the 72 1.7 exactly. However if you just think of it as a closed system after the TB, in that all the lines after the TB "dead end" into a sensor diaphram, i.e. the vacuum advance canister on the dizzy dosen't leak, it is deformed by vacuum, but doesn't leak. Any thing that is open behind the TB raises the idle, i.e. the aux air valve is a vacuum leak to raise the idle until the engine warms up, then it closes.

One thing I was told many times, and I found to be very true, "Replace all the hoses and seals to eliminate vacuum leaks". The TB gasket is usally O.K. but cheap, the intake tube seals (the round rubber tubes) need to be new and have hose clamps on them, the small injector seals dry out and crack (also be careful not to pinch them), and aux air regulators often go bad.

Check out Dave Darlings 2.0L Vac hose diagram, and Kjell fuel injection article on the "914 fan's web site" www.914fan.net/

[Paul914]

Hmm .. either we're misunderstanding each other or we just disagree ..

First off, the oil filler neck DOES go to the plenum AFTER the throttle body (just next to the pressure sensor and 10mm decel valve connections and the temp sensor) .. look at page 40 of the Haynes manual, and you'll see it quite clearly. Also, my idle is not high with the hoses connected this way ..

Secondly, the aux air valve connection is on the OTHER side of the plenum (this is also where the SMALL 5mm decel valve line connects via a funny little "3 way rubber connector/boot") .. and it draws its air from the air cleaner (along with the remaining large connection of the decel valve) ..

This is all pretty clear in the Haynes diagram - I think you've got a 2.0/1.7 hybrid! The 2.0 is a little bit different ..

So just to summarize my original questions: 1. Can you remove the decel valve (or block it off) without sacrificing any performance (or emmissions AT IDLE?)
2. Is anyone familiar with the air plenum that does NOT have connections for the decel valve or oil filler?
3. If so, where do you route the oil filler hose?

I suppose I could connect it to the "Y-Connector" that usually connects the aux air valve/decel valve to the air cleaner .. hmm .. that might just be the answer if I decide to switch over to that. I just don't like the idea of having the other plenum (ala Haynes manual) with "blocked off lines" .. if I'm just going to block lines, I may as well use the right air plenum!

-Paul

BTW: The oil filler hose connects AFTER the tb on the 2.0 as well - this can be seen on DD's diag (and I can see it on my other cars )

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[JP Noonan]

I think I'll shut up for now.

Let me find my old 1.7L stuff (in a box somewhere) and I'll get back to you.

When I got my car it also came with a 2.0L bus motor. The FI on the bus was not complete put seemed to resemble a L-Jet (it had no pressure sensor, or throttle body sensor) but the intake looked like a 1.7L D-jet. I get back to you as soon as the egg washes off my face. Too many injections man, they are starting to look alike!

[Dave at Pelican Parts]

"1. Can you remove the decel valve (or block it off) without sacrificing any performance (or emmissions AT IDLE?)"

You should be able to. The decel valve seems to serve three main functions. First, it lowers the very high emissions levels that occur on "overrun". (Don't know why they do, the Probst book says something about "uneven cooling".)

Second, it helps to keep the revs from dropping WAY too low when you let in the clutch and take your foot off the gas. The car can stall from this, if the FI isn't in tip-top shape.

Third, but I'm not completely sure I buy this one; it keeps the manifold vaccuum low enough to not stress the diaphragm in the MPS. I'm not sure that too much vaccuum will actually damage the MPS diaphragm, though I suppose it is possible on old and weak ones. But some of the folks on the 914 mailing lists have said that it can happen.

Of course, the 70 and early-71 cars didn't have the decel valve at all.

You might try asking about hose routing on one of the e-mail lists. I know there's one guy on Tim's List (Paul H.) who has a 70 with original FI. He might be able to help.

--DD

[Dave at Pelican Parts]

Actually, the "overrun" condition is probably where the manifold sees the highest vaccuum (lowest pressure).

The definition of overrun is where the RPMs are up but the throttle is closed, right? So if you're at redline, the cylinders are "sucking" air from the manifold (5600*4/2) 11200 times per minute. That's a lot of vaccuum--probably more than at idle, where the manifold air gets "sucked" (950*4/2) 1900 times per minute.

What would the difference be? Probably not a huge amount in terms of pressure. But there would probably be some difference.

The MPS "sees" the manifold vaccuum through its small hose, and if the vaccuum is high enough, opens up a valve between the two larger hoses. That lets high-pressure (pre-throttle) air into the manifold, reducing the manfiold vaccuum at least somewhat.

Just a point in favor of the "save the MPS" argument. Again, I'm not sure I buy it, but some folks on the e-mail lists (particularly those who have had an MPS die on them) believe.

--DD

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