Vacuum Line to Decel Valve Blocked Off

[olafwagner]

I am in the process of cleaning all my vacuum lines, and when I tested them by blowing through them, I realized that one of them was totally blocked. (These are the vacuum lines that go from the decel valve, to both the intake manifold and fuel return valve on my 1978 Type 4 FI bus)

When I pushed in a fat piece of wire from the other side (and applied some pressure), suddenly this metal piece popped out. (the red box shows where it sat)

Based on the force it took to get it out, this was definately 'put' here by a PO. This is on the thin vacuum line that goes to the decel valve (the plug was close to the decel valve) (it was located at the 'red box' as shown in the image below)

Here is a pic from the microfiche showing the location:

So my questions are:
- I assume this is not stock from the factory, so why was this done? Does this 'disable' the decel valve entirely?
- Should I put it back or leave it out?
- I have noticed that when I drove downhill and let off the gas (in gear) I get some pops and bangs - which I assume is related?

Any advice here would be much appreciated.

[timvw7476]

Well I would vacuum check the decel unit to see if it is air-tight. The PO may have inserted that plug so as to appear visually 'emissions compliant' while running around with a failed diaphragm.
Not Public Enemy #1 but if that piece is NLA what does one do?
As far as the afterfire/popping, tons of rides from '72-'85 or so had graft-on exhaust particle burn devices attached to produce a slightly more complete tail pipe. We just put up with it or tore it off in the other 49 state. Big No No in CA.

[BDorsey]

I believe the decel valve leans out the mixture during deceleration to protect the cat. So, if it is not working, you could be going rich and that could be the cause of the pops.
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1982 2.0L Aircooled Country Homes Camper

[dodger tom]

[olafwagner]

Thanks for all the responses. I uninstalled the valve, and first cleaned of 45 years worth of gunk before testing the decel valve:

18 in.Hg: I can now start to blow air from A -> B (or conversely suck air out B from A)
20 in.Hg: I can easily blow air from A->B, but also a bit from B->A
22 in.Hg: I can easily blow air from A->B, but fairly easily from B->A.

Is my assumption correct that this is pretty much in-line with how it should perform?

I am planning on reinstating the decel valve properly, especially if that means it will extend the life of my cat.

[Nitramrebrab72]

[Nitramrebrab72]

The possible reason it was blocked off at that point would be the throttle body butterfly flap not closing properly with ease and speed so to compensate a trick is to block off this line and the now the sudden deceleration vacuum created will be enough to pull the butterfly flap fully home. Preventing erratic idle rpms.

[WhirledTraveller]

[raygreenwood]

[virusdoc]

I also have a disconnected small hose, and rotting large hoses, on my decel valve in the 76 manual FI I just purchased. Before hooking it back up I’d like to test it to make sure it’s not faulty. Is the test used by the OP above appropriate?
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1976 Convertible Super Beetle, "June Bug".
Self-rebuilt 1904
1976 Campmobile 2.0L FI, "Kermit"

[airschooled]

[virusdoc]

Thanks much, Robbie! I’ll get it plumbed up correctly then. I’m not having pops on deceleration, but I suspect I have other vacuum leaks and am running on the lean side all the time.

I’m thinking of getting GeeBee’s kit so I can call it one and done. It’s unclear from his advert whether the S boot is included or “available”. But my S boot looks newish.
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1976 Convertible Super Beetle, "June Bug".
Self-rebuilt 1904
1976 Campmobile 2.0L FI, "Kermit"

[SGKent]

the decel valve does not affect performance and it will eliminate a rich overrun situation. If you have ever been watching a race and see cars slowing for corners that emit large exhaust backfires and flames, that is the excess hot fuel being ignited when it hits extra oxygen. The decel valve gets rid of that. As long as the diaphragm is still intact and does not leak, it is good. The decel valve feeds clean filtered and metered air from the S boot to the plenum during deceleration. Whomever blocked that one earlier in this thread had no idea what they were doing.
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"Most people don't know what they're doing, and a lot of them are really good at it." - George Carlin

[airschooled]

The only "problem" I find with the valves is when engines have been modified or worn out past their original design intent, and the wrong valve is used.

There are two functional versions of the valves; one for SVDA and one for DVDA distributors. (There might be more part numbers, but they all fall into those two groups.) The SVDA engines run at stronger idle vacuum readings, and need a corresponding decel valve to know when the engine is slowing down.

If you use a DVDA valve (seen mostly on 79CA and Vanagons) on a SVDA/009 bus, the vacuum at warm idle will usually activate the decel valve and give you MUCH higher idle speeds. By this time, the mechanical advance comes out to play and before you know it you're at 3,600 RPM in the driveway.

Unfortunately, some people build and purchase engines that go above and/or below Volkswagens design expectations, and every little ingredient adds to the recipe…

So look for other modifications on this engine. Or other things that are broken to compensate.

Robbie
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One-on-one tech help for your vintage Volkswagen:

www.airschooled.com

https://www.patreon.com/airschooled

[virusdoc]

Part number on my valve is 022 133 551B. I think this is an SVDA part number based on what I can find.

This is on a 1976 French market chassis that has a "reconditioned" CJ donor engine (the UK registry lists the donor as a 1976 as well, but the full code is CJ532044 and that number is not in the 1976 date range--I suspect the serial number was transcribed incorrectly on the records I have from the PO).

The VW Engine Company of London claims the donor was out of a vehicle with 21k miles when swapped in 2018. I believe this engine should have had an SVDA carb, but now it is running an electronic ignition 009. That will be one of the first things I fix.
_________________
1976 Convertible Super Beetle, "June Bug".
Self-rebuilt 1904
1976 Campmobile 2.0L FI, "Kermit"

[germansupplyscott]

[raygreenwood]

[mikedjames]

Are you sure that piece of metal is not in fact a factory fitted restrictor for the decel valve, and has a small but dirty hole in it that should be clear ?
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Ancient vehicles and vessels

1974 VW T2 : Devon Eurovette camper with 1641 DP T1 engine, Progressive carb, full flow oil cooler, EDIS crank timed ignition.
Engine 1: 40k miles (rocker shaft clip fell off), Engine 2: 30k miles (rebuild, dropped valve). Engine 3: a JK Preservation Parts "new" engine, aluminium case: 26k miles: new top end.
Gearbox rebuild 2021 by Bears.

1979 Westerly GK24 24 foot racer/cruiser yacht Forethought of Gosport.
1973 wooden Pacer sailing dinghy

[SGKent]