31 PICT-4 - what's this valve?

[Volks Wagen]

Hi,

I'm finding it hard to see what this here is for.. It's a sort of multi piece spring loaded valve that vents into the bottom of the float chamber and seems to serve no purpose at the moment from what I can see. The whole thing is sitting behind a big hex bolt, so maybe it's just blocked off from use... It's difficult to find a 31 PICT-4 diagram, unless anyone out there has one. So, anyone know what this might do?

Sit's behind the big hex bolt bottom left:

Thanks,
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1973 1303 with AB-motor - sporadic
reconstruction as time permits, 1986 ex-Bundeswehr Doka - on the road again.

I'm definitely, probably, the worlds greatest lover.

Aithníonn ciaróg ciaróg eile.

[glutamodo]

I've never really messed with those carbs much, and the ones I have seen didn't look quite like yours. They looked rather like this. I think that stuff is the temperature compensation system for the accelerator pump. However, I've only ever seen the 34 version of it.

[richardc]

Power valve to provide extra fuel during heavy load

[Boble]

Yes, it's for the accelerator pump.

While a 34 PICT-3's accel pump squirts 1.6 ml when pressing the pedal hard, the 34 PICT-4 is temperature regulated: 1.7-1.9 ml on cold engine, 1.1-1.3 ml on warm engine = better fuel mixture

[Edit: The values here are quoted from Bentley book. I found a Technisches Blatt on the net that says that the values are a lot lower - see my post below]

I have seen the type Glutamodo shows in his post, and I have this one on my Europe-spec 76 model (vacuum operated, with a thermosttic valve up in the air filter box).

Your type is new to me, Richardc. Interresting.

[Volks Wagen]

Thanks for the info. I'll have a closer look at it and see how it reacts to temperature. I'll post more pics of the various pieces tomorrow, and confirm the base flange number - I believe I saw '390 1' on there.
_________________
1973 1303 with AB-motor - sporadic
reconstruction as time permits, 1986 ex-Bundeswehr Doka - on the road again.

I'm definitely, probably, the worlds greatest lover.

Aithníonn ciaróg ciaróg eile.

[glutamodo]

390-1 would have been 1973.

[Volks Wagen]

Right I did some 'research' based on your info and I think I understand how it works.

I've had the valve in the freezer and in hot water, and the stubby bit with the needle is definitely the thermo piece (actuator). The needle retracts in cold, extends in heat. Total range of pin is about 5mm. The needle seems to extend at about 20-30*C, but it's out further at about 50*C.

thermostat - stubby bit on left with needle in top

The larger assembly with the spring is the valve. This get's installed in a near fully compressed state, with one end of the non-threaded hollow shaft piece touching the thermostat needle and the other end of the shaft touching a spring-loaded-ball housed in the bigger threaded bit on the other end of the spring. The ball is pushed (opening the valve) by the hollow shaft when the thermostat needle extends as the carb body warms up.


And this is the bit that stumped me because I could not see what good it was doing to open the valve, cos it seemed to deadend in the valve chamber. I finally found by eyeing the lines of the carb and my photos, that this valve opens a channel between the bottom of the float chamber (red arrows) and the accelerator pump chamber. (the green arrows, lines, star - photos below, the blue arrow is the thermostat)

Based on the theory that this system reduces the amount of fuel squirted when the engine is warm, and that this channel is opened only when the carb is warm, I'm going to hazard a guess that some of the fuel in the accelerator pump chamber will now get squirted back up the opened channel into the float chamber, and the rest down the carb throat. When it's cold, the channel is closed and all the fuel gets squirted down the carb throat. I also suspect that the way the thermostat extends by degrees, and maybe the length of the notch on the springloaded shaft, that there may indeed be a temperature variable rate of fuel squirted down this channel.

Does that mode of operation sound right?

_________________
1973 1303 with AB-motor - sporadic
reconstruction as time permits, 1986 ex-Bundeswehr Doka - on the road again.

I'm definitely, probably, the worlds greatest lover.

Aithníonn ciaróg ciaróg eile.

[Boble]

Very interresting, and thanks for this great and thorough lecture. Makes sense all of it.

Wonder why they changed this valve? I guess they had some issues with it, since it reacts to the temperature in the carb itself, obviously heavily influenced by the temperature of the gasoline in the fuel chamber. The later type used the temperature in the air filter. Makes more sense...

Also I wonder about the motivation for it all. Up to 1975, the Solex 34 PICT 3's for the Superbeetle 1600 cc didn't have this temparture regulation of the accelerator pump.

Was the motiviation for it to reduce emissions only, or was it better engine performance?

[Volks Wagen]

[glutamodo]

I think it could have varied depending on what country or region you were in as well. The picture I posted above was from a USA 34PICT-4.

Here, I finally dug this up - I knew it was around somewhere. It's in German though: Technical Sheet K-80 from Nov 1972 and covers the 31PICT-4.

[Volks Wagen]

Wow! Fantastic Glutamodo, thank you. I understand German, so this confirms operation, and gives the full details. I'm now even more convinced that my '73 super 1285cc dualport AB engine and this 31 PICT-4 '390 1' model have been together for 40 years in this car! Of course it also says that one should not fiddle with the valve as it's factory set, but it's too late for that, and I'll just have to measure the squirt as per test thats outlined. They say that the squirt is up around 2.1cm3 on cold weather to improve the 'drivability'.
_________________
1973 1303 with AB-motor - sporadic
reconstruction as time permits, 1986 ex-Bundeswehr Doka - on the road again.

I'm definitely, probably, the worlds greatest lover.

Aithníonn ciaróg ciaróg eile.

[Boble]

Glutamodo, thanks! Great info!

Some more German technical sheets: Not for the 31 PICT-4, but the later 34 PICT-4 for the 1600 CC engine.

Interesting fact is that these engines were supposed to be tuned a lot leaner, with the following squirt values for the accelerator pump:

0.75 - 1.05 (above 29 Celsius/84 Fahrenheit)
1.25 - 15.5 (below 21 Celsius/70 Fahrenheit)

Guess VW was trying to please some tough emission requirements in the mid seventies. The earlier 31 PICT carburetor had max value 2.3 ...

[/img]

[Multi69s]

Cool info.

Yeah it was the emission requirements of the 70s that were the final nail in the VW coffin.
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[glutamodo]

Ah, the tech bulletins from the Workshop manual on the VW-Classic website. You left out the page with the info on the dashpot and the dual thermostat vacuum valves though. I had previously gone and excerpted and translated part of that page though:

And I just re-discovered where there was some more vintage German documents about the PICT-4. Instead of loading this page up with a bunch of additional huge size images, I'll post a link - look at pages 19 to 23:

http://vwbus2.dyndns.org/bulli/michaelk/vw_bus_d/solex30-34PICT3-4/index.html

-Andy

[Boble]

Wow, great, Andy!

[herbie1200]

Great post!

Let me add something to this.

Unfortunately I was not able to unscrew the big valve from carburetor, it is too tigh and I ruined the threading.

How to check if the whole assembly works and eliminating the doubt that the accelerator pump sprays always at its maximum?

I found this way: just at the bottom of the carb there is a small screw, circled in red in the image borrowed from Volks Wagen.

That screw if removed exposes the volume between the accelerator pump chamber and the big valve.

So the check is simple.
At first fill the carb with gasoline or other fluid less aggressive.

Then remove the little screw.

Test n.1
- no gas should drip off.
- with a termal gun or similar (not a torch) apply some warm to the thermostatic unit, following the blue arrow.
- when the thermostat is warmed enough it opens the big valve so the gas should exit from the hole, see red drops in the picture.
- The path is the red curved arrow.

Test n.2 (The other half of the channel)
- carb filled
- act on the accelerator pump
- also in this case (no matter for the temperature) some drops of gas should exit from the hole
- this indicates that the path from diaphragm pump and the hole is clear.

[FreeBug]

Like those writings probably explain, it was an economy-boosting/ emissions-reducing gadget to modulate the accelerator pump volume. You see them a lot here, and on type 4 twin PDSITs. They also used a cam system to operate the accelerator pump, but I don't know if that was before or after, or on the same carbs...

To measure accelerator pump volume, I do it like this. Zero out a sensitive scale with a large enough tray to catch the spray, and then pump the carb accelerator 10 times. Give it a second between squirts for the pump to have time to re-fill completely.

I roughly calculate with gas at 3/4 the density of water, and 1cc water=1 gram.

[TK-CS]

[glutamodo]

Only one year and one state, California 1974.
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"What is Glutamodo, Horror of Toyko, besides total, fiendish, quality-a-you brain!"

[vamram]

I just got one from Volkzbitz for my '73 1600DP:

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