TheSamba.com :: View topic - What is this? (something on a 34 PICT-4)

Sat Dec 26, 2009 3:25 pm

Merry Christmas, friends!

I have a 76 German vert with a SOLEX 34 PICT-4 carburetor, and I'm just doing some refurbishing of the carb. Been studying how it works, but one thing I can't find anything about:

On the bottom corner of fuel chamber there is some sort of valve which is connected to a vacuum hose up to the air filter.

What is is? What does it do?

Can I remove it or disable it? (... the standard 34 PICT-3 doesn't have it)

Thanks!

Mon Dec 28, 2009 12:25 pm

Bump, anyone?

Mon Dec 28, 2009 12:38 pm

I'll bet keifernet knows!!!! :D He knows all that stuff...

Mon Dec 28, 2009 1:03 pm

Well I don't see many of these 34 pict -4 style carbs. I personally have never had one in my hands that had that item on it.

I can't be sure but I think it's either a device that controls the amount the accelerator pump squirts or some type of an altitude compensator/auto adjust for main jet mixture.

Mon Dec 28, 2009 2:40 pm

I believe the 34-4 had a temperature controlled device for fuel metering. I've never actually seen one either but have heard about it. I assume it ties into the vacuum line out of the warm air switch on the air cleaner?

Tue Dec 29, 2009 9:36 am

keifernet wrote: Well I don't see many of these 34 pict -4 style carbs. I personally have never had one in my hands that had that item on it.

I can't be sure but I think it's either a device that controls the amount the accelerator pump squirts or some type of an altitude compensator/auto adjust for main jet mixture.

Thanks Keifer!

I've read in the Bentley maunual that the PICT 4 has a temperature regulator for the accelerator pump - maybe the connection to the air filter temperature device from this "thing" has something to do with it!

Hmmm...

Tue Dec 29, 2009 9:39 am

DrDarby wrote: I believe the 34-4 had a temperature controlled device for fuel metering. I've never actually seen one either but have heard about it. I assume it ties into the vacuum line out of the warm air switch on the air cleaner?

Thanks, Darby.

I'll look again at the connections. My air filter has 4 inlets for vacuum lines (unlike the other ones that have 2). I'll check if the "thing" is connected correctly to the air filter.

Wed Dec 30, 2009 3:24 pm

Hi - here are some more pictures.

My question is: How does this device work?

First this is the little thing - a small vacuum operated valve (when you suck, it closes):

It fits into a hole in the bottom left corner of the fuel reservoir:

The vacuum comes from the air filter box:

And there is a channel from the valve to the accellerator pump:

The channel ends up in the accelerator pump (the little brass hole):

So - what do you think?

Wed Dec 30, 2009 4:53 pm

My guess- a "two speed" accelerator pump that has a larger discharge volume when the air is cold and the flapper is closed. GM used "two speed" accelerator pumps in some of their Quadrajets on GM trucks in the '80s, electrically controlled. They had an electrical plug on top of the float bowl that mimicked the later CCC electronic carbs

Good Luck!

Wed Dec 30, 2009 5:30 pm

Bruce Amacker wrote: My guess- a "two speed" accelerator pump that has a larger discharge volume when the air is cold and the flapper is closed.

Makes sense... I guess I have to re-think how the hoses are connected. I'm not sure that the setup I have now is the correct one.

Thu Dec 31, 2009 7:52 am

It looks like you've got a vacuum feed coming from the rear of the carb that "tees" and feeds both temp senders. Each temp sender controls one output- the left one controls the accel pump, and the right one controls the thermostatic door in the air cleaner.

It appears you have the vacuum hoses connected correctly, but it is possible to have them reversed at the temp sender(s) themselves. Using a vacuum gauge on the outlet hose should show vacuum on a cold start up with the engine running. Remove the hose from the blend door and insert a vacuum gauge into it, likewise remove the vacuum hose at the accel pump controller and insert a vacuum gauge there as well. You should read vacuum at an idle in either situation. My Ghia has the single blend door setup and my hoses were reversed, not allowing vacuum to reach the blend door. It makes a huge difference in how the car runs when the blend door is in the heated position in the winter. Does the car have any hesitation or stumbling on acceleration? This should correct that problem.

Good Luck!

Fri Jan 01, 2010 3:42 am

Thank you for your comprehensive reply, Bruce! I must admit to you that I do not understand all details on how this works, though. Please be patient with me… ;-).

Some questions for clarification:
- When you say “temp sender(s)”, what is/where is that, actually?
- “Outlet hose”, is that the one that goes from “E” to “D”?
- “Blend door” is that “E”?

Good idea to get a vacuum gauge, will do that. If I understand it correctly, vacuum suction appears at “2” when the car is running. When the temperature is warm, there is something at “E” that controls vacuum to the system (cold=closed, warm=open?).

Again, thanks for the help!

Fri Jan 01, 2010 9:59 am

Boble wrote: Thank you for your comprehensive reply, Bruce! I must admit to you that I do not understand all details on how this works, though. Please be patient with me… ;-).

Some questions for clarification:
- When you say “temp sender(s)”, what is/where is that, actually?

Perhaps I should have called them temperature controlled valves, which are the AB and CD things in the air cleaner.

- “Outlet hose”, is that the one that goes from “E” to “D”?

Correct, but I'd call it going from D to E to show the direction of control.

- “Blend door” is that “E”?

Yes.

Good idea to get a vacuum gauge, will do that. If I understand it correctly, vacuum suction appears at “2” when the car is running. When the temperature is warm, there is something at “E” that controls vacuum to the system (cold=closed, warm=open?).

Again, thanks for the help!

"2" is your vacuum source, or the "start" of the system. "2" feeds vacuum to the temperature controlled valves, AB and CD. The temp valves sense the temperature of the air inside the air cleaner housing. When the air is warm, they do NOT feed vacuum to their servos. The servos are the blend door (E) and the (remember, I'm guessing) "two speed accelerator pump", (1). When the air is cold, the temperature controlled valves feed vacuum to their respective servos. Servo "E" has a small vacuum servo (AKA vacuum motor, vacuum pod, diaphragm, flapper, controller) that is hooked to a small blend door in the air cleaner snorkel. With vacuum applied, it closes the flapper door and allows warm air to be fed to the carb. Warm air in the carb is very important for proper operation in cool climates.

(Sidebar- fuel droplets will not burn, causing severe hesitation, stalling, and massive HC emissions. Fuel MUST be changed from the droplets a carb makes into a vapor for it to burn properly. Vaporization is incredibly important to proper operation of a carbureted engine, and several things must exist for vaporization to be complete- vacuum and heat are two of the most important. That is why VW guys are so anal about having the carb heat crossover working under the intake manifold, and why it helps considerably to have the intake heater flap working correctly in the air cleaner. I built a 2180 with a 32NDIX and the stock air cleaner so I would have the warm air feature functional.)

I'm a little surprised that VW used two vacuum valves to control these two outputs. It would have been much easier (and cheaper) to "tee" the two outputs from a single vacuum valve. This leads me to believe the two vacuum valves operate at two different temperatures. On carbureted American V8s in the '70s-80s, if it were under about 50F the flapper would be closed feeding warm air to the carb, and if it were not working correctly, the car would hesitate and possibly stall on acceleration. It was very common for any of the components to fail or the sheetmetal shroud on the exhaust manifold to rot and fall off, with the driver complaining of a hesitation. This all changed with EFI, but that's a complete separate lesson. :)

I assume this system is not covered in your factory service manual?

Possible problems you may have:

-Hoses at A and B are reversed, not allowing vacuum to be fed to the servo.

-Hoses at C and D are reversed, not allowing vacuum to be fed to the servo.

-The left temp valve should be hooked to the warm air flapper, instead of the accel pump.

-The right temp valve should be hooked to the accel pump, instead of the warm air flapper.

To check the system:
Remove the vacuum hose at 1, it should show vacuum when the car is idling on a cold day.

Remove the hose at E, it should show vacuum at the hose when the car is idling on a cold day.

If the warm air flapper were to be closed on a hot summer day, it would contribute to vapor lock. If the "two speed accelerator pump" were to have vacuum fed to it on a hot day, it would contribute to higher emissions and slightly lower fuel economy.

Good Luck, and Happy New Year!

Fri Jan 01, 2010 1:10 pm

Bruce, thanks a lot! Great to have such knowledgeable persons around who are willing to spend time and energy sharing with the rest of the VW community!

Your explanations here definitely makes sense, and I finally understand how this vacuum hose arrangement is intended to work! As you indicate, neither Bentley, John Muir, nor the Haynes manuals cover this subject.

I'll get myself a vacuum gauge (or rather, I'll make myself one from a clear plastic hose filled with fluid) to check out the system on my engine.

Thanks!

Boble

Sat Jan 02, 2010 8:49 am

I forgot to mention another quick easy check. After running/driving for several minutes on a cool day, the air cleaner snorkel (inboard of the flapper) should be warm to the touch. With an IR gun, mine measures 95F to 110F on a 20F day. The air cleaner housing is a bit cooler for some reason, only measuring in the 50s to 70F. Put your hand on it, and it should feel warm to the touch after running a while.

I'd fart around trying to "figger out" the "2SAP", too. Look down the carb throat with it idling and see if there's a difference visually in the accel pump discharge volume with the vac hose on/off, or with/without vacuum. You "might" be able to tell the difference of volume, which would help you understand the system a bit better. There's a slight chance it could work backwards, too- giving a larger volume with no vacuum and a smaller volume with vacuum.

Good Luck, and Have Fun!

:ghia_red:

Tue Nov 02, 2010 1:32 pm

Boble wrote: Merry Christmas, friends!

I have a 76 German vert with a SOLEX 34 PICT-4 carburetor, and I'm just doing some refurbishing of the carb. Been studying how it works, but one thing I can't find anything about:

On the bottom corner of fuel chamber there is some sort of valve which is connected to a vacuum hose up to the air filter.

What is is? What does it do?

Can I remove it or disable it? (... the standard 34 PICT-3 doesn't have it)

Thanks!

That is CLEAN - I just finished cleaning mine and now it looks dirty compared to yours. How'd you get it so brand-spanking-new looking?

Tue Nov 02, 2010 1:47 pm

Ok My standard reply on this type of thing is dont start me to telling lie's ?
But I have a theory ?
Excess from the squirt is siphoned off into a EGR valve or manifold, maybe the air cleaner if you lett off the pedal suddenly and in doing so closes the throttle plate abruptly ?
I have a 34 pict -4 camm and wheel carb but doesnt have that on it ?
Good post as Ive never seen one also .
Sean

Tue Nov 02, 2010 3:10 pm

emu88 wrote:
That is CLEAN - I just finished cleaning mine and now it looks dirty compared to yours. How'd you get it so brand-spanking-new looking?

Hehe. Cleaning it in solvent, i.e., diesel or kerosene, removes most of it, then this:

My wife didn't like that too much, but I survived!

;-)

Tue Nov 02, 2010 3:41 pm

hahah I'm gonna try that!!! Must remember to take all the small bits out..

Thu Nov 04, 2010 6:23 pm

Boble wrote: emu88 wrote:
That is CLEAN - I just finished cleaning mine and now it looks dirty compared to yours. How'd you get it so brand-spanking-new looking?

Hehe. Cleaning it in solvent, i.e., diesel or kerosene, removes most of it, then this:

My wife didn't like that too much, but I survived!

;-)

Oh man!... :shock: I would have got my ass kicked!
I have a 34 pict 4 on my workbench just sitting there. Mine needs throttle bushings.