Primer on throttle positioners from The Samba

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asiab3
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Primer on throttle positioners from The Samba

Post by asiab3 » Sat Oct 31, 2015 1:05 pm

I wrote this and cross-posted it on The Samba. Feel free to ignore, comment, or point out errors. :blackeye:
---

“What is this thing? Do I need it?”
The throttle positioner. Yes.

Image
Photo courtesy of wcfvw69.

Background:

In 1967 (model year 1968 for our Volkswagens) the Federal Air Quality Act was enacted. In addition to allowing California to set its own stricter emissions regulations, due to its larger pollution base, it amended the 1963 Clean Air Act and focused on regional issues and stationary power produced emissions.

From www.arb.ca.gov:
Federal Air Quality Act of 1967 was enacted. It established a framework for defining "air quality control regions" based on meteorological and topographical factors of air pollution.
http://www.arb.ca.gov/html/brochure/history.htm - accessed 11/1/15

However, the 1967 act did not go far enough, as the public did not feel the pace of improvement was acceptable. Enter the Clean Air Act of 1970, (1971 model years for us,) which moved far more aggressively to limit emissions of mobile sources. (Remember, Volkswagens were still new then, so they were in fact mobile! …since no one was using them as chicken coops yet.) The Environmental Protection Agency was also founded this year, along with the holding of the first Earth Day, on April 22nd, 1970. What kick started this year’s movement, was the signing of the National Environmental Policy Act. This is considered by scholars to be the “environmental Magna Carta.”

Raw gasoline practically belches hydrocarbons, which can combine with other gases to create a majority of smog-based health hazards. I am not a chemist, so I will not attempt to explain the course of chemical evils, but just remember that more than 99% of pollutant gases are invisible.


What it does, and how it works:
The air required to run an internal combustion engine must flow through the carb in such a way that it draws in just enough fuel for a complete burn. However, we know that different engine conditions require different mixtures, so carburetor technology has changed over time to allow the proportional mixing of air and fuel when the engine requires it. When the engine does not require much air and fuel, like when you release the pedal and the engine returns to idle, carburetors are extremely ineffective and inaccurate at mixing air and fuel.

When a piston goes down and “sucks” in intake air and fuel in, it creates negative pressure in the intake known as “vacuum.” The level of vacuum is dependent on engine speed, (how fast the cylinders are pulling air in,) and throttle position, (how much air volume is allowed in due to suction.) Imagine that you are traveling at a high RPM on the freeway, and you let of the pedal immediately because you were hastily cut off by a Prius driver who looks at the fuel economy gauge instead of the speedometer. “But it says 65!” The pistons are sucking air mightily, and the throttle plate is trying to cut off airflow, so the vacuum, or suction level, in the intact tract is extremely high. This creates a sudden influx in fuel, because of the sudden increase in air suction. But your Volkswagen doesn’t need the fuel!

Enter the throttle positioner. By using a mechanical linkage to detect these spikes in intake vacuum, the throttle positioner actually holds your throttle arm and throttle plate open a tiny bit while the engine slows down. This reduces the fuel dumped needlessly through the engine, preventing raw gas from fizzling out your tailpipe.

The mechanical aspect of the linkage is simple. There is a vacuum diaphragm inside the positioner arm, and when your intake manifold vacuum is high enough, it pulls the arm. Just like a distributor vacuum advance can. The amount of vacuum produced by the engine depends on many factors, like age, wear, cam profile, and timing, so adjustments can be made. Inside the assembly underneath the brass screw is a bellows that can compensate for vacuum changes. Bentley calls this the “altitude adjuster,” as vacuum levels greatly change with “elevation.”

Differences in models:

1968-1969 model year bugs and buses came with the one piece style positioner, which includes the actuating arm/diaphragm and adjustment screw/bellows in one housing. The one-piece throttle positioner mounts directly underneath the carburetor, and requires an additional gasket to the intake manifold. There is only one vacuum port, and it receives full manifold vacuum from a nipple on the intake manifold.

Image


An example of a correctly functioning one-piece throttle positioner can be seen here, courtesy of wcfvw69:

[youtube]Https://www.youtube.com/watch?v=5y2FsKHqdKk[/youtube]


1970-1971 model year bugs and buses came with the two-piece style throttle positioner, which functions almost identically, but is split into two pieces. The two-piece unit has two vacuum hoses as well; one hose goes to an intake manifold vacuum port, but now the other hose goes to a new vacuum nipple on the 30pict3 and 34pict3 carburetors.

The actuating arm is still mounted to the carburetor, except now it looks like this:

Image


The adjusting part is now located on the left side of the engine bay, in both bugs and buses.

Image
Photo courtesy of busdaddy.

Image
Photo courtesy of celticbob.




What else it does/alternate names:
The throttle positioner can also be called the shift enhancer, smog device, ride smoother-outer, backfire eliminator, and my personal favorite, unnecessary. It is all but one of those things.

By slowing down the throttle plate return on a carburetor, the engine stays at a slightly higher RPM when you upshift. This creates a smoother transition into the next gear, creating greater comfort and pleasure for drivers and passengers alike, IF you care about such things. This also matches the transaxle input shaft speed more closely to that which is required for the next gear. This puts vastly less wear on your synchronizers in the transaxle, and you SHOULD care about that.

We’ve already discussed how it can avoid dumping the gas out the tailpipe for emissions reasons, but does your car gurgle and fart on long downhills with the throttle shut? The throttle positioner purifies the air/fuel mixture on throttle overrun, so these little queefs of the car will be reduced or eliminated as well. Unless you have an exhaust leak. Get that shit fixed.


Installation and adjustment:

As a general rule, your car can use the one-piece throttle positioner if you have a vacuum port below the carburetor mounting flange on the intake manifold. As the second general rule, you can use the two piece throttle positioner if your carburetor has the angled vacuum port on the right side, (shown here in red,) AND a similar vacuum port on the intake.

Image


It is worth paying special attention to the mounting brackets, regardless of which type of throttle positioner you choose to install. The 34pic4 carburetor has a larger base flange, so it requires the 1971 one-year-only mounting bracket. All other carburetors interchange mounting brackets. In addition, both the one-piece and two-piece positioner use the same “ring mount” style, so you can mix and match parts to get a mount and a positioner that works with your setup.

With the one-piece positioner, the factory provided a vacuum port just below the carburetor flange for your hose to reach. Since parts get swapped and all three of us that care about throttle positioners can’t be evangelical enough, your vacuum port may be on the left side of the intake down pipe, or on the far left by the cylinder head part of the manifold.

With the two-piece positioner, the angled carburetor port is always near the diaphragm connection, and the factory vacuum port for the second connection is on the left side of single port manifolds, and cast into the #3&4 end piece of the dual port intake manifold. Regardless of where your hoses end up, notice how the two ports are of different sizes? Standard distributor vacuum hose works for the smaller size, and 5mm (fuel hose size) vacuum hose works for the larger connection.

Adjusting the throttle positioner is detailed quite thoroughly in Bentley. Whaaaat you thought I was going to spoon feed it to you? HA! I will say your carburetor needs the secondary throttle arm to attach the mechanism to; without it you're out of luck.

Image


Final thoughts:
Your car will run without the throttle positioner. Most take it off and throw it away, but some sell them to me for a couple bucks. I have enough spare to last my lifetime, so now it is time for you, good reader, to peruse the classifieds and junk yards of the world and get your engine working like it is supposed to.

wcfvw69 has a thorough article on rebuilding the one-piece positioner. The two-piece positioner will be rebuilt the same. Usually, I see two-piece positioners need nothing but a cleaning and one-piece positioners needing a diaphragm of some kind. Nitrile sheet gasket would be ok, but viton would be best for recreating the diaphragms using the old ripped ones as templates.

http://www.thesamba.com/vw/forum/viewtopic.php?t=637886


Remember how I said the two types function almost identically? Since I am feeling tired of typing, I will offer a prize to anybody who can tell me WHY Volkswagen went to a two-piece system. Answer in complete sentences, and perhaps explain the major functional improvement please. :P

Love and good air quality,
Robbie
1969 bus, "Buddy."
145k miles with me.
322k miles on Earth.

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Amskeptic
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Re: Primer on throttle positioners from The Samba

Post by Amskeptic » Sat Oct 31, 2015 7:15 pm

asiab3 wrote:WHY Volkswagen went to a two-piece system. major functional improvement
I am feeling tired
Robbie
Single vacuum hose system used atmosphere to bleed down, ambient contaminated air.
Double vacuum hose system with the remote positioner used already filtered air within the carburetor.

Now, go research this supposition. Look at the cross section of the single positioner and see where the vacuum release occurs.
Colin-I'MTIREDTOO.
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

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asiab3
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Re: Primer on throttle positioners from The Samba

Post by asiab3 » Sat Nov 07, 2015 8:42 am

Amskeptic wrote:
asiab3 wrote:WHY Volkswagen went to a two-piece system. major functional improvement
I am feeling tired
Robbie
Single vacuum hose system used atmosphere to bleed down, ambient contaminated air.
Double vacuum hose system with the remote positioner used already filtered air within the carburetor.

Now, go research this supposition. Look at the cross section of the single positioner and see where the vacuum release occurs.
Colin-I'MTIREDTOO.
Yes yes, but there's also a vacuum-related functional difference.
...Visible in Bill's video, too! :thumbleft:

VW knew about the air contamination too, when they put the foam/spongy piece in the one-piece diaphragm vent hold.

Those smartypantses.
Robbie
1969 bus, "Buddy."
145k miles with me.
322k miles on Earth.

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Elwood
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Re: Primer on throttle positioners from The Samba

Post by Elwood » Mon Jan 04, 2016 8:02 pm

Hi Robbie
This may seem like a indirect way to reach you but it is all I gotssss Im a computer nerd ans also the brain damage from tumor certainly does not help. anyway onward-----

Your email before christmas Made me Joyas and all the above and below feeelings I was having that timeTHank You

I would like to share many more adventures Ive had with my Elwood , still my only car and many thanks to Colin for his help and support.

Peace and Lots of Love,

Barb and El
'69 weekender ~ Elwood

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asiab3
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Re: Primer on throttle positioners from The Samba

Post by asiab3 » Tue Jan 05, 2016 1:12 am

Barb, so good to see you here again! Glad you got the message, I hope you're staying warm this season.

Still enjoying everything I said, and more,
Robbie and Buddy
1969 bus, "Buddy."
145k miles with me.
322k miles on Earth.

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Amskeptic
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Re: Primer on throttle positioners from The Samba

Post by Amskeptic » Tue Jan 05, 2016 7:04 am

Elwood wrote:Hi Robbie
This may seem like a indirect way to reach you but it is all I gotssss Im a computer nerd ans also the brain damage from tumor certainly does not help. anyway onward-----

Your email before christmas Made me Joyas and all the above and below feeelings I was having that timeTHank You

I would like to share many more adventures Ive had with my Elwood , still my only car and many thanks to Colin for his help and support.

Peace and Lots of Love,

Barb and El
Merry Christmas and Happy New Year to you, Barb!

I have been remiss in my communication with you but still love the photograph of our outing to the reflectorized windows at the pizza joint. Looking forward to Lunch With Barb 2016!
Colin
(
Elwood wrote:I would like to share many more adventures Ive had with my Elwood
well GET ON IT, TIME's A' WASTING :cyclopsani: )
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

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Amskeptic
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Re: Primer on throttle positioners from The Samba

Post by Amskeptic » Tue Jan 05, 2016 7:08 am

asiab3 wrote:there's also a vacuum-related functional difference.
...Visible in Bill's video, too! :thumbleft:
Robbie
Use words to describe the functional difference you speak of.
I know of no functional difference.

I'd also like the opportunity to edit your Primer after your reply to the above.
Colin
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

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asiab3
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Re: Primer on throttle positioners from The Samba

Post by asiab3 » Tue Jan 05, 2016 10:41 am

Amskeptic wrote:
asiab3 wrote:there's also a vacuum-related functional difference.
...Visible in Bill's video, too! :thumbleft:
Robbie
Use words to describe the functional difference you speak of.
I know of no functional difference.

I'd also like the opportunity to edit your Primer after your reply to the above.
Colin
My biggest observation this far, is the split second it takes to build "enough" manifold vacuum during throttle closing of the one-piece positioner. You can see this in wcfvw69's video, where the instant he lets the throttle shut, it snaps 100% closed for a click, then the positioner holds the throttle open the correct amount of time. Pay attention or you'll miss it!

You'll see that a working two-piece positioner does not have this delay. I think this is a key moment in cleaning up tailpipe emissions; especially if you consider how quick you'll let off the throttle if you get cut off by a Prius merging onto the freeway at 47mph. The two-piece positioner uses a modified ported vacuum nipple to slow the throttle arm until manifold vacuum is enough to trigger the diaphragm and secure the arm in the open position. The relatively slower closing speed of the two-piece setup might help smooth the transition back to idle. (Handy if you have a DVDA that wants to grab the timing and suck it back quick if the throttle lever closes.)

The difference is not noticeable on the road, and my LM-1 doesn't react fast enough to show a spike in either direction. But I do think this is a designed-in difference.

Robbie
1969 bus, "Buddy."
145k miles with me.
322k miles on Earth.

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Amskeptic
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Re: Primer on throttle positioners from The Samba

Post by Amskeptic » Wed Jan 06, 2016 9:16 am

asiab3 wrote: the split second it takes to build "enough" manifold vacuum during throttle closing of the one-piece positioner.
a working two-piece positioner does not have this delay.
But I do think this is a designed-in difference.
Robbie
It is not a designed-in difference, I promise.

All of these positioners are adjustable in three areas,
a)pull rod length
b) pull rod stop
c) vacuum bleed down rate.

Deviations amongst the three parameters can give you the difference you observed.
I have a tenacious throttle positioner that pulls the pull the pull rod any time the engine is above about 1,800 rpm. I have adjusted to be sensitive, but I limit its effectiveness with a tightly limited pull rod travel. If you have adjusted the bleed down rate to be quicker, then it defaults to rest position most of the time and it waits for high rpm throttle closing vacuum. I think that gives you the "Bill effect".

While here, let's tighten up the reason for the throttle positioner. It was introduced on the 1968 model year Volkswagens. It was hydrocarbon reducer technology introduced AFTER the closed crankcase ventilation hydrocarbon gains had been realized. Its purpose was to prevent throttle-off high rpm fuel dumps into the carburetor. It was only so blindingly complex with that expensive aneroid bellow ( if a simple dashpot would do the same trick, why this?) so that the throttle closing would be consistently delayed at any elevation. Only after the introduction of the bypass carburetor with the vacuum retard at idle in the 1971 model year was VW able to pass emissions with a simple dashpot, and damn if all carbureted VWs didn't have that simple dashpot henceforth.
Colinforthwith
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

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