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airschooled
Air-Schooled
Joined: April 04, 2012
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Location: West Coast, USA
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 Posted: Sat Oct 31, 2015 1:31 pm Post subject: Introduction to throttle positioner and altitude adjuster! |
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“What is this thing? Do I need it?”
The throttle positioner. Yes.
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:
| Quote: |
| 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.
An example of a correctly functioning one-piece throttle positioner can be seen here, courtesy of wcfvw69:
Link
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:
The adjusting part is now located on the left side of the engine bay, in both bugs and buses.
Photo courtesy of busdaddy.
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.
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.
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. 
Love and good air quality,
Robbie
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airschooled
Air-Schooled
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Location: West Coast, USA
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airschooled
Air-Schooled
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Location: West Coast, USA
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vectorsprint
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Joined: December 29, 2011
Posts: 36
Location: Beaverton, Oregon
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| Posted: Fri Nov 13, 2015 5:41 pm Post subject: Re: Introduction to throttle positioner and altitude adjuste |
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| Neat stuff. You mentioned 68-69, and 70-71, I've got a 74 super; would mine have come from factory with one of these? And if not, would it see much benefit from it? I ask cause I'm the one bug owner who doesn't have to smog it, but still hooked the EGR back up because I want the best fuel efficiency possible. |
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Cali_Army_Guy
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Joined: March 19, 2015
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Location: Stockton, CA
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| Posted: Fri Nov 13, 2015 6:47 pm Post subject: Re: Introduction to throttle positioner and altitude adjuste |
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| Hmm. My '71 super has the piece on the carb that is supposed to have the two vacuum lines. They aren't there and the nipple on the carb is plugged. The other piece is gone altogether. Will hooking up the piece on the carb make the car run better? Seems to run fine without it being hooked up. |
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Juanito84
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Joined: March 17, 2012
Posts: 2436
Location: Colorado Mountains
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| Posted: Fri Nov 13, 2015 6:50 pm Post subject: Re: Introduction to throttle positioner and altitude adjuste |
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| vectorsprint wrote: |
| Neat stuff. You mentioned 68-69, and 70-71, I've got a 74 super; would mine have come from factory with one of these? And if not, would it see much benefit from it? I ask cause I'm the one bug owner who doesn't have to smog it, but still hooked the EGR back up because I want the best fuel efficiency possible. |
Yours should have a dashpot. It also slows the closing of the throttle. But it doesn't keep the throttle open "engine braking". But it is still beneficial because it helps avoid going rich when you let off the accelerator pedal.
I don't think you can have both a throttle positioner and an EGR valve. They go in the same place, so it's either one or the other or you'll have to do some modifications.
The dashpot:
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If a water cooled engine cools its water with air, isn't it just an overcomplicated air cooled engine? |
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Juanito84
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Joined: March 17, 2012
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| Posted: Fri Nov 13, 2015 7:15 pm Post subject: Re: Introduction to throttle positioner and altitude adjuste |
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| Cali_Army_Guy wrote: |
| Hmm. My '71 super has the piece on the carb that is supposed to have the two vacuum lines. They aren't there and the nipple on the carb is plugged. The other piece is gone altogether. Will hooking up the piece on the carb make the car run better? Seems to run fine without it being hooked up. |
You have to have the whole system for it to work right. This includes the altitude adjuster on the firewall.
It won't make your engine run better. However, besides reducing emissions, it does promote longevity by virtually eliminating cylinder wall washing every time you let off the accelerator pedal.
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If a water cooled engine cools its water with air, isn't it just an overcomplicated air cooled engine? |
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Cali_Army_Guy
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Joined: March 19, 2015
Posts: 2094
Location: Stockton, CA
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| Posted: Fri Nov 13, 2015 7:44 pm Post subject: Re: Introduction to throttle positioner and altitude adjuste |
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| Juanito84 wrote: |
| Cali_Army_Guy wrote: |
| Hmm. My '71 super has the piece on the carb that is supposed to have the two vacuum lines. They aren't there and the nipple on the carb is plugged. The other piece is gone altogether. Will hooking up the piece on the carb make the car run better? Seems to run fine without it being hooked up. |
You have to have the whole system for it to work right. This includes the altitude adjuster on the firewall.
It won't make your engine run better. However, besides reducing emissions, it does promote longevity by virtually eliminating cylinder wall washing every time you let off the accelerator pedal. |
I don't have the firewall piece. Don't care about emissions. Oh well I guess. Good info though. |
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goober
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Joined: May 03, 2003
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| Posted: Fri Nov 13, 2015 10:56 pm Post subject: Re: Introduction to throttle positioner and altitude adjuste |
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"...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."
I think VW went to the two-piece system on the sedan in 1970 because of the drip-tray used on the new vented deck lid. No major functional improvement I can see.
What's my prize? |
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airschooled
Air-Schooled
Joined: April 04, 2012
Posts: 13501
Location: West Coast, USA
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| Posted: Sat Nov 14, 2015 12:40 am Post subject: Re: Introduction to throttle positioner and altitude adjuste |
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| Juanito84 wrote: |
| vectorsprint wrote: |
| Neat stuff. You mentioned 68-69, and 70-71, I've got a 74 super; would mine have come from factory with one of these? And if not, would it see much benefit from it? I ask cause I'm the one bug owner who doesn't have to smog it, but still hooked the EGR back up because I want the best fuel efficiency possible. |
Yours should have a dashpot. It also slows the closing of the throttle. But it doesn't keep the throttle open "engine braking". But it is still beneficial because it helps avoid going rich when you let off the accelerator pedal.
I don't think you can have both a throttle positioner and an EGR valve. They go in the same place, so it's either one or the other or you'll have to do some modifications.
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Good info, thanks Juanito. My post is a copy from the bus forum where I wrote this originally. The years are applicable to Bay Buses delivered to the USA. They may apply elsewhere, but I do not know. Dashpots came in '71 for bugs too? Do you know when they deleted the throttle positioner in bugs? Thanks!
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airschooled
Air-Schooled
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Location: West Coast, USA
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| Posted: Sat Nov 14, 2015 12:42 am Post subject: Re: Introduction to throttle positioner and altitude adjuste |
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| goober wrote: |
"...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."
I think VW went to the two-piece system on the sedan in 1970 because of the drip-tray used on the new vented deck lid. No major functional improvement I can see.
What's my prize? |
Ha! Good thoughts! I don't have a '70 deckled with drip tray nearby, but I'll ask YOU to be the detective on that one!
 My biggest observation this far, is the split second it takes to build "full" manifold vacuum during throttle closing. 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. Pay attention of 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 or hold (I can't tell which yet) the throttle arm until manifold vacuum is enough to trigger the diaphragm and secure the arm in the open position.
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mcdonaldneal
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Joined: June 13, 2013
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Location: Gullane, Scotland
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| Posted: Sat Nov 14, 2015 2:00 am Post subject: Re: Introduction to throttle positioner and altitude adjuste |
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While both of these threads are still active I'll add this post from our 1970 Ghia resto thread, it describes adjusting the two piece positioner, with pics...
Speaking of tinkering, after the basic tuning steps the Bentley gives instructions for setting up the throttle positioner/ altitude adjuster. I've never known how to get this working so I thought I'd give it a go.
The throttle positioner is attached to the carb. There is a lever inboard of the throttle arm which is attached by a threaded rod to a diaphragm unit that stares out at you as you look in at the engine. There is an adjustment screw to limit the travel of the lever, (black, directly above the threaded rod in the pics), and two vacuum lines on the diaphragm unit, one to the carb and one to the altitude adjuster next to the voltage regulator.
Bentley says to pull the lever against the adjusting screw to check the fast idle speed, and adjust this to 1700rpm. Mine had been disabled by extending the threaded rod to force the lever against the carb, so it wouldn't move. I loosened the locknuts and experimented with changing the rod length. Spinning it clockwise shortens it and stops the lever contacting the carb body. Spinning it anticlockwise lengthens it and stops it from touching the adjustment screw.
At this stage it worked well to unscrew the big screw on the altitude adjuster, which is the setting that does not slow the return to idle.
Bentley says adjust the rod length so that, at idle, the lever is not touching either the carb body, or the normal throttle lever (where the throttle cable attaches). My experience was that if the rod was too short, both levers got stuck and wouldn't return to idle (with the engine running). As the pic below shows...
Turning the rod anticlockwise at this stage suddenly got to a point where the levers sprung back to idle. This seemed the best length.
Having done that, the return to idle speed from 3000rpm is still quick, as the altitude adjuster is unscrewed. If you turn the screw in you get to a point where it will slow the return to idle to about 3-5secs (I needed about 4-5 turns).
After all that I took a test drive, and you can tell the difference in a slower return to idle. Is it any use? No idea! But it did help with double declutching from 4th to 3rd and 3rd to 2nd as you approach a sharp bend, holding the revs up. (Nobody does this any more, and the car will shift down without it, but it is kinder on the engine/ transmission!)
How's that for some useless information?!
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Juanito84
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| Posted: Sat Nov 14, 2015 3:13 pm Post subject: Re: Introduction to throttle positioner and altitude adjuste |
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| asiab3 wrote: |
| Do you know when they deleted the throttle positioner in bugs? Thanks! |
From what I understand they came out in 1968. Then:
| Volkswagen, Official Service Manual, Super Beetle, Beetle & Karmann Ghia, 1970-1979, by Bentley Publishers wrote: |
A throttle valve positioner is installed on all 1970 and 1971 cars equipped with manual transmissions. For 1972, it was installed only on cars sold in California equipped with manual transmissions. The positioner automatically adjusts the throttle closing rate for minimum exhaust emissions and prevents the carburetor throttle valve from closing suddenly when the accelerator pedal is released -again to reduce emissions. Beginning with the 1971 models, this latter function is supplemented on some cars by a dashpot installed on the carburetor.
Instead of a throttle valve positioner, an Exhaust Gas Recirculation system is used on 1972 and later cars sold in California with Automatic Stick Shift, on all 1973 and later cars with Automatic Stick Shift sold in the U.S., and on all 1974 cars sold in the U.S. |
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Juanito84
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| Posted: Sat Nov 14, 2015 3:15 pm Post subject: Re: Introduction to throttle positioner and altitude adjuste |
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| Cali_Army_Guy wrote: |
| I don't have the firewall piece. Don't care about emissions. Oh well I guess. Good info though. |
You can achieve a similar effect by simply letting off the accelerator pedal slowly every time you need to let your foot off the accelerator pedal.
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Failer
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Location: South Austin
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| Posted: Sun Nov 15, 2015 9:48 am Post subject: Re: Introduction to throttle positioner and altitude adjuste |
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| Juanito84 wrote: |
| Cali_Army_Guy wrote: |
| I don't have the firewall piece. Don't care about emissions. Oh well I guess. Good info though. |
You can achieve a similar effect by simply letting off the accelerator pedal slowly every time you need to let your foot off the accelerator pedal. |
True, but when they were new, they, of course, weren't "classic cars" The average driver probably wasn't going to change their driving habits for environmental reasons. Maybe for engine wear reasons, if they were the informed, mechanically minded driver.
It seems sometimes we forget that these classic cars we drive were once just everyday economy cars, and they were driven by the average driver. Good ways and bad ways. |
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RobKnox
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Location: Knoxville, TN
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| Posted: Wed Jul 27, 2022 9:51 am Post subject: Re: Introduction to throttle positioner and altitude adjuster! |
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| I have a 1972 Super Beetle. Both these pieces on my engine but are not functioning so my next project is to get them working correctly. Can somebody show me some pictures of what a 1972 setup should look like stock with all the hoses in place, etc? I would be VERY appreciative for the help. |
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