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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:
http://www.arb.ca.gov/html/brochure/history.htm - accessed 11/1/15Federal 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.
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:
[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:
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. :P
Love and good air quality,
Robbie