VW part number 311 129 501 B (for single carburetor models)
More than a few people (including myself) have been surprised with the existence of this item. This made-by-Solex "corrector" replaces the main jet in your side draft carburetor.
The theory is...as you travel up and down mountains the unit adjusts your fuel mixture to its optimal setting.
Does it work? I will let you know. Where I live (Lake Tahoe, USA) is a great testing situation as I can travel more than 4500 vertical feet in twenty minutes.
Email from Scott Perry regarding the above:
Date: Thu, 15 Jun 2000 16:26:27 EDT
Subject: 8/64 - Workshop Bulletin K-99
I was rummaging through my old workshop bulletins and found one (K-99) for the High Altitude Corrector Paul wrote an article about. It was introduced in 8/64 for the single-carb Type 3 engines from #0 220 137, carb #208 500 (and Type 2 1500s).
It also states that the "high altitude correctors and main jets are adapted exactly to each other. The size of the main jet in a high altitude corrector must not be selected at random. It is not possible to readjust the pressure unit to a jet size other than the prescribed size."
There it is. A bit more info.
Here is what the Type 3 Workshop Manual has to say:
As the height above seal level increases the air becomes thinner, that is to say, it contains less oxygen. If the fuel quantity remains constant, the composition of the mixture will change. In order to maintain the proper proportions of air and fuel and thus ensure the formation of the correct mixture, a high altitude corrector with a suitable main jet can be installed in place of the main jet carrier.
The main jet (1) of normal size is screwed to one end of the high altitude corrector. The fuel is supplied to the main jet through four oblique holes, and through a small hole between the main jet and the four oblique holes. The fuel also flows into the vacuum element chamber (3).
The vacuum element (4) is supported axially and fixed in position at one end by means of an adaptor which is screwed into the house. The conical end of the needle (2) which is mounted at the other end of the vacuum element slides freely in the passage leading to the main jet.
When the vacuum element (4) expands due to the changing atmospheric pressure at high altitudes, the needle progressively throttles the fuel supply for the main jet. The fuel supply through the small hole remains the same.
The vacuum element is set during assembly by means of adjusting nut (6) which is secured with a lock plate (5). This ensures that at sea level, the needle will be in a position which does not affect the normal fuel consumption.
The adjustment of the vacuum element must not be altered under any circumstances.