Needle Valve Operation – VW 105-1 Carb

Envision, if you will, a toilet tank.  If you’ve looked inside one, you see water. And, you see some other things in there.  Hopefully, none of them alive!

When the tank handle is pushed down, a rod inside the tank pulls a chain which, in turn, is attached to a flapper.  For our discussion, the handle, rod and flapper are not relevant.

What IS relevant is the manner for filling the tank to the desired level—and no further.  A toilet tank might have various methods for achieving a desired water level—not allowing the tank to over-flow or to have just the proper level of water to cause the commode to flush. 

In the 30 Pict-1 Carburetor, the operation of filling, then stopping the inflow of Gasoline, is quite similar.  There is a Float inside the “tank”, which is called the Fuel Bowl.  The Float operates simply by being suspended on a Cross Pin which allows the Float to pivot.  As the Fuel Level rises, so does the Float.  When the Fuel Level drops, the Float follows the Level of Fuel.  Most Carburetors are patterned in this manner.

But, how can we operate the Carburetor without its over-flowing?  That’s the interest of this article.

We have established that the Float pivots on its Pin Axis.  But, Fuel Level must be controlled.  If not controlled, the Fuel Bowl will continue to fill until it over-flows.  Yes—Fuel Bowls can over-flow.  There is the Discharge Tube which connects the Fuel Bowl to the Throat of the Carburetor.  The Discharge Tube is clearly visible when the Carburetor Top has been removed. 

But, we do not want the Fuel to reach the Level of venting to the extent that it over-flows down the Throat of the Carburetor.    At that point, so much Gasoline will be over-flowing that it will cause the Combustion Process to be super loaded with Fuel.  The Engine will begin to choke and sometimes dies altogether!

During its pivoting, the Brass Tab of the Float is in contact with a Valve which it activates to meter the flow of Fuel into the Carburetor.  This Valve is called the Needle, or Float, Valve.



The Needle Valve consists of a Housing, a Seat and a “Needle”, or Pin, with a Conical Rubber Tip.  The Pin travels up and down inside the Housing.  When forced upwards, the Conical Rubber Tip contacts the Seat (or Fuel Inlet Orifice), blocking incoming Fuel.

The Brass Float Tab contacts the Lower End of the Needle Valve Pin, pressing it upwards as the Fuel Level Rises—or, relaxing pressure as the Fuel Level drops.  This is a constant process.  Unless Fuel Pressure drops drastically or the Fuel Supply is interrupted, the Float Tab and the Needle Valve Pin are in constant contact—up and down (the Float Dance LOL).  Thus, the Fuel in the Bowl is kept at a viable Level

Moving parts can and do fail.  Some time ago, I talked with the local VW Engine Builder about the Needle Valve.  He asserted that the Pin inside the Valve must have a rubber tip.   A rubber tip would best be suited to fit and to close the Fuel Inlet Orifice.  That made sense to him—and to me.  BUT—I wanted to see for myself.

I went to my  Carburetor Parts Cabinet and produced a couple of the usual Needle Valves.  I was determined to disassemble one of them.  The kicker is that the Brass Inlet Piece (the Seat) is pressed into the Valve Body.  No removing it, that I could discern.  So—to the grinding wheel I went.

I began grinding as carefully as I could, grinding-examining-grinding examining until the fitted Inlet Piece actually popped out on its own!  At last, I could remove the “Needle”. 

This was, for me, an “aha moment”.  Sure enough, on the end of the Needle was a firmly attached, Conical Rubber Tip.   So, this was how the Needle could staunch the flow of Gasoline.  As the Gasoline fills the Bowl, the Float rises ever higher forcing the Rubber Needle Tip into the Inlet Hole of the Valve.

Here, we must recognize that the Rubber Tip eventually will wear.  Ethanol in the Gasoline contributes to damaging the rubber—until the Needle’s Rubber Tip no longer can seat firmly enough to stop the flow of Gasoline into the Bowl.   Wear is inevitable! Maybe that Rubber Tip will harden–rubber does that.  In any case, the Tip ceases to do its job of closing the Inlet Hole of the Valve—and Gasoline flows with little restraint.

I’ve seen Needle Valves whose Pin does not move smoothly—for whatever reason. Movement that is unreliable means that the Fuel Level in the Bowl may also become unreliable.

With each Carburetor Rebuild Kit, a new Needle Valve will be included with Two Washers of different thicknesses.

Either Washer can be used.  I recommend using the thicker of the two.  For my own Carburetors, I use both Washers to further lower the fill-height of Gasoline in the Bowl.  Available Gasoline is not a problem, if there is Fuel coming from the Gas Tank and the Fuel Pump is operating properly.  Our small Engines are not going to use so much Fuel that they will out-run the supply in the Fuel Bowl.

Clean the Float.  Make certain that it has no cracks which would allow the Float to fill with Gasoline and cause the Float not to be buoyant.  Test a Float by submerging it in water and holding it down to see if any bubbles emerge.  Then let go of the Float to see if it pops to the top.

If the Float is damaged—obtain a new or a good used one.

Carefully clean the place on the Float Tab where the Needle Valve Pin rests.  If it is corroded or the Pin has badly indented the Tab—obtain a good used or new Float.

Polish the Brass Cross Pin to ensure smooth movement.  It also must be straight.

Having all things in order, you should have carefree Fuel Bowl operation and smooth driving in your 1967 Beetle.

Posted by Jay Salser

My wife, Neva, and I have been driving and working on VWs since 1976. In fact, we raised our family in these cars. Now, we are retired and enjoy VWs as a hobby. The ’67 Beetle always has been our favorite year. We own a '67 Beetle and a '68 Karmann Ghia.

  1. Great article, JK!

    Reply

  2. Thanks, Eric! This was a “fun” article–a fact-finding-mission for me, actually. I’d always wanted to know more about operations inside that closed chamber! jay

    Reply

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