[T3] Dual Carb Shift Vacuum

Jim Adney jadney at vwtype3.org
Wed May 7 07:21:27 PDT 2014


On 7 May 2014 at 7:11, Craigs List wrote:

> As most of you know I have been chasing a shifting issue lately. To try and
> get some answers I hooked up a vacuum gauge to my manifold vacuum reading
> last night. The gauge was all over the place. It was a blur all across the
> face of the gauge. When you would rev the car it would settle a little bit
> but not to the point that I could get a reading. What causes this?

It's normal, and caused by the fact that each time a cylinder sucks 
in air the vacuum increases. At idle, the intakes are rather widely 
separated in time, so the gauge tries to respond to the up/down in 
vacuum as each intake valve opens. The blur you see is the result.

At higher rpm, the needle doesn't have time to respond, so what you 
see is less blur and more of an average reading.

> It was brought to my attention that there might not be enough vacuum at
> higher RPM's to initiate the shift from 2nd to 3rd. In my mind, I though a
> LACK of vacuum initiated the shift. I could be backwards on this thinking
> because closing the butterflies is what finally tricks the car into
> shifting. Closing the butterflies would increase the vacuum.

Your second thought is the correct one. Increasing vacuum causes the 
VM to push the upshift.

> I'm just looking on some pointers as to where to move from here. I was
> thinking of a few things; vacuum canister between manifold and shift
> modulator, drilling and tapping the second manifold to try and smooth the
> signal or blowing the car up......just kidding.

A vacuum cannister would smooth out the vacuum pulses, making your 
gauge reading more steady, but, depending on the sizes of the hoses 
connecting things, it would slow down the vacuum response. I don't 
think you'd like the result.

At first I didn't understand what you meant by "second manifold" but 
then I realized that you are trying to make this work with 
aftermarket carbs. The OE carb setup came with a balance pipe 
connecting the 2 carbs and the AT VM was connected to this balance 
pipe, just to the right of center. IIRC, the early pipe was about 
1/4" OD while the later one was nearly twice that diameter. It was a 
steel pipe, bent to fit around the front (transmission side) of the 
engine with rubber connections at each end.

With your setup, you're getting only 1(or 2 unevenly spaced) vacuum 
pulses per cycle (2 revolutions) of the engine, depending on how your 
vacuum ports are done. This makes your vacuum level even more ragged 
and might be causing your problem.

I'd suggest adding a balance pipe between manifolds and then Tee the 
VM hose off that pipe. The connections to the manifolds should be 
such that you get even vacuum pulses from both cylinders on that 
side. The latest OE manifolds have 2 vacuum ports on each manifold. 
Those connect together and then connect to the balance pipe.

-- 
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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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