You may not have a dump valve. I installed one, as it is recommended.

See how the vacuum source is connected to a tapped point on the mixer? This is because using manifold vacuum would be too sudden and violent to continually pulse to the converter. It also means that when you open the throttle suddenly, you expose the FCV, and therefore the converter, to more vacuum, which of course affects the mixture.

There you are, cruising down the highway at correct mixture, then you hit the gas. You open the throttle, introduce more air (lean condition), and also introduce more vacuum to the converter. Converter diaphragm lifts (another lean condition), and your engine almost stalls because it suddenly gone super lean.

The dump valve is connected on a T-piece in between the mixer tube and FCV. It simply allows air in if vacuum gets too high, and is supposed to keep the FCV seeing the same vacuum regardless of throttle position. They don't always work perfectly, and the CDP Oz doesn't react fast enough on throttle response regardless.

It would be much more effective to use a delta throttle kit, or simply a microswitch mounted somewhere on the throttle body to disconnect the FCV during such conditions.

not sure that i understand the operation of the system but would the dump valve go somewhere like in this picture?

would you have a pic of your setup?

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Found my instruction book and scanned these for ya

I forgot to mention, you'll need a calibrated orrifice for the t piece or 4 way block as well. I have no idea what size this is supposed to be, but it is important you have it. Best speak to an LPG pro to see if he can supply the parts.

Heres a pic of my mixer. You can see the brass 4 way block to the right of it (bolted to the old intake pipe stand-off). The dump valve is a tiny black valve screwed into the right of it. The rest is a blur (old, bad photo)

The FCV is screwed into the 4 way block, so is the elbow (with the orrifice incorporated into it), the dump valve and the mixer vacuum. The converter gets connected to the other end of the FCV. The FCV's usually only work properly connected in one direction, as they have a seat which may leak if vacuum is applied to the wrong end.

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one thing i dont understand is where does the vacume come from...its not connected in any way to intake manifold so how is the vacume being created?

not sure either what a 4 way block is...is just a way of connecting 4 openings together?
so is just connecting the 4 ports together:

The vacuum comes from the thin black tube connected to the mixer. Manifold vacuum is way too strong to opearte the converter diaphragm. The point on the mixer that the black hose connects to provides a gentle vacuum to the FCV. The orrifice is there to release the vacuum to the converter when the FCV is off. If you have a 3 way FCV, this may not be necessary. If it was not present on a 2 way FCV, the vacuum would just keep building up and keep the converter lean.

The brass block is eactly as you say - its just a 4 way port to connect everything up. My FCV was threaded, so was the orrifice and other connections. It was just easier this way. You can just see the FCV as the black section to the left of the 4 way block.

so the vacume is created by some sort of venturi effect through the mixer?

this "orrifice" is that the dump valve? and also "the vacume building up" is that only when the FCV is closed? which side of the fcv would the vacume build up? converter yeh?

The dump valve is seperate from the orrifice.

Follow this diagram and you can't go wrong.

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BTW, there is no dump valve in that pic, as it is optional to improve acceleration response.

If there was a dump valve, it would go between the FCV and the converter T piece. This would allow air into the tube if vacuum became too great from abrupt throttle movements.

The orrifice is just a small cylinder of metal with a hole drilled in the centre of it.