The needles and seats on Kadron carbs aren't very good (to be kind). There is no rubber on the needle valve, so the seal is just brass on brass, which is why you're only supposed to run 1-1/2 psi of fuel pressure with them. Maybe they’ll seal, mabe not
Regardless, 1-1/2 psi is really, really low pressure, since a stock VW fuel pump will develop about 4 psi. The "solution" (such as it is) is to keep stacking up gaskets under the fuel pump, so that the arm of the fuel pump makes progressively less contact in the case. It's a really cheesy way to go about adjusting fuel pressure, but it's the commonly accepted manner.
The problem could be as simple as too much fuel pressure. But lots of times Kadrons' needle valves will only be able to hold gravity pressure (from the tank sitting higher), and won't seat while the car is running, even if they have almost no pressure on them.
Knowing if the car idles for more than 30 seconds to a minute, without feathering the throttle to clear it out is really is the first step to diagnosing this problem.
We can keep guessing all week, and won't get any closer until we know the answer.