Quote Originally Posted by FMS88 View Post
That’s a vacuum delay valve to the 1st stage port for the choke pull off. From your description it’s operating correctly. You could try using a hose without the valve. If that has no effect with a hot engine, then the delay valve isn’t the issue.

Have you checked the vacuum advance, distributor advance plate, TPS and the high altitude compensator to see they operate smoothly and within specs?
I thought i would mention that the TPS seem to be a common failure (at least in my case) with these carbs of the 3 carburetors I have all of them tested bad and I had to order a new one.