Choke not opening soon enough

I have a 89 Caprice Estate wagon with the Olds 307 engine. When I start the car, it runs fine for the first 2 minutes, then it starts bogging down. I opened the air cleaner and found the choke is
staying closed. I adjusted the dashpot that pulls it open, but that dashpot is connected to a vac hose that goes to a "contraption" on the valve cover with a whole lot of vac hoses going to it. After a few minutes, the dashpot suddenly pulls the choke open. Therefore, something in that "contraption" must sense the engine is warm, and allows vacuum to that dashpot. Until then, there is no vacuum on that hose. However, if I attach that dashpot to another vac hose, such as one coming right out of the carburetor, it pulls the choke open.
Can anyone explain what is going on here, and how to fix this. I'm pretty much stuck with having to manually jam something into the choke everytime I start the car.
One other thing, has anyone ever tried to put a manual choke cable on one of these. It would be so much easier to control it myself. Automatic chokes are a pain in that butt.
Thanks
George
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A "quick fix" would be to rig something that keeps the choke open at least slightly even when cold. I did this once on a Caprice, until I replaced the "choke pulloff valve", which fixed the problem Sounds your problem is not the choke pull off valve, (if your car has one) though, since eventually the choke does open.
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Agreed... sounds like that "contraption" on the valve cover may be partially to blame.
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