Chevrolet Repair: fuel system, ported vacuum switch, plastic valve

Question
My son has a 75 malibu 350hp 2bbl. Ran out of gas. Thought fuel pump was bad. Was not. Changed fuel filter. Got it started, but very rough. Put a carb kit in, had someone check it all. Runs poorly. He broke a plastic valve of some kind, while working on it and don't know what it is. Sits on top of exhaust manifold and has two vacuum hoses coming off of it. One goes to carb. Timing OK. Fuel pump OK. Someone said this mysterious part is a "thermal riser", but cannot identify or find. Any suggestions?

Thanks.
Jack

Answer
Runs poorly? At idle? Under power? Miss, or shakes alot? Won't hold an idle? Misses badly during acceleration? Plastic valve should be on the intake manifold, as plastic would not hold up to the temp. of the exhaust man. More than likely a "ported vacuum" switch in/on the intake, coolant passage near thermostat outlet. Check fuel lines clear back to the tank for cracks, etc. Lifters should never make any noise what-so-ever, if they do...there's a problem. Let me know and we'll go from there!