[Mike_The_Grad]

Wow, that video helps for sure. Something is definitely amiss with your setup. Steeveedee is on to something. I've not seen a steady rise in intake vacuum as the throttle is opened like yours does. Then immediately drop to zero like that. Read up some more on vacuum gauge reading. There's something to explain the sudden drop like that. Also sounds like you got a knock or something that is rpm related. I could be wrong and just hearing things. Either way, I'd start at the exhaust manifold and work your way back to the end of the tail pipe. But the heat riser valve is probably gonna be the culprit. Do you have it connected properly? I couldn't tell you what it's supposed to look like or how it functions. Never had one and I just put headers on my dads 292. My guess is that its malfunctioning or inoperable. Especially because you say that youve let the engine warm up and it still reacts the same regardless.