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Check the color of your plugs. Reading plugs is a science. But in short, you want them to be tan. White is lean black is rich.What engine do you have? What carb and intake are you running? What is your ignition timing?If your exhaust manifolds have the AIR ports you could check to see if your exhaust has a restriction, or too much back pressure.
If you raise the RPMs and keep them steady, what happens to the vacuum reading?
760°F shouldn't make cast iron glow.Interested to find out what is going on with this.