[Mike Simard]

Is your vacuum really bouncing around that much at idle? What size cam and engine do you have? It might be helpful to tape the vacuum gage to the windshield and see what the vacuum is at light cruise, that way you can get an idea of when you want the power circuit richened and match a power valve to that. Remember that there could always be other spark, valve or common engine gremlins present that may need to be dealt with before you start serious carb tuning. First work on getting the idle circuit set before worrying about jet sizes. The idle circuit handles most of the street driving and can make a car seem pleasant to drive or not. The main jets are relatively easy. A typical Cobra is probably overcarbed and needs tuning beyond adjusting the idle screws but can be made to work nicely with patience. There are restrictor 'jets' for the idle circuit on the metering block and air bleeds on top of the carb. The air bleeds for the idle circuit can be opened up to lean out the idle circuit and it's easy to change them. I've bought bags of undrilled Holley restrictors (or whatever they're called) and drilled them in .002-.005 increments to tune the air bleeds. As far as your main circuit goes, it sounds like your 2.5 power valve and main jets aren't letting enough fuel in. 2.5 is a pretty low number. What do you mean "hard time staying running"?