Sudden Rough Idle Syndrome.

[MrMagoo]

Here's the scenario: Cobra sits for about 3 months without being started. First startup: runs exactly the way it did last summer. Idles perfectly. After about 45 miles through second tank of gas (about 200 miles of driving this season), car goes in garage. Next morning, start it up, idles low and rough. I turn the idle speed screw just to keep it running, and drive it at highway speeds, where it seems OK, normal power etc.

After getting to my destination, I put the idle screw back where it was, and investigate. Still with the rough idle and low idle speed.

Fuel Pressure (post filter) is 6+. Good. Float Levels good. Choke open at temp. Good. Didn't check fuel filter, it has a few thousand miles on it and it;s clearly not blocked.

Carb is Quick Fuel 780 (Holley 4150 style). MSD ignition.

My first guess is that I got some bad fuel. Or perhaps some debris got into the system. I'm not flooding, so I am guessing the needle valves are not the problem. Maybe somewhere in the metering blocks?

I think the ignition is OK, cause it starts right up, cap looks normal, plugs normal.

Anything seem obvious to anyone here? I might try some fuel additive before pulling the carb and cleaning it out.

Thanks