Slick 60's

My truck had a Carter YF carburetor and it ran ok but whistled like a tea kettle at half throttle.
Was driving me nuts (not that I would have to pack a lunch as the trip is not that far) so I purchased a Carter YF NOS from
e-bay. Would not run at all. Rebuilt NOS carb still would not run. Sent carburetor to The Carburetor Dr.
Would not idle. Sent back to TCD and it would run but not well.
Spent the last 4-5 months adjusting, double-checking and triple checking.
Carburetor would intermittently flood, act as if starving for fuel at high rpms, cough, gag and was generally boggy.
Pulled top off carburetor last night to check float for the zillionth time and removed needle valve seat and found object pictured.
Don't know if it is a piece of plastic or fossilized carburetor varnish but it was almost invisible in the brass seat.
This morning truck ran the best it ever has on my way to work (53 miles) with good acceleration and no sputtering or
stumbling!

did it whistle Dixie?

Great that you solved your problem. It does look like varnish in the gas to me. My problem was a loose float seat on my 1904 Holley allowing the carb to flood at times. I also found some recommendations on the IH site to run the float lower than you would normally because the carb to overflow and leak out through the throttle shafts at shudown due to heat soak back.