1986 B2000 backfire

I just purchased a 1986 b2000. It had over 300,000 miles. I replaced the engine with a low millage Japan (2.0) import engine. The truck has new plugs, new vacuum hoses, new O2 sensor. The truck runs well when warmed up at moderate to full throttle. There is a small backfire up thru carb when accelerating from idle. There is also a single electrical connector that goes from the side of the carb that isn't hooked up. It has a yellow blue wire and I believe it is coming from the choke. I have a service manual and I cannot find out if it is a test connection or if it needs to be connected to something else.
Thanks All
Brad