[M Nelson]

It's not uncommon for a "rebuilt" carb to malfunction immediately after being bolted down, and Q-jets are the most common to get a bad re-build. There are these little "soft plugs" in the bottom of the bowl on Q-jets that tend to leak,and drizzle raw gas directly into the intake manifold. Some rebuliders put a little spongy thing under them to soak up the gas, but that is not a permanent solution. Take the top and bottom off of the carb and put it on a clean 2x4 or something similar. Fill the bowl with gas and see if it leaks. By using a piece of wood , you can let it sit overnight and check if the bowl goes dry,and also if the board is wet or stained.
I wouldn't buy a rebuilt. Get a new one. The lower price is more attractive, but sometimes you really do get what you pay for.