I replaced my cat with the bonez test pipe on my 83gsl. The tube pictured with the cat goes to something under the carb.

Should I plug that off or leave it open?

 

You need that. It supplies air from the air pump to the cat; without it the cat may burn out prematurely.

The Bonez cat should have come with a short hose (mine was blue) that connects it to the new cat. The little flange at the end of the metal pipe needs to be cut off to attach it.

 

The bonez straight pipe (cat delete), not catalytic converter. But thanks for the explanation.

 

I failed reading.

Yep, chuck it. You could also get rid of the air pump.