While it is idling, get an assistant to stick his hand over the tail pipe, or stick a potatoe in it, and the leak will become obvious.

You're best bet is to toss away the junk headers and fit a stock twin secondary arrangement on. B20E twin 1.75 into 2.125 gets the job done. Only a good head, twin DCOE's & a BIIIG cam can use a a set of quality 4-1 & then you need a big exhaust behind it of a tuned length.

Don't be tempted by vpd's headers eaither, they are far too big in diametre, wrong length and feature a very crappy collector. They sure look pretty but.....