I just rebuilt the two barrell Holley 2100 that's on my '55 Fairlane. I had pulled it off due to a leaking gasket and decided to rebuild it. I put it back on today and fired her up. She sounded great while warming up. I was waiting to ger her nice and warm so I could adjust the idle screw setting and idle mixture screws when she started to lope and run rough. I put some gas to it and it sounded like it was missing. I also heard a hissing/sucking sound. i checked the vacuum line and it was tight at both ends. Any ideas what may be causing the issue? Thanks.