Hello all. I’m looking for help because I’m lost. I just bought a 1977 K5 Blazer 350 engine. It had some fouled plugs, which I replaced, and a leaking valve cover gasket. When changing the valve cover gasket I noticed the #7 intake rocker arm was loose. It has roller rockers. I followed the steps and adjusted the rocker arm. Fired it up and it drove great. After about 20 miles, it started backfiring out of the carb out of nowhere. I found a small vacuum leak on a cap on the carb. Replaced the cap and it’s no better. I don’t hear or see any other leaks. I checked the rocker and it’s still snug. It only backfires once per rotation leading me to think i have an issue in one cylinder. Any suggestions would be great.

 

I'd do a dry and wet cylinder compression test, were I you. It's an old engine, so it would be a good start.