Yesterday I took the Chevelle out for a test drive. The car has been sitting since November, but it fired right up. After driving about 20 miles, I left a stop light and the engine started "popping" through the carb at about 30 mph. I pulled over immediately to investigate. After pulling the valve covers, I found the #5 intake rocker to be very loose. I attempted to tighten, but the polylock didn't seem to have enough travel. I visually inspected the rocker, pushrod, spring, and lifter as much as I could. I then swapped a polylock from another rocker and got it to tighten up. The backfire continued. I limped the car 15 miles home, trying to keep it below 2000 rpm.

Today I checked the compression in all cylinders. Cold, all at 170 psi, except #5 which was 190 psi. I changed plugs and rotor. I inspected the cap. I checked the timing, 12 deg initial advance. Oil pressure is good and oil condition looks OK. The engine never exceeded 180 deg during the drive. The engine starts fine, but the backfire continues. Mildly during idle and more violently as rpm increases.

I was planning to check for a possible valve spring or cam lobe problem next.

Any additional ideas would be appreciated.

My engine is about 8 years old with lest than 5000 miles on it. The combination is: 454 w/solid cam, 780 vac sec holley rebuilt last year, HEI ignition, stock 290 heads.

Thanks,
Jeff

PS The loose rocker was not a big surprise. I had not set the lash for the last two summers due to very limited driving.

Jeff, I would have to guess you may have a collapsed lifter or bad lobe on number 5 cylinder, was it an exhaust rocker or intake that came loose? Also when a cam goes bad the cranking compression will go up in that cylinder due to the fact the duration will decrease building higher cylinder pressure. With the engine running and watching the rockers, does the problem rocker have as much travel as the rest? Good luck, keep us posted.