Small block Chevy V-8's are notorious for wearing out camshaft lobes, usually the exhaust lobe on #4 or #6 cylinder. It was most common on mid-1980's 350's and 305's. The engine gets progressively rougher-running, yet it will pass a compression test, and until the cam goes totally flat, a cylinder balance test is inconclusive. The exhaust gas that remains in the bad cylinder due to the partial valve operation dilutes the intake mixture when the intake valve on the bad cylinder opens, resulting in low manifold vacuum, intermittent bad mixture in other cylinders caused by exhaust gas dilution, and enough effect on the carburetor power valve to result in very rich mixtures.
Pull the valve covers, and monitor the valve lift, particularly on the passenger's side exhaust valves.
Jerry
