Move the timing-light pickup closer to the distributor and check for a faulty light or a cylinder misfire if the mark flickers. Because small-block Chevy balancer rings can slip, verify true TDC with a piston stop in the number 1 plug hole, mark both stop points, and use the midpoint as TDC. A mid-to-late-1960s 327 reference calls for about 4 degrees before TDC initial timing, but confirm the exact engine and symptoms before setting it.
The owner of a 327 with a medium cam, 650 carburetor, and camelback heads sees timing marks jumping around and asks whether to black out extra marks and what timing to use. Experts say flickering can come from a bad timing light or a misfire and suggest moving the pickup closer to the distributor. They also explain that the balancer's outer ring can shift, so true TDC should be found with a piston stop and a new mark made at the midpoint. A mid-to-late-1960s 327 reference is given as 4 degrees before TDC, and the owner is asked to clarify the exact problem.
