I suspect a tiny exhaust leak, they can be difficult to detect and produce a tick that is difficult to locate.
If the rockers or a rocker is hitting a valve cover it will leave witness marks, but it is common for roller rockers to contact the intake side of the covers and we've had to remove material from than area on some types for clearance.
If you're engine doesn't like vacuum advance at light part throttle the cam isn't big enough, or on a tight LSA with advanced/early intake closing.
I've also found engines that don't like much timing from the vacuum unit are also not wanting or liking a lot of total mechanical timing and a quick advance curve........Cliff
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https://cliffshighperformance.com/
73 Ventura, SOLD 455, 3740lbs, 11.30's at 120mph, 1977 Pontiac Q-jet, HO intake, HEI, 10" converter, 3.42 gears, DOT's, 7.20's at 96mph and still WAY under the roll bar rule. Best ET to date 7.18 at 97MPH (1/8th mile),
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