What plugs do you run in your HP 273?

Spark gap also matters. The factory 0.035" gap is only if running points. If you move to electronic ignition like HEI, you can go as wide as 0.060" gap. That will help ignite leaner mixtures too. If it misses under wide throttle at low rpm, reduce the gap a bit. Little reason to use points today since electronic ignition is easy and cheap.