Briggs flathead 8hp horiz - instant start at first then dies, backfires

Briggs 8hp flathead horizontal shaft. New carb, New plug, good spark visible, flywheel key good & properly positioned. Gas in float bowl. No change in symptoms:

- "starts" instantly from cold with recoil starter on choke & runs for 3-5sec then dies.

- Subsequent starting attempts ineffective; backfiring through muffler

- motor ran for several hours of use in the fall and then began exhibiting these symptoms. I've changed the carb since then with no different result.

Is my exhaust valve sticking open? Haven’t pulled the head yet; prefer not to if avoidable... Other possibilities?

If it is stuck/not sealing valve: next steps?

Incidentally the new carb has a brass nipple under the choke - nothing to attach to it. Is this where a primer bulb would go? Should I seal it or? Doesn't need bulb as tank is above the carb.