Mike,
<p style=”text-align: left;”>Some progress then, but backfiring through the carb? ….. if timing is correct this would suggest a sticking inlet valve, possibly closing enough to avoid fouling the piston but leaving a gap allowing the combustion to leak into the manifold leading to ignition of fuel in the carb.</p>
Suggest remove rocker cover and check operation of all valves before making any changes to timing.
B.
Bernie