Sounds about right on the welded up squish heads. Measure squish clearance on the heads with soft solder or putty or such all the way to the edge of the piston.
I have 24cc TR heads on one bike and 1mm squish.
28cc on welded up heads on the other with 1mm squish.
Both run 3.0mm BTDC timming and the TR head bike uses B10EGV plugs and the Welded head bike uses B9HN.
2.4mm BTDC is retarded way back and a safe start but really knocks out the bottom end. Try working your way up and keep the bike a little on the rich side for saftey.