what year is your engine? looks like 86-87, the 86 run a bit higher compression and wont have any decompression slots in exhausts sub ports, also a later dome piston may up the the compression a little , not sure how to accurately calculate the compression ratio with a dome piston and do u alllow for ur 1mm head gasket and 0.5 gap the piston usual has to TDC ,
if u are running a too high compression ratio u will get pinging at low revs
i think u have a problem with decom valve , the valve is not beeding enough compression or the hole in the head is to small , [try kicking over with valve not instaled]