after decomposing
my spare head i found out,
indeed the ports, are relatively big, but then - in the area close to the cylinder , velve seat, there is something like a narrower diameter - doesnt it last to speed up the air a bit (venturi effect...)?
have you tried to fit a restrictor somewhere in the inlet? how about the total volume of inlet - the effect you desribed would suggest that its too big/long and the pulse effect are too small
(my english is crap but i hope you understand)