need some help

have pu;lled the motor out and torn it down a sit needs a rebild.
77 z650.
when i removed the inlet rubbers i was shocked to see the inner dia is smaller than the ports on the head.
the carb sits into these and then were they bolt to the head the dia is samller than the ports in the head.
to me this is planely wrong but is it the norm on these bikes?
bike has never revved out proberly and im guessing this is one of the reasons why.
can i fit bigger carbs and manifolds and from which bike can i get them?
cheers rob

any one have any ideas or help on this?
looks like i have got myself some high lift cams, just waiting for them to turn up.
what bigger carbs are going to fit my engine?

Mikuni VM26, VM28 and VM29 smoothbores can fit, as well as other type of BS carbs and probably more.
The manifolds will need to be changed as you mentioned.
Have a search in the carb section and elsewhere as there have been discussions on this in the past.
I'm not really good with carb info.