The two photos below are examples of poor assembly by a motorcycle shop. If you look at the exhaust port you will see the late model mesh gaskets have been used and have crushed into the port by about 1/8th inch all the way around creating havoc with exhaust flow.

On the right we can see where late model heads have had the manifold installed using early model intake flanges and the manifold has been misaligned to the ports by 1/8th of an inch.

It is important that installers use performance exhaust gaskets and make certain manifolds are properly aligned so the special porting will be effective.