There are lots of reasons that can cause the issue. Is this a stock, rebuilt or performance engine? Reasons could range from cam bearings, head gasket, head milling, head bolt, rocker wear or shaft incorrectly installed.
As long as you have good flow in the left head, unless your flooding the valve cover on the right, there's not much reason to mess with it. If you're flooding the valve cover I'd look at the rocker set-up before just restricting it. If everything looks good, shafts installed correctly, rockers not overly worn, then there's no reason why you couldn't restrict one side more than the other.