Hello cussbud...
You shouldn't need to re-shim for 20%
If the engine is new, and the needles are still at the factory recommended settings, it could be way too rich? How much total run time is on the engine? If it's new, you might need a hot plug?
You will find much more info on the GRP in the car engines section...
http://www.rcuniverse.com/forum/forumid_269/tt.htm