The Zen G26 is an absolute gem of an engine but when new can exhibit a few tuning issues. First off it really prefers a bigger prop than that 16x8 as an 18x6 is considered the norm here which would also aid in a more reliable idle. As Al mentioned if the engine is new with little or no run time on it then don't expect a great idle, it just won't happen until it its been run a few times, so as already suggested get a decent tune on it and go fly being careful on the first few flights to keep wide open throttle or long up lines to a minimum.