G'day Mate,
3 things, first off, the engine is still running in, & also you are under proping it, a Saito 180 should have at least a 16x10, or up to an 18x8, Check out this Saito site, for more info.
http://saito-engines.info/prop_sizes.html
And last of all, although there may be more, I would check the valve clearance, Check the above site, under maintenance.