I bought this boat last year,it's ALWAYS idled rough.I rebuild carbs even though they were spotless clean.I used Yamaha rebuild kit.STILL idles rough.Compression is perfect,new fuel pump and hose,I've tried different premix ratios ,everything from 50:1 to 100:1,same thing,idles rough.Its at a boat mechanic now and he's tried everything,STILL idles rough.Its a 1990 Yamaha cv40eld twin cylinder ,2 carb ..Any ideas??It runs great at higher speeds,just idles rough and hesitates just a little while taking off..