The manual says, http://www.tohatsu.com/tech_info/own_man_pdfs/Toh_MFS2A_3-5A.pdf
Re: Poor idling, on Page 32, it lists
o Deformed or damaged fuel hose
o Closed air vent on the tank
o Clogges fuel filter, fuel pump, or carb
o Improper engine oil
o Improper gas
o Poor carb adjustment
o Dirt or soot on spark plug
o Weak Spark