the timing isnt your issue here... your valves may be out of adjustment... in that case you would need some very expensive tools... probably not a good financial choose considering you only need to set your valve lash  every five to ten if ever...  i would pull and clean the carburetor and any other sight glass bowls or pumps the unit may have..  lucas oil also makes an ethanol stabilizer that i am recommending  to all boaters.  keep it around.. put it in with your gas every time it works great..  ive even noticed a slight increase in hp after you run half way through the first tank.