I have a brand new 2006 30hp honda that had the same problem last september. The culprit-bad gas. It had been two weeks since i was out and i did not run the gas out of it. Started right up but acted rough esp at idle. Took it in and the shop said that water was in the carbs from bad gas and to always run the motor out of gas if i wasn't going to be using it the next day. No problems since but i also do not buy gas from the place where i had the problem (gas station/car wash....hmmmmmmmm...wonder were the water came from).