I inspected the spark plug yesterday and found the vento is running lean and there is burned oil in the ceramic. I drilled 4 small holes in the airbox to let air flow more freely to the carb but probably this is causing the leaning. I don't remember but probably the spark plug has 600 miles. I will replace it this month as soon as my vacations begin. Here is the link for the spark pics.
http://s123.photobucket.com/user/hrm...y/Spark%20plug