Don't know any mechanic in that area however I have had that problem among others on motors. With something like that it always turned out to be fuel related. First thing I would do is the basics...make sure the tank, fuel line and filters are clean and fully operational and it has clean gas. Next thing would probably be remove the carbs and clean them throughly, installing a rebuild kit which should include needle tip, needle seat, O ring for jets etc... Also, just for kicks, do a compression test just to make sure you are working with a solid powerhead.
There are alot of guys on here who are way more inclined then I, so standby for better advice. Good luck man