im not an engine expert but it sounds like your low idle needs to be adjusted. maybee someone can tell you wich way leaner or richer. i dont know about the v25 but heres a link to the 25fx manual wich may help. and you may wih to check your glow plug and make sure it has not gone bad.
http://www.osengines.com/manuals/15c...2sx-manual.pdf