Dolmar blower vac?

Dolmar blower vac wont start,think the last fuel mix was a bit rich. Have since re fuelled cleaned spark plug and cleaned filter, it went for 5 seconds then no go.
Have re done the above steps several times with same result.
Its a Dolmar pb-250 has allways worked fine for three years without a problem untill now.
Any help would be appreciated. Thanks..