You could try taking the exhaust box off and looking to see if there is a spark arrestor mesh fitted. If yes, remove this, re-fit the exhast and re-try. Many countries now insist that these guards are fitted to stop any sparks from setting off brush fires. These always get oiled up after about two weeks and contractors normally remove them or poke a screwdriver through them to make a hole big enough to release the pressure.
If it's under guarantee, beware of voiding any warranty.