dirt in the carb or gummed up could also be water in the fuel filter it also acts as a pump the filter is on the end of a pipe inside the petrol tank you should be able to pull it out (the pipe/rubber hose) it may also need re tuned but if you are not clued up leave the tuning alone
the tank should be drained out when not used for long periods ask your local stealer how much it is going to cost you could buy a new one and sell the old one on flee blag there is always a numptie out therethat will pay more than new price there was a sthil 08s sold for £245 recently
Remind me never to buy anything off you Rob
, I used to help run a plant hire repair business.
Sudan ,
it was interesting to hear how you described the problem , it does sound a classic case of barrel wear mainly due to incorrect lubrication and your getting crank case conmpression as a result .
Which blows back un burnt fuel mix up the worn side of the piston and barrel and backout through the carb .
If you can remember the filter being well oiled up after the use or you can see evidence of lots of oil /fuel mix on the fikter side of the carb you might just as well go for a new machine and be done with it.
But if you want to play and see whats what ....
As its been a few years since I was playing in the repair game things may be slightly easier/different due to design changes ....sometimes you can use a torch when the carb & silencer are off to look down into the cylinder through the spark plug hole when the piston is at the bottom of the stroke and see the exhaust & inlet port area of the cylinder wall wear looks like . If it is worn will look dull drey area around the holes in the cylinder wall . Frequently you will see vertical scoring around these ports and in first 2/3 of the cylinders downward travel where all where the piston rubs against the cylinder wall when the engine is turned in the correct direction .
if you can see wear again go for a new machine
Alternativly if your really happy to play and dont mind the fair likelyhood of the machine dying from some other malady such as a worn bar & chain sprocket , failed safety brake etc . .........
Do you have a screw in compression tester for the plug hole .. if you can get hold of one look for compressions in excess of 18 pounds 18 to 21 is good .. if you can't reach that add a level dessert spoon of clean engine oil to the barrel via the spark plug hole connect the tester pull the cord several times and see if it comes up in pressure ..... if it comes up to around 24 psi , your barrel and piston are badly worn & need replacing and its nearly as cheap to buy a new machine .
You could confirm this by carefully stripping the barrel and piston out and looking inside of the barrel for a really dull mat grey area by the portholes when compared to the rest of the shiny barrel
Take lots of pictures if you do this and don't feel happy doing it , mark all sides and surfaces with matching & corresponding marks so you can reline things up if needed .
Other thoughts in the fuel tank there should be a pick up fuel tube with a compressed brass micro ball filter device or a fine nylon mesh filter on the end . See if you can use a bit of clean wire to ease the end of tube and filter out the tank , remove the filter and wash it out in meths ( this will dissolve the oils and absorb any water in the filter then let it dry narurally for a couple of days.
Like Robert said about the fuel ......never store any two stroke machine or any other small plant with fuel in it .
Not only does modern low octane fuel decompose / oxidize within a few weeks of being decanted from a fuel container it also evaporates out of the two stroke mix leaving you with a greater two stroke oil density .
This often sticks the reed valves & diaphrams in the carb , easing them off frequently distorts them .
The only long term effective way I found to overcome this prob was to replace all reed valves and rubber diaphrams etc.using the manufactures own carb repair kit , after air line blowing out every drilling on the carbs & cleaning the complete fuel lift tube including replacing the old fuel filter..