Whilst I cannot offer a definite solution to this problem, it is worth disassembling the carburettor looking for dark brown material that collects if you don't drain the fuel tank after each use. I have started running all of my petrol engines dry to avoid this problem of fuel evaporating leaving coagulated oil in the carburettor as a membrane. You won't damage a good motor by doing this since as you should have lubrication for the piston rings right up until the agonal stroke.
As an addendum to the above, don't hold back in adding enough two-stroke oil to the fuel. I use a ratio of 1:25 for pretty well all my motors (40 millilitres per litre or 200 mls per five litre should be enough). A new plug is around $6 on special and you'll will be surprised how few you go through if you drain your tank and run the motor dry as well. Don't forget to clean the air filter (gentle rinse in clean petrol in an old tin or even a zip lock bag then shake it out - take less than a minute if you are any good fixing things).
Nota bene: you can also add semi-synthetic or full syntethic two-stroke oil at a ratio of 1:100 (10 mls a litre of petrol) to small four-stroke engine fuel to lubricate the top end such as in a lawn mower or ride-on mower four-stroke engine. DO NOT TRY THIS IN YOUR CAR OR ANY OTHER COMPLEX MOTOR HOWEVER AS THE FUEL SYSTEM IS ENTIRELY DIFFERENT.
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