Have you put any fresh petrol in? Petrol that lyes in the tank can go off and spilts into water and the other chemicals and will never be the same as free petrol. Also the fuel filter may be a bit dirty as the old petrol may have rusted/corroded the inside of the tank allowing small bits of dirt into the fuel line blocking the line up a bit.

The misfiring feeling sounds funny maybe this could be to do with the oil or the bike not being up the temperature. Not too sure. Kev will be more of a help.

The other thing could be to make sure the air filter is cleaned in case any nastys are inthere, ive seen birds nests make in them sometimes.

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Plough on