The mower starts fine, usually on the first or second pull and after a few seconds starts to mis- fire (the best way I can describe it).
I have cleaned the filter and changed the plug. It sees to run better without the filter being on i.e. hardly mis-fires.
Is there something else that could be causing this?

Hi,
Usually it is either the plug or the fuel. The plug will not work correctly if gapped incorrectly. Generally 0.030" on most; 0.020" on B&S OHV engines.
Peace,
Clarke