Nick,
Did you have it tipped with the carby on the lower side, if so the filter may now have oil in it and if this is the case they won't start. Try starting it with the filter removed. Also pull the plug out and pull the motor over a few times and see if any oil comes out the plug hole.The plug may be soaked with oil, try another plug and make sure you have spark. Then if it still won't start give it a quick squirt of starter fluid in the carby and it should start
