I have an old Hayter petrol lawn mower with a Briggs and Stratton engine which will not start. It is getting fuel but I believe the problem is electrical. I cannot get a spark. I have changed the points and condensor, changed the coil and used a new spark plug and disconnected the stop cable. I do get a tingle if I hold the plug lead when I turn the engine over but if I take the plug out and hold it against the engine with the plug lead connected and turn the engine I cannot see a spark. I think I have covered most things and there is not much more to the electrics so I am stuck. I have also checked the magnet on the flywheel and set the points and plug gap and checked the air gap of the coil at the flywheel. Help!