[ecomodded]

Mort I have a idea for adjusting the timing.
The Briggs & Stratton small engines are set at the factory, the timing is determined by the placement of the armature (coil) and position of flywheel on crankshaft.
Perhaps by modifying the flywheel key position you could have control over the timing.

To filter tap water you would need a ceramic or equivalent water filter to remove all minerals and organic matter.