1) As an electrician the 1st thing I would do would be to check the
earth
connections. The usual sort of thing paint/rust/corrosion under the
connections or even just a loose connection!.
2) Also try cranking it over without any spark plugs and see if this
makes
a difference: not a definitive test but sometimes a no-load test like
this can turn up the odd surprise. If there is no difference pull it out
and try bench running it or get an auto electrician to do it for you: DO
NOT
DO SO IN SUCH A WAY THAT YOU ARE MAKING ANY SPARKS NEAR THE BATTERIES -
THERE
IS THE EVER PRESENT RISK OF A HYDROGEN EXPLOSION!
3)How about the brushes in the starter motor? Are they so warn that they
only
just touch the commutator?
Regards
Doug Greville
Australia