I've seen the technical bulletins but it doesn't fully explain why.
The twin had a problem equalizing the fuel supply to both cylinders Norm,I haven't seen it completely explained anywhere but I think the rear cylinder being lower to the front cylinder makes the rear cylinder run richer (the fuel mix will take the easiest path) being a richer mix this would foul up the rear plug so a higher compression piston was used in the rear cylinder to help burn the excess fuel.The crank was also modified to help in equalizing the supply of fuel to both cylinders.
