NO expert here but I would venture to guess a 1" tapered 4-hole spacer would work well.Porting the intake should show increase as well.I believe increasing the plenum volume would be a good idea. The sd should do well on that engine.They have responded well on other 500c.i. engines so no reason why it won't work with yours. Try to make one change at a time so you know why it improves(or doesent).
