My scout runs again-sort of

["CDesPlanques" <adesp@domain.elided>]

OK guys here goes,
I finally got my '67 Scout running again, well sort of.  It idles like a swiss clock.  Then, I press the pedal a bit, and the
backfiring sounds like it's going to blow the hood off.  I've played with the mixture to no avail, then I went to the distributor,
took of the vacuum advance and retimed it and it runs better, but not good.  My neighbor seems to think that because I can't get
passed 2000 or so rpms that the valve springs are weak.  Were do I go from here?

Chris
'67 800-makes a lovely lawn ornament