I'm no fan of the downdraft webers... not many folks here are I think. Have had a bunch, and they usually seem to run pretty nominally. Acceptable, but not to potential.

That said, I've had a couple that ran absolutely fantastically. If the weber is in good order, and jetted correctly, they are great. But very few are. If you have access to SUs, go for it... otherwise, live with it, or get the weber tuned properly. First though... make sure all else is in proper tune. Ignition system first, fuel system second.

I'd say your speedometer needs an adjustment or replacement.. but, if it works for you as is.. well what the heck. Go with it. I never look at mine anyhow. I trust my eyes seeing the road go by more than I trust a speedometer.
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-Matt I ♥ my ♂