The throttle cable is very sensitive to proper routing. It's really easy to have it routed incorrectly and consequently pull the throttle open. Make sure that when you work the throttle, that the mechanism returns to its base position. Visually confirm this at the carb end. Mine makes a definite click when I release the throttle telling me that the spring is returning to fully closed.
I suppose it's possible, too, that the carb synch is so far out that one or more of the throttle butterflies are open enough to cause it to rev, but you'd have to be pretty far out.
heading to the PO today, btw...