Same with mine, I just live with it. It's just a little tricky at manouvering speeds, but great when things get a litle bumpy.
I found that the springs on each of the Carbs all add upto my problem and there's nothing I can do. Mabe it's the same with yours, try to operate by hand the throttle control lever where the linkage from your remotes terminate ie:bottom of carbs
Looks Slow but is Fast
Member of the ebay Blue RIB cover club.