Many times this is caused by not adjusting the carb correctly.
Get something to check the true RPM like a dwell meter don't use your tach. (too much fluctuation)
Start engine get warm and set your RPM to desired speed 800-900 maybe.
Then hook up a vac gage and then adjust your curb idle screws in or out at a 1/4 turn at a time until you get the highest reading on the vac gage. DO-NOT rush this.. 1/4 turns only and then let motor run out a little. Then make another adjustment. After each adjustment re set RPM,, YOu may have to go around 3-4 times..

I have done this for many a friends,, They tend to rush this or set by ear.. Do it right and take your time..
My BG carbs had 4 screws and on twins this took me about 2 hrs,,but purs like a kitty..

Update:

Looks like my problem was a bad carb. We had rebuilt the one that was on there but it still ran the same. I finally just grabbed a carb off my other boat with 330's and rebuilt that one. It is now running better then ever. There may be some vacume leak in the old carb some place that we are not seeing? At any rate for me it was the carb. Thanks for all the help guys!