I once tried to spin a 25-lb suckling pig that I had taken considerable time and effort to truss and mount in as balanced a manner as possible. No dice. The roti motor groaned noticeably from the instant I turned it on, which suggested to me that the weight was excessive regardless of the balance issue.
So, I had to take the pig off the spit and cook it w/o the rotisserie. It came out delicious, just the same!