I recently replaced timing belt and related components of my 2001 Honda Accord 2.3L automatic. For the Accord, this requires holding the engine up and removing the driver side engine mount. It has 2 balancer shafts that are run off a serpentine belt.
The car runs better than new, but between roughly 2700 and 3100 RPM it vibrates badly. Moving or stationary, irrespecitive of engine or ambient temperature. The two things that I am most suspcious of are the engine mount(s) and the balancer shafts.
Engine Mounts - I lifted the engine from the oil pan with a block of wood and hydrualic jack. Over the course of an hour, the jack had lost enough pressure that the engine was supported by the other 3 mounts only. I checked all the mounts afterwords, none of them appeared damaged or loose.
Balancer Shafts - the Balancer Belt is allegedly tensioned by letting the spring attached to the tensioner push down on the belt. This was so loose that the belt popped off when I turned the engine over a few times by hand. I ended up going with a total guess of how tight it needed to be based on my personal opinions about belts. Could I have gotten it too loose/tight?
So would either of these explain my vibrations? I'm guessing its the engine mounts, but I haven't really figured out a good way to find which one is the problem. I looked at this question, but given the... weird... arrangement of the engine/trans mounts on the Accord, I am not super-confident about it.
And before anybody guesses, the spark plug wires were all labled and ended up back where they needed to be.
EDIT: So dlu was 100% right but I wanted to add a KEY element of my solution up here so that somebody else having this problem doesn't get stuck like I did.
The rear balancer shaft on the car has 1 timing mark on the pulley and one on the engine. However, turning the pulley 360 degrees only turns the shaft 120 degrees. This is difficult to discover without an intimate knowledge of the balancer shaft or a borescope. So it is possible to have everything lined up to the factory timing marks and for the rear balancer shaft to be in the wrong place. I highly recommend the video in dlu's answer, it illustrates everything perfectly.