I have a 88 base model with a 87 N/A block installed 2 weeks ago. I am currently experiencing some evil vibrations. My mounts were used but decent in shape...but my tranny mount got the full weight of the trans in an awkward position for 3 days solid. Would bad mounts attribute to bad vibratiosn during all modes of driving?

 

Considering the mounts job is to reduce vibrations from the engine/tranny to the chasis, I'd say yea. There was a good test for motor mounts, if you search im sure you'll find something.

 

For what isn't worth, I've pulled engines to find that both motor mounts were broke. The only thing that was holding the engines in the cars was the weight of the engine & worn out tans. mounts. Oddly enough, nether of the two cars had vibration problems with the mounts like this. Not to say the engine wouldn't of been jumping around under hard driving, but under normal driving, couldn't tell.

When they brake, they still sit on their respective rubber. Though the engine moves more when driving, it wasn't enough to cause any extra vibes.

You said you just put the engine in? Using the correct flywheel & front counter weight combo? Using the correct rotors for the flywheel & counter weight?

Does it only happen when the car is moving? Might need to rotate the drive shaft a bolt over, may help. If it does, but not completely gone, rotate again. There's a sweet spot.