5IVE.OH

New 306 shortblock just fired up for teh first time and it has a bad RPM vibration. i have changed the balancer and it made no diff at all. The vibration is still there even with the belt off and no accessories running. It does get a bit better if I remove the bottom pulley. Think maybe the engine builder screwed something up when it was being put together?

50Trippin

flywheel could be your problem too. I would definitely get the builder involved before you destroy something internally. i would not run the engine anymore until you figure out what's causing the problem. if the vibration is bad enough you can spin or destroy bearing which will start destroying your rotating assembly.

BlackMustang02GT

check your firing order, that could be a really fuc|<er upper for bad vibrations.

1994sn95

what flywheel do u have in that thing? is it still the stock one? i believe its a 50 oz on the 302 and i think it is the same for 306. But yeat it is most likely that or it could even be the crank. but probably not likely. where did u get the rotating assembly from?

5IVE.OH

Firing order is good. I ordered a new flywheel just to be safe. I have those damn GT40P heads and the wrong headers. My number 7 plug waas broke a few times from the header being pushed up against it so I am thinking this is the root of my problem. I have dimpled the header a bit to allow for some clearance but it's not enough. Are there any inexpensive headers that clear the plugs for these heads?