I have a problem which i hope someone can shed some light one. I have just installed a 302W EFI throttle body engine ('84 vintage I think) but have gone back the a carbie setup. I have used a early 10'' flex plate & C4 box. The original flex plate was a 11''. The harmonic balancer is original. The problem is I have a vibration from 1000 to 2500 RPM. I have been told that the later EFI windsor had a different size counter balance weight than the early windsors. Has anyone hear of this? I think they said the early windsors had a 28oz weight while the later EFI windsors have a 50oz weight. Is this fact? Could this vibration be anything else? Either way I have a s**t job pulling this motor out this weekend to find the problem.

were you using the same flex plate and torque converter before? maybe one of them could be out of balance?

yes, there are different flexplate and harmonica balancer weights for old and new windsors

Yep early have 28 ounce while latter [1995 on ] have 50 ounce...

'82 up 5.0L's have a 50oz imbalance
Pre-'82 have a 28oz imbalance like all other smallblocks.
You'll need a flexplate with a 50oz imbalance to go with your engine.

Pete.