84 Chevy excessive engine vibration!!!!! Need help..

I've had an 84 Chevy 4x4 w/what i think is a 454 for the last 5 yrs. I used it as a daily driver w/o vibrations of any sort. I started having starter grinding problems associated w/ a weightless flywheel being worn. So i replaced it w/ a weighted flywheel and started having vibration problems. I then swapped out flywheels, the weighted for the weightless, yet the problem persist. The local mechanic said that i threw out the harmonic balancer by adding a weighted flywheel and running the truck. So i had the balancer changed and paired to the weightless flywheel. BUT i'm still having vibration problems!!! It's isolated to the engine compartment, a sort of humming from the engine bay. The vibration starts at idle and increases with rpms, then levels out around 3/4 throttle, then starts again at full throttle. I'm thinking the cranks out-of-round...any suggestions??? My only other options is to get a rebuild from somewhere. Also, where is the VIN located on the block??  

You have a engine balance problem. A 454 is externally balanced with proper flywheel and damper. It all went sour when you changed flywheel. You need to figure out if you have a 396 (which looks the same block wise but is internally balanced) or a 454 and get the right flywheel and damper for it.  

What is obvious difference between a 396 and 454?? I can't find casting #'s anywhere. I looked on both fronts and sides.  I did find a "Hi Perf Pass" on drivers side side of block. Are the heads the same? On a different note, when i pulled off the worn flywheel, someone chiseled off the counterweight. That tells me someelse might have thought it was a 454, purchaced the weighted flywheel, and chiseled away to make it work right when it didn't. Do think it would be fine if changed the harmonic balancer to a 396 balancer??  

There is a casting number on the block behind head on bell housing flange on drivers side. The blocks look the same as do heads as even 8.1 shares same basic block and head design. I suspect that you have a 396 but casting number would clear it up. It will be a 7 digit number cast on block in raise numbers and rather than giving you the numbers (as they is a lot of them) get them and post them and I will tell you want you have and what approx year it is too.  

first determine what it is...

and ya can get the 2-3 lettters off of the pad under the alternator. that'll be listed in a motor manual and you can determine the year, displacement and what it was in from the factory.

high perf pass, doesn't really mean much, they put the blocks in alot of different rigs.

what tranny is in it?
factory bbc?  
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Casting number tells you more about the engine block itself as there was several 396 blocks and it is easy to decode. Same with a SB. In the old day when I used to lok for engine a lot, I always started with casting numbers when looking for building blocks.  

Okay. I looked on the block and found the #'s ?61959. Guess that means it's a 454. So does anyone have any suggestions?? I'm thinking it's the torque converter..  

959 casting number can be a 427 or 454 they have the same block and yes you could have messed up converter but you most likely craked the crank shaft running it with the vibration you should pull it out and have it rebuilt this would be the best in my opnion then you could find out for sure what it is.  

My info states that this casting number was only used for a 454 and from 1973 to 1985 too. (long after 427 car motor was no longer made) The last 427s made were tall block truck motors and casting number was 364776. They were made from 68 to 85.  (3969858, 3999294 and 3955276 427 tall blocks were made from 68 to 84) Last 427 car block was 3782870 made from 68 to 76.  

here are some crank #s for 454. Had to break out my napa bearing book for this, good info. Snoman excellent info on chevy stuff. 454external balance and 400chevy.  396,402 internal balance. I'm a ford guy also but messed with chevy for years and mudraced nmro circuit. N853039,N853638,3521,353039,3963523,3963524,3967463,3975945,7416,833039. On the 366,396,402,400,427 big blocks chevy casting numbers. 3804816,3863144,3874874,3882842,3882847,3882848,3882849, 3887114,3904815,3942411,6223,7115,9115.
Gonna have to pull the pan to get these numbers, good luck and the bigblock stuff and hope I could help. Nice to have a cool question to make me think a little and dig up some info. Trying to figure this kinda question out on your own really sucks bad. Been their done that.  

Thanks for the info. I'll try to check it out this wkend. Do you think the engine could have been rebuilt and replaced the externally balanced crank for an internally balanced one. I was searching on web and found several. It could explain a weightless flywheel and a smaller damper...guess i'll find out soon enough!!!  

wow this makes me wounder about my truck, my k5 originally had a 400 which is externally balanced? and so i put on 400 flywheel on it when old flywheel started squeling from starter, but just recently i found out my motor is actually a 350 which is internally balenced? so far the motor is fine with a little shaking when its ideling but im guessing that just cuz its needs some more timing tuning, but what could happen in a situation like this?  
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Hey geo45racer, sometimes in extreme situations you could trash your rear bearing or possibly crack a flexplate for automatics and that makes a strange noise itself. Its like a little chirp at first. Your thrust bearing in middle is taking some heat due to vibration probably with manual tranny. Your thrust and vibration dampner up front are taking the most abuse. If I'm correct with a 400 flywheel on a 350 your overweighted at the rear instead of underweighted? Just more stuff to worry about on top of what we have to worry about now when trying to build a better engine.  

This will sound stupid and you have probably already checked it but if it's an automatic it could be your Torque converter. If it's not tightened down it will give you a vibration.

Not sure on the VIN #