Ok I bought a 383 stroker motor out of the local classifieds. I wasnt given any info on it cause the guy bought a firebird with the motor in it and wanted a pontiac 400 so he sold it. He told me it was 11:1 compression but he wasnt 100% positive. Ok now for what I do know. The motor has Eagle H beam rods, JE pistons, Stock .194 heads, performer rpm intake, 1.5 roller rockers, hydraulic lifters, not sure on cam lift and duration, looks like its been decked, looks like a balanced rotating assembly cause I can see where weight was added and taken away from the crank in some places, also has an externally balanced harmonic balancer and flywheel. I can tell that the motor must have had good heads cause I could see the stock heads were put on and then spray painted. The problem is that the motor gets a bad shake at around 3k rpm's when you rev it up. Ive asked around and been told it could be the valve train??? Thats the only thing I can think of since it seems like it has a big cam in it at idle, and the heads are bone stock with stock springs. I know thats not a good combo..........any ideas or suggestions?  thanks for the help.  

What I suspect is ugly but just to cover the bases I need to go into this in baby steps.

- Let's start with the question, is the motor mounted in anything and if so what?

- Are the engine and transmission mounts proper and OK, no rips, tears, delaminations?

- Torque converter or clutch? Are they fastened properly to the crankshaft?

- Is the chassis secure no cracks, missing parts like a "K" member, broken welds, and loose bolts in the mounts or cross member?

OK what I really think after you've confirmed the installation, which is something is loose or broken will be a lot less difficult and cheaper to fix than where I'm taking you next. 383's are tricky to build if you’re not familiar with the details of 400 type cranks. The original 400 crank was designed for an engine that is externally balanced. So between main number 1 and main number 5 the crank is not quite in balance. To get total balance of the crank assembly a damper on the front and either the flywheel or flex plate, depending on transmission, at the rear are items that have an offset balance. Makers of current 383 kits sell 2 different cranks; one is like the original 400 crank requiring offsetting balance from the damper and fly/flex. Other cranks are internally balanced and they require a neutral balanced damper and fly/flex. If you cross pollinate an externally balanced crank with internally balanced damper and fly/flex you get a shaker. If, and seems like you may have this, you take an internally balanced crank and put an off-set balance damper and or fly/flex on it, you get a shaker. Obviously to figure this out the engine's got to come apart and this stuff has to go to a balance shop along with you or a written explanation.

It is also possible for a balance shop to put an external balanced crank into an internally balanced condition by using a heavy metal slug pressed into a hole drilled in the crank counterweights, if this was done, it's pretty easy to spot. In this case, the engine is treated like any other with an internally balanced crank to where it uses a neutrally balanced damper and fly/flex.

Another fairly common problem with catalog 383 rotating assemblies that are sold in balanced kits is that sometimes some of the rods won't clear some cam lobes. This is mostly common with a bolt and nut retained cap, less so with retention by cap bolts threaded into the shank. But where it's necessary to remove metal for lobe clearance after the rotating assembly's been balanced, it is possible to re-introduce some unbalanced moments into the crank assembly.

So you can see that when building a 383 you've got to keep careful attention on the issue of parts and crank assemblies for external and internally balanced versions.

But before delving into the motor; first make sure it's fastened into the chassis properly and that the chassis isn't structurally compromised.

Bogie  

Great info by Bogie, he's right on target with everything he said. My guess is either the flexplate/flywheel is incorrect and for an internally balanced motor, or when the crank was balanced by the machine shop it wasn't balanced properly. Also, check to make sure the dampner hasn't gone bad, i.e. the rubber insert deteriorating and coming out of the balancer.

Next, I'd pull the crank from the motor, take it to a good machine shop, and have them check for balance.  

The motor is in the truck and everything is bolted up and no worn out bushings. The rotating assembly is balanced for sure. On the back side of the balancer theres a date stamped 9/12/2005 so im assuming thats the build date and that it was all balanced with the 400 flywheel and balancer. Could it be the torque converter or the stock heads with stock weak springs and big cam doing it?? The motor came out of a nice car and inside the motor looks very nice as well how everything was clearanced. thanks for the input so far.  

Your problem is definitely NOT in the valvetrain. If the springs were weak for the motor, you'd have all sorts of flat spots int he power curve where the engine woudl fall flat on its face, a sign the valvesprings were surging.

Your problem is in the rotating assembly. Is the harmonic balancer a factory unit or aftermarket? If it's a factory unit, does it have a notch cut out of the outside of hte balancer? If it's an aftermakret, what brand and part number? Is it right for your engine? Also, did you check the flexplate to make sure it's externally balanced also?  

I had the same problem,The problem is the BALANCER and FLEXPLATE.
The reason why is that if there is grind marks on the crank like weight was added or taken off then the rotating ass,is probably internally balanced.
Try a balancer and flexplate off from a 350.
                      GOOD LUCK !!!