skyhigh4by

iTrader: (1)

Ive been chasing this for awhile now... 305 bored .40 over, forged pistons. The vibration starts very subtly at about 1800rpm and clears up at around 3100rpm. Its at its worst around 2200-2600 but really isnt very noticeable if the engine is under a pretty good amount of load (really bad if no load). First I tried rotating the TC with no luck so then I totally unhooked the TC and still no luck (ENGINE ONLY VIBRATION) Motor mounts are in good shape. Im using a 8 inch damper with the stock ring gear originally from the car. I just replaced all the plug wires with no luck.

This engine was running before I put it in the car and never had a vibration. I used the intake,carb, and heads from the car and the shortblock is from an early 80s truck.

Im all out of ideas..... PLEASE HELP ME

skinny z

If your sure the engine is healthy, (no weak cylinder) then you can check the rest as you've been doing. Your replacing of the plug wires makes me wonder if you're suspicious of the engine condition.
I see you've tried clocking the TC ( 120 degrees and check, then another 120 and check again ) with no improvement.
That being the case, and you say the engine ran fine before, this leads to only a couple of possibilites that I can think of.
The balancer. If it's not the original, then there could be a problem there. You say you're running the original flexplate. Just be certain that these parts are meant to be used on your application. One piece rear main blocks have different flexplate requirements than the traditional two piece. Lots of different balancers too.
Other than those two bits of the rotating assembly, contact somewhere on the frame may cause your vibration. Look for headers (or exhaust anywhere on the chassis) or the oil pan against the k-member.

skyhigh4by

iTrader: (1)

Thanks, I guess my next step will be a compression test and see where that leads me.

The reason I replaced the plug wires was because I found one had melted on an exhaust manifold a bit and I thought that maybe it was arcing. Also they were old and in need of replacement.

I cant remember if it was one piece or to piece rear main. Is there anyway I can tell what should be matched with what in that aspect?

skinny z

The casting number off the block and a trip to Mortec.com should tell you if it's a one piece rear main.
An inspection of the flexplate may find a counter weight welded on. That would be for a one piece rms block (or possibly a 400 but that's another story). Two pice rms blocks have no counter weight on the flexplate.
A compression test is always a good idea.

skyhigh4by

iTrader: (1)

The casting number is 14010203

All the info I can find on it is.....80-85 305 w/2 bolt caps. I cant find anything to tell me if its 1 or 2 piece seal.

I just got under the car and spun the flexplate around. I cant find any weights on it.

skinny z

anything pre86 will be a 2pc rms. That means no counter weight on the flexplate. Looks like you have the right part. That leaves the balancer or contact somewhere. Sometimes the balancer can delaminate and the inertia ring around the outside rotates relative to the hub. That may can a vibration issue not to mention being dangerous. If the ring seperates completely it may going flying off the balancer hub. Probably worth checking.

iroczracer07

iTrader: (2)

You might want to pull your oil pan and check your main bearings with a feeler gauge. They will cause the vibration in the rpm range you're talking about.