engine vibration...where to look ?

[chargerdon]

Chryco...It was professionally built and internally balanced (paid about $250 to have this done... when first installed it was very smooth...  so this has definitely crept in.

Yesterday, i did compression test on 7 of the cylinders and all were in the 195 +- 5 range.   Done while engine was warm but not hot... Throttle held wide open and two plugs at a time out.   Couldn't get the compression tester screwed into the #7 cyl as the header prevents the compression tester with its host to go in straight...will try again today.  But probably ok... 

Also, going to try today to advance/retard static timing to see if that makes any difference.   If not i think my next step (in order of simplicity) is to try replacing each spark plug wire...one at a time to see if it makes any difference.   

Then next ill take the inspection plate off of the transmission (of course have to remove starter to do this), and try to visually inspect the flex plate and torque converter to see if anything obvious..  and of course make sure the torque converter bolts are tight. 

After that will take off the engine mount to make sure it isn't broken.   I hate to do that because i have to remove the header bolts and move the header to get out the main 4 1/2 inch bolt that secures the mount to the frame.   Then ill probably have to replace the manifold gasket.   Note:   My mount i think i got from Mancini or maybe Schumaker has the safety bolt thru the rubber so that if the rubber breaks the bolt will still hold the engine in place.   

[MoparLeo]

My comment about making several changes was not about the conversion.
Obviously you have to change many things to do it. But making several changes at once when you are trying to chase down a problem.
Do you have access to a reputable garage That has a good tune-up tech ? They will have scanners, tools etc. that most people won't have at home and the expertise to find engine related problems quickly. It can cost a lot more in time and unneeded part replacement to try to find it yourself. Just an idea.
The suggestions that you get on these websites are generally from people who have had years of experience and access to a wide range of tools.
You can't buy experience but you can pay someone who has it.
If you are going ahead, then start with the engine. Recheck the tune. Don't fool with advance curves and such, just the basics first.
Pull and check all plugs, gaps etc.. Check inside distributor cap, rotor, that all wiring primary and secondary are clean and tight. Timing,
.
Just good basic inspections. Most problems are found to be simple things.
After you eliminate the engine  vibration, then you can go after the drive line problem. Most likely loose or misaligned parts. 
Verify everything is installed properly again, never assume. Good luck with your quest.[/font]

[chargerdon]

Decided to check my spark plug wires next.   They are Mopar Performance Blue carbon graphite suppressor 8.0 mm wires.   I have no idea what the ohms rating is supposed to be on these wires...  I got readings of between 3500-3800 ohms per foot for each of them except ONE.   That one read 330 ohms per foot.   Looking at that one it is NOT the same wire as it is blue but with no markings on it, so not part of the original set.  Could that much of an ohms difference cause the engine to be "slightly mis-firing" on that cylinder??? 

Anyone know what the older Mopar Performance 8.00 mm carbon graphite wires ohms rating is supposed to be ?   

[MoparLeo]

According to your readings, all the wires are bad. Should be closer to 150 Ohms per foot.

[Bullitt-]

According to your readings, all the wires are bad. Should be closer to 150 Ohms per foot.

Sorry but that's not accurate. While some wires do have much lower ohms per foot the typical wires spec out pretty much at what he's seeing. Case in point the 1st set I pulled up states 3500ohms per foot.  https://www.summitracing.com/parts/tay-70051

[dodj]

According to your readings, all the wires are bad. Should be closer to 150 Ohms per foot.

Depends on the type of wires.

But, A misfire will not manifest itself as a consistent vibration. On again, off again variable maybe, but not what you describe.

[MoparLeo]

"Typical" is subjective. I clicked on your link and the first wire I found had 50 ohms per foot....
"A wire with high resistance can be in the 5000 ohms per foot range for especially high-output ignition systems, whereas a low resistance performance wire could easily get double digit ohms per foot. Lower resistance means more of the ignition coils energy is going towards bridging the gap or the spark plug. "  We are checking "Ohms" but did we already check the simple stuff ? The cap, rotor, connections etc.. more likely to cause misfires and rough running. Don't try to make simple things complicated..  K.I.S.S.    look it up if you don't know what it signifies.

[chargerdon]

Last week i removed the transmission inspection plate...  and inspected the flex plate and the torque converter and checked the bolts.   All looked good...no cracks or anything of note. 

So yesterday went the other direction and removed the harmonic balancer.   It is a Scat brand for internal balanced engine (my engine was internally balanced when built).   Nothing noticeable on it either...    One thing that bothers me is that if you scribe a line from the keyway to the outside it lines up about 10 degrees to the RIGHT of the mark for TDC.    Is this correct ?   Or did it move on the elastomer ?   Stated another way the TDC scribe mark is about 10 degrees counter clockwise from where the keyway points.  This correct or a problem?   

[Filthy Filbert]

put a piston stop and a degree wheel on it.  find true TDC on #1.  Then compare the timing marks

[Dakota]

Since you're still searching for answers, please take a look at this thread.   It was more about noise than vibration and at a lower RPM, but just maybe....

https://forum.e-bodies.org/engine-transmission-and-rear-end/4/potential-causes-for-rattle-noise-as-my-340-approaches-3k-rpm-fixed/22987/msg275892#msg275892

[DeathProofCuda]

Last week i removed the transmission inspection plate...  and inspected the flex plate and the torque converter and checked the bolts.   All looked good...no cracks or anything of note. 

So yesterday went the other direction and removed the harmonic balancer.   It is a Scat brand for internal balanced engine (my engine was internally balanced when built).   Nothing noticeable on it either...    One thing that bothers me is that if you scribe a line from the keyway to the outside it lines up about 10 degrees to the RIGHT of the mark for TDC.    Is this correct ?   Or did it move on the elastomer ?   Stated another way the TDC scribe mark is about 10 degrees counter clockwise from where the keyway points.  This correct or a problem?

I don't know how it is for a small block like yours, but on the original damper for my 383, the keyway and TDC scribe mark are 90 degrees out of alignment.  Maybe someone with a smallblock damper can chime in.  Seems worth taking a closer look.  Good luck.

[chargerdon]

Thanks...   nobody has a small block damper on the shelf to look at ?? 

[DeathProofCuda]

Thanks...   nobody has a small block damper on the shelf to look at ??

I did an image search for "small block Mopar damper" using Google, which came up with a number of pretty good photos.  To me it kinda looked like the keyway and TDC scribe mark were in alignment, but 10 degrees isn't that much, so hard to tell for sure from the photos.  You might get more traction if you start another thread asking if someone has a small block damper to look at.

[FSHTAIL]

So at one time, you didn't have these issues and then after a while, they just started up?   ​

When you feel the vibration at speed, can you let off the gas and coast, does it still feel the same?

[73_Cuda_4_Me]

Just checked my spare 340 externally balanced damper with a degree wheel, and the crank keyway is 10 degrees clockwise to the TDC timing mark...