You are correct. If it were externally balanced then, in addition to that side weight you have, there would be a larger imbalance weight on the face of the converter.

My thoughts are something wrong in the balancing. It was most likely always there from the start up. You have done just about everything else possible. Was this the original engine and crankshaft to the car?

Maybe it has developed a misfire. Have the plugs been looked at.

Not sure if you've already gone down this road, but maybe the trans mount? Its been a long time but I had a vibration, though I think only when driving, that went away when I replaced the Energy Suspension red poly mount with a stock trans mount.

I also am starting to think that the problem has been there since the start and I just did not notice it. It is the matching numbers 383 block. It is the crankshaft that was in the engine and I believe it is original, although when we tore it down for the rebuild a few years ago, it was obvious that it had been gone through before, so anything is possible. It is a forged crankshaft stock stroke for a 383. The shop that did the machine work for me balanced the rotating assembly. They had to take material out of the counterweights to get it to balance with the lighter pistons that I used. I did not get any data with the balance job, just a note on the bill that it was balanced. The shop is very respected and reliable, so it is hard to believe that the balance is off, but mistakes do happen.

At the beginning of the search for the source of my problem I did a RPM drop test for each cylinder by pulling each plug wire one by one. Cylinders #3 and #5 did not drop as much as the others. They were not dead, just a bit less. IIRC they were about 80 RPM and the others were around 100 RPM. I'm going to repeat this test and verify the previous results. Also going to try again to verify that the vibration is not the result of a misfire. I did pull and inspect all the plugs and did not notice anything that would suggest a misfire, but will do that again. The vibration does not feel like a misfire at least based on my experience. I don't feel or see the vibration in the engine like I do in the steering wheel and through the seats.

Yesterday I found an issue with the left motor mount insulator that I thought might be the problem. The stud on the aftermarket insulator was too long and the end of it was gouging slightly into the block. The right side had clearance. I found an original insulator in my parts stash that was in OK shape and also the rubber seemed much softer, so I put that in place to see if it made any difference. There was no noticeable change in the vibration.

Thanks again to everyone that has provided their thoughts and experience to help me figure out this problem.

Regards,

John

I was referring to this type of weight that is used for cast crank external balance engines.

Are all 3 of your mounts rubber? Poly has been known to transfer some harmonics.

Have you brought the car to the shop?
A quick demonstration of the vibration might get some address by them.
I'd expect a properly balanced rotating assembly should not be the source.
Do you have a magnet oil drain plug, or have cut open an oil filter to see if there is any obvious bearing or other metal showing up?

I have not yet been in touch with the machine shop. They are about 3 hours from me, so taking the car to them while not out of the question is not real easy. I will be in the area where the shop is located next week and plan to go talk with them. Agree a properly balanced rotating assembly should not be a source. I don't have a magnetic drain plug and have not taken the oil filter apart. However, I did remove the oil pan and it was very clean. I did not find any obvious metal or other debris.

The insulators and trans mount are rubber aftermarket items that are less than OEM quality IMO. I think I got them from NAPA. The driver side insulator is now an old rubber insulator that was on the engine when I got the car. It has some numbers on it, but don't think it is an original MOPAR part. I installed it yesterday to see if it would solve the problem. It did not. I am going to install an old right side insulator that has a MOPAR part number on it just to see if anything changes. I don't have an old trans mount to try.

John

have you looked really, REALLY, close at your transmission mount ?
sometimes they can contact the crossmember and be hard to see where, making one think all is good.

Have you done a compression check?

Wow - amazed this has not been figgered-out yet!

For entertainment sake, here's an old true Mopar story my old business partner told me about many years ago - I was in business with him from 1984-1992. A buddy of his bought a NEW 1967 Charger with a 440 in it back in the day. From day 1, the car had a strange vibration coming from somewhere. The car was back to the dealer numerous times - they replaced: the wheels, the tires, the driveshaft & U-joints, the fan, etc. They COULD NOT get the vibration figured-out. Last resort before buying the car back, was to schedule a meeting with the factory rep from Chrysler. So the day comes & my old partner & his buddy - the car owner show-up to the dealer. The factory guy is there & he says "let's take it out for a ride." The Chrysler dude is driving & they finally jump on the highway - he mats the gas pedal until the motor blows. He says: "looks like we found the problem!" The car gets a new 440 & problem was solved. True story............

That solution has crossed my mind.

I replaced both motor mounts with old used ones. One has a MOPAR number the other is an old aftermarket I believe. The rubber in them seemed much more pliable than the rubber in the new mounts. Anyway, no change in vibration. I removed the transmission mount to take another very close look at it. It looks like what it is, a brand new rubber mount. When I bought the mount, the metal U-shaped bracket that holds the insert was made from much thinner metal and looked flimsy. I removed the rubber insert with its steel shell and installed it in my original bracket. The original mount rubber was junk. So, I don't have an original trans mount to try.

I ran the engine with the trans mount removed, and the vibration was pretty much gone. Possibly completely gone. I had the car in the air on my lift with the trans supported on an under lift stand, so I had to use a ladder to get in the car and start it. I did not run it very long since my garage door was closed. I also did not get under the car while it was running to see if I could see/feel any vibration in the transmission. I may repeat this test with some additional help, but not sure what more I could learn from that. I think I've learned that the vibration is being transmitted from the engine/trans through the trans mount into the body. I suspected that was the case, since I could feel the vibration in the seat and steering wheel ( and see it in the steering wheel) but could not feel it anywhere under the hood.

I'm thinking I'll pull the valve covers first and do some inspecting of the valve train to make sure all is well there. Then maybe the intake and valley pan to get a good look at the cam and lifters. Could a bad lifter, bent push rod, bad rocker, or even a cam that is going south on a cylinder cause an imbalance in the engine firing that transmits through the mounts to the body? It is a stock flat tappet hydraulic cam with the original pushrods and rockers.

R/ John

Years back my buddy had a similar situation and i noticed he didn't have those rectangle washers under the two trans. bolts. The bolts were bottoming out and mount was ever so loose because of it.
Just another thought.
Ron

A vacuum gauge will quickly answer if you have an issue in the last bolded paragraph. A cheap often overlloked diagnostic ? tuning tool that everyone should have

Myself I would repeat the test on the lift with additional help. Might be able to buy rent or borrow exhaust hose or find something else that will take the heat HOSE LINKY That will help tremendously with the exhaust fumes.
I started out with with a kit $$, a door port and a cheap exhaust fan (for green houses?) Worked well until the fan gave it up. So I bought a Dayton Commercial unit that i set outside, hook the hoses up and I can run a car for quite some time. Great for heating the shop up if it's cold LOL 🥶☃🥶

I think pursuing your path MAY expose the issue. Do make sure the trans is at the same height as with the CM in place. one can then raise or lower it a bit, Keep us posted

glad to hear about the transmission mount investigation.
do you suppose the mount you had there was a tick too short, causing the transmission to hit the crossmember, or was it just the bad rubber it was made of ?
have you tried to space the mount away from the transmission ?
i forget. is this mount a spool type or the earlier block type ?

Unless you do this exact same test with only the trans mount installed being different, it really means nothing. You can't compare results of a test between a car on a lift with the transmission mount removed to a car on the ground with the transmission mount bolted in place. The very concept of putting the car on the lift compared to it not being on a lift just doesn't work, lots of stuff is different between those two comparisons.
Unless some serious measuring was done between the position of the tail shaft of trans on a lift stand, and careful measurements of the trans mounted on the mount, your results could be inaccurate. If the differences between the tail shaft height with out the mount and with the mount are only 1/8" different, the entire problem could be as simple as a driveshaft u joint angle bind, or a bind between the two motor mounts and the trans mount. With the trans supported on a stand, its free to move side to side and up and down as much, or as little as it pleases, and only eliminates the fact that securing the trans stops the vibration, it doesn't put the blame anywhere.
You could put the mount back in, with the bolts slightly loose and retest to see if the vibration is gone or still present. If its gone, then I would tighten the bolts to the trans and test, then tighten the bolts to the mount and test, then maybe change the order in which the bolts are tightened, and see if one or the other solves (or at least helps) the vibration. If the vibration is still present with the bolts loose, you may want to add shims (washers) to lift the trans above the mount.to see what that might do.

PB,
I agree but he didn't say whether it was a 2 or 4 post lift, If it were a 4 post with jack trays or a trans jack would be a big plus IMO

I agree that removing the trans mount does nothing to help figure out what is causing the vibration in the first place. My objective of the test was to try and figure out how the vibration that was produced in the engine/transmission was being transmitted through the seats and steering column/wheel. Since replacing the motor mounts did not help at all and I did not have another trans mount I thought that removing it would isolate the rear of the transmission from the body. I don't believe that there is anything wrong with the transmission mount that I have. There is no contact anywhere between the mount and the crossmember or body. Since the vibration occurs while running the engine in Park, driveline geometry does not come into play. I removed the mount on a 2 post lift, left the crossmember in place and removed the through bolt and two bolts into the transmission without lowering or raising the end of the transmission appreciably. I then started and ran the engine only in Park. I don't think any transmission mount will be able to dampen the vibration that is coming from the engine/trans so I am going to focus on finding the source of the vibration.

I have used a vacuum gauge a lot during this search. Yesterday I went back through the ignition system. The distributor is an original points unit that I refurbished. It does not have the electronic vacuum advance on it, it has a conventional vacuum advance can. Timing at 750 RPM idle with VA disconnected is 15 BTDC, vacuum is 16" and steady. Vacuum was measured off port on rear of carb. VA is hooked to a ported signal from the front of the carb. Carb is an original Holley 4368. Timing all in at 2850 RPM is 35 w/o vacuum advance.

With the engine idling I took a RPM reading from my timing light and then removed the wire for a cylinder at the distributor cap and recorded a second RPM reading. I repeated this for each cylinder and at an initial ( all wires in place idling normally) RPM of around 1040 and a second test at a RPM of around 740. For the 1040 RPM test the RPM drop varied from a high of 115 RPM to a low of 55 RPM. For the 740 RPM test the high was 50 RPM and the low was 30 RPM. With a plug wire disconnected the RPM's jumped around a lot, so I had to try to make an estimate of the average RPM from watching the numbers on my timing light. This obviously introduces some errors in the data. Other than confirming that each cylinder is firing I'm still trying to figure out what the data is telling me.

I also used a IR temperature gun to measure the exhaust temperatures as near to the outlet of each cylinder as possible. They varied from 640F to 540F on all cylinders but #1. It measured 470F to 490F on multiple tests. Again, not sure what the low temperature on one cylinder is telling me. Temperature variation could be just leaner or richer mixture at each cylinder since I'm relying on a carb.

I've been working on cars and engines as a hobby and made my living as an engineer working in and around lots of industrial machinery for more than 50 years. I've never run into a problem like this that seems to have no answer. I'm sure the answer is there and I will find it eventually, but it is at this point trying my patience. Thanks again to everyone that has helped me try to figure this out.

Regards,

John

Couple other things to try to isolate.
Take a half empty bottle of water with the cap tight and hold it here and there in a manner the you can observe the "rings" formed on the water surface. My mechanic mentor (in the early 70's) showed me this and it surprisingly helped isolate location of various vibrations (end to end, side to side)..
Ratchet straps over transmission and cinched tight to cross member can help eliminate mounts as source. put one over the bell housing to uni frame, and they can also be used to pull the front of engine down (up or side to side). Try different configurations.
With all that my gut feeling is it is internal, or t.c. Also not mentioned is fif there was any major structural replacement of the uni frame assy that may need inspected.