Chaining that left mount down would be an interesting trial just to see if I have a mount issue. The only thing I've done as far as checking mounts is put the emergency brake on and feather the clutch out to watch the amount of engine lift I get which didn't seem to be allot but it was only visual. Definitely food for thought.

If you get the shudder on a feathered start or a dumped start, I'd start looking elsewhere. On a dumped start the clutch is going to lock up quickly. If you have a shudder after that occurs, how could it be clutch related? It is interesting that putting the traction bars solid against the springs virtually solves the problem. You wouldn't have a broken spring or a loose differential by any chance?

I put new Super Stock springs on and didn't see any difference in shake. As far as the differential goes, I talked to Dr Diff for quite some time about the issue tapping his knowledge about differential operation and his contention was that it would not be the carrier. To go a step further a fella on A Bodies Only site that followed my problem also has the same problem with his 68 383-S car.

I also need to add to this mix that the car does not shake every time you let the clutch out hard or soft. I would say 70% shake if I put a # to it.

Also to answer a previous question, I can't say I've checked crank play but wouldn't you think I would have allot of other things going on to with motor vibration if the crank had an issue like that? It's part of thinking out of the box at this point though which is what I'm trying to do. I very much appreciate all this food for thought.

So this isn't a consistent problem? So what's different between the 30% and 70% cases? Track, weather, time of day, amount of fuel in tank........?

Nothing is consistent.

Any unusual sensations when up or downshifting when let clutch out, or does your quote of no shifting issues cover this?

Any shudder stsrting out in 2nd gear.

Assume transmission mount ok as you have removed it several times.

Does sound like plain old ordinary clutch chatter caused by defective clutch or flywheel surface or motor mounts. Pilot bearing fit of trans mainshaft ok?

What happens when in reverse? If nothing, would suspect again engine or transmission mounts

Don't suppose rear axle posi could cause this???

No issues up or down shifting.

No shudder with 2nd gear takeoff.

New tranny mount.

2 new pilot bushings. Only because I already had it apart and they are cheap.

No shake in reverse.

Dr diff says no on the posi and a contact from A bodies site has a 68 with the exact same problem. He rebuilt his posi. Did not fix.

I think I will chain down that left engine mount side and see what happens.

Was once ok and now is not?

How, when, after what did it start to shutter.

No shutter in reverse sounds like motor mounts or possible cracked K member or chassis. All bolts there torqued properly?

With handbrake on, ease clutch out in low gear. Still shutter? Can you get it on a drive on lift and have someone under car while easing clutch out to perhaps observe what may be occurring?

Driveshaft too long, causing a bind as axle winds up as clutch is released?

The car has shook the two years I've had it. According to the last owner who had it 30 years and it's been on stands for about 15, It has never shaken for him till just before he stored the car away. He got the car running then I bought it but I didn't drive it down the road before hand. He did report the fact he felt it needed a clutch.

During assembly all bolts have been torqued correctly.

I have raised the rear of the car on stands and had someone in the car so I can observe clutch, drive train action. No issues. Car has to be on the ground, taking more torque/resistance to highlight the issue. When handbrake is on, car back on the ground and easing the clutch out, no shake.

Wayne Brewer suggested also measuring the drive shaft. Shaft is the correct length.

Hey...I'm the guy from for a bodies only forum. I have a '68 Barracuda 383, 4 speed, 3:23 sure grip convertible. Unlike Tim's car, my car is not numbers matching. It is an original 383, 4 speed convertible car but it was missing the big block and 4 speed and the big rear end. Anyhow, everything on my has been rebuilt or is new. Had the tranny rebuilt, got all the rest of my 4 speed stuff from Brewers. Had the 3:23 suregrip rebuilt, etc. My car acts exactly like Tim's. I originally thought the clutch was the problem and replaced it (new fly wheel). I recently filmed the rear in slow motion with my phone in my driveway and the leaf springs jump up and down pretty violently which is what the shaking is. I was about to buy new springs when I read Tim's other post and saw that he tried that and it didn't help so I don't know what to do. I put this car together myself and it's super frustrating and embarrassing that I have to take off with no acceleration until the car is rolling or it will shake pretty badly. Someone else must have had this problem before.

Have you talked to Wayne Brewer at all about this? Especially since you got all the 4 speed stuff from him as I did. If you do, tell him you've talked to me as well. He'll remember me from last years Norwalk Monster Mopar. That's the last time I talked to him about mine.

What you describe could be caused by clutch chatter as the assembly grabs/ slips/grabs. As the power is applied the front of the differential raises as the pinion tries to climb the ring gear. how much it raises depend on how much power is being transmitted.
In order for it to act as you describe the power has to be varying for one reason or another or the front of the springs are extremely weak. One could try an adjustable pinion snubber pre loaded to stabilize the front of the differential. Doing so would minimize /eliminate the differential movement , u joint angles changing etc. on a normal take off.
The chatter if being caused by the clutch may be being amplified by the differential movement. It would still be there but should be a lot less noticeable.
I'm still thinking something in the clutch area is at the root of the problem. Another thought that might be a possibility is a bent input shaft.

I almost wish one of the cars was closer as I love resolving issues like this

Another thought I just had would be to check the air gap as the disc is being engaged with say a feeler gauge looking for irregularities. While this would not expose contaminated / irregular surfaces it might expose a bent disc, warped PP, or bent input shaft

I removed the stock snubber & tried a new adjustable pinion snubber. Played around with the setup. As far as the distance from the floor. I even set the rubber snubber against the floor. Still shakes.

Checked the tranny input shaft when I replaced the front bearing. Straight as an arrow.

You mention weakness in the front of the springs. I've already tried installing brand new super stock springs. Still shakes.

Have you tried moving driveshaft yoke as it enters rear of transmission up and down to ensure no slop at this bushing. Unlikely the cause but certainty the cause of unexplainable vibrations from driveline cruising down the freeway.

Slim chance of your problem, good news is easy to check. Should have no movement if all is well and snug.

Back to your original title, has bellhousing been throughly dialed in! Running out of ideas, had considered that, but assumed had already dialed in when replacing clutch. However do not see anywhere that it had been.

Wonder if mainshsft bearing out of transmission into clutch might cause your shutter if sloppy. I believe you said had replaced pilot bushing/bearing.

Are rear of transmission and differential shafts running true and straight left to right? You have mentioned that driveline angles are ok up and down. Am assuming U Joints are in order.

I had missed earlier comment that you had checked driveshaft for proper length.

Are you certain that rear springs are quality products with correct number and positioning of leaves. Rear axle properly positioned in springs?

I would think that a tweaked pinion shaft would cause a wealth of issues besides a shudder in low gear,,,,vibrations, broken u joints etc.

Interesting no issue in reverse,,,takes me back to engine mounts. Is engine setting at proper height in K Member. Not one side higher than other or tweaked so tailshaft from trans is not pointed directly left/right to pinion at rear,,,same as mentioned earlier in this post.

With hood up, can an observer see shudder in engine?

If you just slip the clutch gradually to move forward in low,,,,do you still get shuddder. If let out rapidly with sufficient gas not to stall, will you have shuddder starting in 2nd.

Wonder what you might see on a chassis dyno with full weight of rear wheels on the roller.

Yes I have checked for drive shaft yoke slop as it sets in the tranny. Non there. Compared to my 71 Cuda and it's pretty much the same.

Bell housing has not been dialed in. I wonder though that if that were the cause wouldn't I have other issues going on?

Replaced tranny front bearing.

Checking drive train alignment left to right would be something to pursue. I haven't checked that. U joints are good.

Mopar Performance super stock springs.

Yes gradual clutch engagement will still shake. If you just get it rolling you can boil the tires after that but hang onto your hats if you try this from a standing start.

Don't think we discussed this but does your car also shake in reverse or from a 2nd gear take off?

I am still struggling with the fact that if you set the traction bars up so they touch the springs all is good. If it was clutch chatter, how could setting up the bars affect that. I also go back to the fact that typically Mopar rear springs don't need traction bars just spring clamps. I hate to get fixated on something but this seems to indicate spring wind up. Too bad you don't have another set of springs, even lighter ones, just to see how it will react.

Spring wise though, I put a new set of Super Stock springs on it. It still shakes. Also all isn't good when the bars are set to the springs. That just makes the car bearable to drive. It still has some shake just not violent shake. Shook with my worn out stock springs too.

After doing all that to the rear of the car with the springs, pinion snubber and pinion angle. I believe it is up in that clutch area somewhere, especially since take off is good about 30% of the time. It's inconsistent.

I didn't go back and read every post but early on you state that you got a new clutch set from Brewer. What did you tell him you were going to be using your car for? Did they sell you a Hi performance disc? such as a Kevlar one? Those will grab,not slip on take off and will cause the symptoms you have. Other than that , mounts, motor and transmission. They might look OK but if they have never been changed out there 50 years old. Shocks , the same , they may look good but don't last forever.