Anyone any good at finding vibration problems?

I have a '69 with a 434" smallblock, built Dynoed and broken in by a very good shop.
It runs good but there is terrible vibration at 60 in 3'rd gear.
It's a low frequency vibration, like 3000-5000 hz, not wheel vibration.
It seems like the engine and the drive shaft are vibrating together which causes a beat frequency of about 1 hz.

It really Buzzes the car and rattles your fillings!
I've tried changing the driveshaft 180 degrees front and rear but no difference.

It only happens under load in third
smooth in neutral
Fine in second

I have Poly engine mounts
a flat tappet hydro cam with 239 degrees of duration and 108 deg lobe seperation.

And side pipes

This engine / trans has always done it, the previous engine ( the stock 350) / trans never did it

I'm thinking about changing back to Rubber mounts, but don't know it it would matter

HELP!

 

Is this engine internally or externally balanced?

 

Internally ballanced
The dampnercrank and flywheel were all balanced as one.

Makes me think tho, maybe I should rotate the Converter a notch...

 

Manual or automatic tranny? If manual, maybe the shop forgot to install a transmission input shaft bushing on the end of the crank.
John

 

Maybe your driveshaft needs to be balanced. I'm having a similar problem in mine and that's about the last thing I haven't tried.

 

tshort
when did your's start vibrating?

 

Worth a try.

A cracked flex plate could do it, too.

The dampener hasn't gone to hell, has it? Outer rings come undone from the rubber sometimes.

As mentioned above --> How about the bushing in the tailshaft?

How about the yoke that bolts up to the pinion gear?

How about your u-joints? If one of them is on the way out . . .

 

It's started a "while" back. (It's not been run very much in the last year and a half). Anyway, while having some other work done I had them look at the vibration. They said the rear bearing coming out of the transmission was totally shot and replaced it. It then ran smooth for about 1,000 miles and the vibration has come back. It only happens in fourth gear and starts in around 65-70mph. I've had the tires balanced and it doesn't feel like an out of balance tire. The whole shifter will start vibrating real bad. I plan on taking it to a driveshaft place and have them balance it and check out the u-joints.

 

You said your old 350/trans/converter did not do this. Is the trans/converter new to this project or is it the one you had with your 350? Was it rebuilt? You can easily try the rotation of the converter, but it can still cause a vibration.

Did you change the mounting angle or height of the trans in reference to the crossmember and yoke? You mentioned the poly mounts, did you put one on the trans or just the motor? I have them on the motor only and can feel more motor, but not the vibration you speak of. It would not hurt to have the drive shaft checked. Vibrations I have found in the past stemmed from broken, loose or modified components. Of course I found them after eliminating any out of balance rotating assemblies. Now, I check for tightness and inspect my work, before I look at anything else.

 

Have you tried putting it nuetral when you feel the vibration? I had a problem with vibration at 3500 rpm's with the engine that came in my vette. When I pushed the clutch it went away so I figured it was engine related. Long story short, I blew a piston getting on the highway at half throttle at, you guessed it, 3500 rpm's. Put a nasty hole in the sleeve. Any opinions on resleeving the 72 ZR-1 block? Can it be done and is it worth it? It is a 255 horse LT-1 block based engine.

 

My feeling is that it is the tailshaft bushing. I've had that happen before. Has the driveline been balanced and checked for straightness?

 

The tailshaft bushing is something I've considered, It was replaced in the rebuild but the yoke could be worn...There is a little play in the tailshaft bushing, but the transmission guy swears it's within spec...

The Flexplate is a 2 year old SFI rated job

Dampner is a Fluid Dampner

All new U joints

Rebuilt Diff

All new poly engine and Diff mounts

The driveshaft has not been checked for straightness nor ballanced, it wasn't happening before I rebuilt everything...Kinda makes a guy wish I had done one thing at a time!

Here's one thing I just thought of,
When I reinstalled the trans I had to pull the trans over to the passenger side to make it line up with the mount holes. So now the trans in not centered in the tunnel side to side, it's off by about an inch...

 

I kind of lean towards the poly mounts. For what ever reason they don't make these the same height as the factory mounts. I have tried to use a poly trans mount on 2 different cars and both times caused virbrations. They are taller than the stock ones and change the algiment. Check this out.

 

Well for what it's worth, here is a little something I have learned first hand. Check your half-shaft mounting bolts, both at the side yokes and at the spindles (if you don't have French locks). I have seen them loosen. While you're at it, check all the U-joints. I know they are new, but if you got a bad one, it could be causing this. Next, try the above mentioned drill -- take it up to 60 or so (till it starts vibrating) and knock it out of gear. If the vibration stops, it is engine/trans related. If it stays, it is something else. If it is not engine/trans related, that is good news as it should be fairly easy to trace down from there. I would definately check all driveshaft/half-shaft bolts first though. I have had mine almost back completely out. I keep a very close eye on them now. Needless to say, I will be installing french locks on them upon rebuild.

Good luck and keep us posted.

dl

One other thing, I just reread the thread and I see that the poly mounts aren't the same height. That could definately cause the problem. The principle behind it is that your pinion angle and your trans output shaft should be parallel. If they aren't, you will get a wicked vibration. I read about this in the Chevrolet shop manual for my car -- the theory behind this alignment. If you have a shop manual, it should be in there. Anyway according to this, one side of the u-joint is accelerating while the other side is decelerating at any given time IF there is an angle present between the output shaft of the trans and the driveshaft. This creates different velocities from one half of the joint to the other. These harmonics have to be offset exactly in the opposite direction on the other end of the drive shaft. Just to make another point, the engineers had to overcome this dynamic when designing a viable front wheel driven car because of camber changes of the front wheels. Their solution was a CV joint or Constant Velocity joint.

Sorry for being long winded but if the poly mounts changed the angle of the centerline of the crank/trans output shaft, that could very well be your problem.


[Modified by dladd74roadster, 2:45 PM 4/1/2002]

 

While sitting still, in park or neutral, can you slowly raise the RPMs to a point where you can feel any vibrations? Or does the vibration only occur while rolling down the road?

After swapping from my old L-48/TH350 to a 389/700R4 I ended up with a bad vibration. As it turns out, the new 2000 stall converter had been welded together "crooked". It had a high side. I have not ran the car with the second new torque converter (2400 stall) yet but the converter was the likely culprit in my vibration problem.
Good luck in finding the source of your vibration.

 

Keith:
Could be a driveshaft u-joint. I hope it's something simple that got overlooked. Keep us updated.

 

Thanks everyone for the advice.

The thing that makes the most sense to me right now is the trans centerline, shouldn't the driveshaft be centered in the tunnel?
I have a drive shaft loop and I had to grind on it because the drive shaft was hitting it on one side it's so far over...
It could be the poly mounts that is causing the problem by changing the engine position.

Tomorrow the car is going to a frame shop to check the frame for straightness, and fix if necessary. then the next issue is the vibration. It HAS to go!

 

Just another possibility: I had a vibration like that once when a driveshaft U-joint went bad.

-Noel

 

The tranny's don't come out in the middle of the tunnel. I made a slip yoke with a good slide fit rod and slid it in the back of the offset transmission tail stock and by sliding it out it went straight to the center of the rearend. So looking at this offset in the tunnel is no way to check this alignment. I also had problems with a drive shaft loop and ground the fiberglass a little on one side to clear the larger slip yoke and everything was aligned.
I had to convince myself that this alignment while looking bad was in fact right.
It was.

 

I'm with Norval on this. Our engines are mounted .75-1" to the right (passenger side) of our cars. This is the way they came from the General. Driveshaft balance, u-joints would be the first things I looked at, along with anything changed from before when the vibration wasn't there.