rhythmic vibration

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PostalBGraham

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I'm having vibration issues with my Barracuda. The car was originally a '67, 273/904. When I bought it 3 years ago, it had a hipo273/904 and ran with no vibration. This past winter, we put in a rebuilt '70 340, 727, headers, drive shaft, and an 8 3/4 rearend. The car vibrated horribly the first time out of the garage and had some noise. We checked compression and it's good in every cylinder - 150-160. Most of the vibration worked itself out, until it setttled into what I call a "rythmic vibration". It felt as if I was driving over mild rumble strips. It was irritating, but livable. The first thing we attacked was the drive shaft. I had it cut down and rewelded in Titusville, PA. I called the guy and he said he has no way of making sure it's balanced. I took it up to Erie Drivetrain, in Erie, PA. They checked it out and found it 110 thousandth's off. He recut and welded it, and added a small weight. We put things back together. The noise was gone, the vibration remains. Actually, shortly after this the vibration grew worse. It got to where the steering wheel was vibrating. I was running 323 gears in a 489 case. I had a set of 294's in a 741 case, so we swapped center sections. The vibration disappeared, for awhile. I first noticed it again on decelleration. Within 100 miles, the vibration is back fulltime. We are puzzled. As part of my overall upgrade, we also put in green axle bearing from Randy's Ring and Pinion. I called Randy's, and they said I need to either take the "buttons" off the center section, or have my axles ground.

Additional info - When going down the road, I shift into neutral, and the vibration goes away. When we had the car on the lift, with the rearend up in the air, I revved the engine in neutral, and the rhythmic vibration is present. When driving, the vibration changes according to rpms.

Our plan is to grind the axles. Has anyone ever encountered this before? A little bit of direction would be greatly appreciated.
 
Revving the engine in neutral and you get the vibration and it varies with rpm ?? Sounds like something in the engine or front of tranny (converter and pump) is out of balance.
Pop the driveshaft out and try it again.
 
Sounds like resonation from exhaust to me. Are there rubber isolaters in all the hangers ? Only takes 1 dime size spot where header or tubing or most anything meets the body to send a vibration all through the car. I hunted one for 2 days before I found the top of the 727 touching the trans tunnel, actually the helper spotted it. A bar and a hammer fixed it.
 
Okay, the vibration that we felt while the car was on the lift was at a constant speed, no matter what the rpms. While on the road, the vibrations increase with rpms. The faster I go, the less interval time between vibration. The opposite is true, the lower the rpms, the longer the gaps in between vibration.

We thought the vibration was engine related, except when we put in the 294 gears, the vibration disappeared. It first recurred on decelleration at 50 mph and slower. It has gradually worked it's way to being constant, regardless of speed, and regarless of accelleration or decelleration.

Exhaust has been an issue. We ran dual exhaust from Summit. All hangers have rubber insulators. Occasionally, I hear exhaust rattle while idling, but it dissappears once I get moving. Could it be there in the form of this rythymic vibration? We installed Gabriel Air Shocks to lift the rear of the car for more clearance, but the passenger side is still tight going over the axle and passed the gas tank.
 
This is just something to try to narrow down which end of the car it's coming from. Disconnect all the belts at the front of the engine. Start it up and idle it and also rev it to different rpm's. If no change then you can eliminate any pulleys or the fan or fan clutch.

One thing you can try only if you have a place to take the car and try it is to try going down a long steep hill, put the car in neutral and shut off the engine and see what happens. That takes the engine and trans pretty much out of it and then you can focus on the driveline. If it goes away you know it isn't the drive line. If no change then look to the driveline. I had a similar ordeal with my Dart and mine ended up being a out of round driveshaft. It started out small and got progressively worse. I had to have a new shaft made and that took care of it for me. At least it will give you and area to look to by eliminating engine and trans.
 
Listen........Been there, done this a number of times...........

Chances are good that you're looking at a mismatched balance between the motor and 2 components..........

The 340 you mention should be internally balanced.......There may be a counterweight bolted to the inside of the harmonic balancer up front....If there IS a counterweight, you need to remove it asap........It will cause exactly what you describe......

Secondly, some guys swap out their torque converters too for a higher stall.......if they ordered an externally balanced converter, this will also add to the rhythmic vibration. You need to remove the weight (welded) from the converter, or replace the converter with a neutral balanced unit.

Stop throwing money at it and check that balancer FIRST......then check the converter. :read2:
 
But I like throwing money at my car! It's what I do best!

Actually, I'm fortunate to have a good friend who owns a garage. (He also owns '68 and '69 Barracudas.) He lets me work on my car there and usually helps out when I get stumped.

As far as the balance of the 340, I'm not sure. The torque converter is low stall and neutral balance, no weights. I've run the engine balance issue past the two best Mopar mechanics in the area, including the guy who rebuilt the engine. They both say it's internal, and that cannot be the issue. Here's a pic before installation.


I don't know if this helps, or not...

Engine01 (Small).jpg


Engine03 (Small).jpg
 
The balance of the motor would be thrown off if someone put the counterweight on the balancer.........We took in a 340 Cuda that had the same issue......motor was just built by a Pro Shop, and they installed (or an inexperienced worker IN the shop) the counterweight in the back of the balancer..........That looks like a stock unit on the motor........If you have a counterweight behind it, it's a culprit...........:read2:
 
Here's the actual culprit of our rythmic vibration on a 70 340 that was internally balanced......someone installed a counterweight in the back of the balancer........had to unbolt the weight from the back, tap it out, and the vibration was gone......you can just barely see it in this pic.........I believe it was an allen bolt.

It's easy to check.........and after seeing this on a pro-built motor, I trust NO ONE. 8)

106_0610.JPG
 
I was planning on taking the car in to the garage early next week, we are going to check the balancer.

But, the guy who rebuilt the engine has 30+ years as a Mopar mechanic/Service Manager. He owns and maintains 11 Mopars. His oldest is a '29 Dodge. His newest is a 2008 Challenger. He has a big block Dart, a 340 Swinger, a Hemi Coronet, and so on... His credentials are good. I bought the engine from my brother who is also a lifelong Mopar fanatic. He assures me that the parts are all correct.

That being said, they both could be wrong, and it won't hurt us to look at the engine balance issue a little closer.
 
Well, we finally got around to looking at the harmonic balancer. A friend loaned me one from his spare 340. We took mine off, and there were no weights on the back. We put his on my car and ran the engine, and there was more vibration. The only difference between the balancers was where the "dimples" were located on the face. We put mine back on and ran the engine and the vibration was much less. We put everything back together and I drove home. At first, there was no vibration. I reappeared about 2 or 3 miles down the road. By the time I got home (11 miles), the vibration was back to it's normal irritating level.

I know for a fact the convertor is neutral balance. It's brand new, I put it on, and I still have the box.

We are having trouble understanding how the flex plate could cause the vibration. With so many bolts holding it on, how could it be "out of whack"?
 
I had this problem also when I changed my rear end to a 8 1/4" from a 7 1/4" and my tranny to a 727 from a 904. I also raised the back end of the car a little and this put my drive shaft at the wrong angle You might need to shim the rear end if you have done any of these things. I just wish someone would have told me about this before I spent $1000.00 on crap trying to fix it and all I needed was some $15.00 shims.. your problem sounds just like mine did though check it out...
 
You mentioned that after changing the center section, the vibration wet away, and that it came back in full force after only a 100 miles. My thinking is that your pinion angle may be wrong. It may not be off by much, but could definitely be your problem.
 
The pinion angle was something we looked at early on, and it's correct. Here's what we are thinking...there is some engine vibration, we know that. Both of the center sections I have, need help. The vibration might be a combination of those two factors. The vibration is not so bad that I cannot drive the car and enjoy it. I put over 100 miles on it Saturday. When I put the car up for the winter, I'm going to have the 489 center section rebuilt. It's got 3.23 gears in it now, I'm considering 3.55's. I'm fine with that. My mechanic recommends tearing down the engine to see if I have damaged bearings. I'm not so comfortable with that. I'm tired of throwing big money at this car.
 
Get new straps and bolts for the u-joint on the rear end. They are less than 10.00 I had simular vibration that change with gearsest but never really went away. the straps helped a lot but the biggest was getting the pinion angle correct. My car was "sagging" a little and I put an extra leaf in the rear springs to get it back up and sence have not had the viberation.
 
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