Shakin at 40mph

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AlaskaMoParMan

AlaskaMoParMan

NDT Technician
Joined
Aug 1, 2011
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Location
ALASKA
Hey there

My 68 Barracuda FB 4 Speed that I just got has a terrible shake just as I hit 40mph and smooths out if I let off the gas. I'm just looking for places to start looking to get this under control. The motor is a mild 360 (415hp) and the rear has a 456 geared suregrip. Tires (skinnys in front and fatties in the rear) have all their balance weights and front end rubber is original as far as I can tell. The previous owner tells me the car did not act like this prior to me getting it (had it shipped up from AZ). The car was an original /6 with a 4 Speed (I know, I know) with a 360 crate installed with a new bellhouse. Previous owner drove it through high school with the /6 and 4 Speed and had a 360 put in a few years ago.

I'm losing time (snow will be here soon) and want to drive this thing! Can any of you guys/gals out there help me? Any and all advice will be surely appreciated!!!

Thank you in advance

AlaskaMoParMan

P.S. Car has air shocks in the rear also, if that helps. Drag stance if you know what I mean!
 
If you lower the rear a bit does it still vibrate ? Could be drive shaft angle . Does the vibration go away at higher speed ?
Could be anything from worn LCA bushings to bad tire , wheel .
 
1.Old bias style tires out of round. Cause by the car setting for some time.
2. Weight came off drive shaft.
3. Driveshift. slightly bent.
4. Check lugnuts.
5. Racing wheels? Not good for the street.

Hope this helps.
 
Brand new M/T tires front and back on Centerline rims. Kinda gets better at higher speeds, didn't want to push it too hard...felt like it was coming apart. Haven't let air out of shocks yet, and tires have weights on them. Lug nuts tight. Trueness of drive shaft not checked...weights not looked for. Goes away instantly as soon as I let off the gas and gets back into it as soon as I give it gas. 4th gear usually is where I'm at during these fits of shaky shaky. Just looking for places to start looking, but car will be garaged for the next 7 months so I was looking for stuff I can do while it is cooped up.
 
AlaskaMoParMan said:
Brand new M/T tires front and back on Centerline rims. Kinda gets better at higher speeds, didn't want to push it too hard...felt like it was coming apart. Haven't let air out of shocks yet, and tires have weights on them. Lug nuts tight. Trueness of drive shaft not checked...weights not looked for. Goes away instantly as soon as I let off the gas and gets back into it as soon as I give it gas. 4th gear usually is where I'm at during these fits of shaky shaky. Just looking for places to start looking, but car will be garaged for the next 7 months so I was looking for stuff I can do while it is cooped up.
Click to expand...

Check the transmission mount. Not too likely but check harmonic balancer to see if it could be moving around because of bad rubber.
 
Motor is brand new and seems to be balanced. Will check trans mount though. Thanks for all the help...keep the ideas coming and I will post what I've discovered. Thanks again!!
 
quote:"Goes away instantly as soon as I let off the gas and gets back into it as soon as I give it gas."

I am not a gearhead, but this quote leads me to believe that your problem is not related to the tires and wheels as the vibrations stops when you let off the gas but the tires and wheels are still rolling, so if it was a tire and wheel problem the vibration would continue. JMHO It sounds as if the problem is in the drivetrain rather than an in balance in the harmonic balancer. Just a suggestion, raise the wheels totally off the ground, a lift would be nice. four jack stands will work though, start the car and see if you can duplicate the vibration in your garage and that will be easier to diagnose than going down the road at 40 mph.

Good luck to you. You will find the problem it just takes time.
 
Just got off the phone with my MoPar buddy and he said the same thing, only he said to do it outside...just in case!!! I'll see what that brings me...
 
Is the harmonic balancer after market or factory?
Can you jack the car up and run it thru the gears? See if you still have the shake.
 
That's the plan, got me two sets of 3ton jack stands that I've been wanting to use...wish me luck...
 
Check your motor mounts.
 
Good idea, thanks. Hopefully up on stands will lead me in the right direction...if it don't go scooting down the driveway into the gully at 50mph!!!
 
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