70 aar 340 engine balnace question

[badfish107]

I have a 70 aar 340 six pack, runs very nice, just bought it and around 2800 rpm and above the dash vibrates, are these engine internally balanced or externally balanced, wondering if I have the wrong flywheel or something(stickshift). engine has 650 miles on it

[734406pk]

I have a 70 aar 340 six pack, runs very nice, just bought it and around 2800 rpm and above the dash vibrates, are these engine internally balanced or externally balanced, wondering if I have the wrong flywheel or something(stickshift). engine has 650 miles on it

Welcome to the site! Very nice car you have there! Do you have a numbers matching engine and transmission in your AAR? Somewhere in '72, the 340 changed to a cast crank altering the balance. Is the vibration you see @ 2800 rpm in neutral at rest or at highway speed? 

[jimynick]

In addition to checking to see if you've got a mix-up in balancer/flex/flywheel, see if the vibe is there when the car is stationary. It may be as simple as a wheel/tire balance if it only occurs at that rpm while out on the road. Just a thought. 

[734406pk]

In addition to checking to see if you've got a mix-up in balancer/flex/flywheel, see if the vibe is there when the car is stationary. It may be as simple as a wheel/tire balance if it only occurs at that rpm while out on the road. Just a thought. 

Exactly!

[70chall440]

As another possibility; drive shaft..

[badfish107]

she is smooth at idle and revs in neutral, its a non numbers engine but a real aar 340 engine, hiway cruising she is also smoothe, just when you rev it up 2nd to 3rd and so on the dash vibrate on a hard pull like the flywheel is not right. weird only on a rpm pull

[70chall440]

So it vibrates under load. If it was a flywheel it would vibrate at least some regardless of load. IMO I would look at the drive shaft and pinion bearing. That said, I would start by having the tires all balanced just to be sure, then move on to other things.

[IRON MAN]

If the vibration issue is not resolved after trying the above suggestions check the motor mounts. A friend chased a vibration problem for months and the problem was a bent axle flange. Good luck.

[734406pk]

  I thought maybe you may have a post 72 340 with pre 72 flywheel etc, but since your vibration issue appears only under hard acceleration, engine balance is not an issue IMO. I would suggest (same as above) checking the drive line components and motor mounts (transmission). Trying to find a load related dynamic vibration can be tough but there are several checks you can make with the car at rest. Some examples:

-Pinion angle in relation to the drive shaft. It should be about 2 degrees NOSE DOWN at rest, definitely not NOSE UP.
-Cracked leaf spring(s).
-Worn leaf spring bushings
-Cord separation on the rear tire(s)
-Worn U joint(s)
-Bent/damaged drive shaft   
etc.

It should be pretty obvious when you find it I think. Post back your findings and good luck!

[AARuFAST]

Check your tires if you have BF Goodrich. Known to have ply separation.
Is it only the dash that vibs. May not have all the screws supporting it.
Could be solid motor mounts..