damper converter balance

[rjmorr89]

I have a 79 Aspen 360LA/904 I had the trans rebuilt and the guy got a balanced converter. I know it's balance because it has no weight on the converter. I bought the B&M flex plate with the half-moon shape taken out but when driving I have a slight vibration that happens every 5 to 6 seconds that last for about 2 seconds while driving. Any ideas what might be the problem? I did have the driveshaft shortened because I swapped out the 8 1/4 for an 8 3/4 rearend. Has new U joints and are tight. Any ideas would be appreciated. Thanks

 

[Mikes5thAve]

No if it's out of balance you'd notice it all the time especially at certain speeds.
If it's randomly doing it every few seconds sounds more like an ignition problem that it's dropping a cylinder or something.

 

[rjmorr89]

The motor runs fine it only has 5,000 miles on a new crate motor. Not dure if the driveshaft is the problem being out of balance or the trans mount might be bad?

 

[kkritsilas]

Is it possible that the shortened driveshaft is out of phase? Also, if you have the original torque converter, you can try to re-install it and see if it makes a difference. 360LAs, being externally balanced, normally do have weights on the torque converters, and the B & M flex plate should have addressed that, but maybe this is an "extreme" unbalanced 360 that the B & M flex plate maybe can't balance out. Just thoughts. Does the periodic vibration happen at one speed, all speeds, or just a range of speeds? Speed being either road speed, or rpm if you have a tachometer.

 

[rjmorr89]

It happens throughout the ranges of speed. It happens when I get up to 20 mph aqll the way up to 65mph. I don't have the old torque converter he used it as a core.

 

[Ash_Whole]

an engine out of balance will shake at RPM, not specific MPH. if it shakes on accell, and not an ignition issue, sounds like the drive line angle might be off or drive shaft issue