Guys, I got a real head scratcher here.
1970 Dodge dart with 71 340 block and internals. The car vibrates badly at highway speeds in a pulsating vibration. vrrruummp........vrrummmp....vrruummmmpp. like that. it's done this ever since I was a kid and my dad owned the car.
here is what I have done to try to fix this problem.
1. found out the motor was a 71 external balance motor, but it had the wrong harmonic balancer and flexplate on it. so I change to the correct external balance harmonic balancer and B&M flexplate with the notch cut in it. this did not solve the problem 100%......so I moved forward with number two below.
2. I had the transmission rebuilt by a very very reputable shop and a new "zero" or neutral balance torque converter with 3000 stall made for it. At the same time I also had a new driveshaft made from yoke back....new u joints, new yoke...and computer balanced. Also at the same time I had the third member rebuilt and went from a 3.23 to a 3.73....new bearings, new gears, new pinion, new pinion bearing, etc....same yoke though.....it was a good yoke....7260 u joints.
none of that has solved my vibration problem. not a dang thing I've done above. the motor is timed right...no misses. it runs pretty good around town. there is no vibration at low speed and around town....but when I get on the highway...3500-5500 rpm....it vibrates like crazy.
wth am I missing here? the motor has not been touched when my dad bought the car in 1989, it always ran fine. it is stock 340 only 71 block with cast crank (no idea what happened to the original) it has a crane purple shaft cam and that is it. nothing fancy. stock 340 headers.
could I have a slightly bent rod or maybe a piston that is way heavier then the others? I didn't think piston weight mattered too much on an external balance motor. I really appreciate any insight you guys may have.
for pictures...go here Henry's 70 Dart Swinger
1970 Dodge dart with 71 340 block and internals. The car vibrates badly at highway speeds in a pulsating vibration. vrrruummp........vrrummmp....vrruummmmpp. like that. it's done this ever since I was a kid and my dad owned the car.
here is what I have done to try to fix this problem.
1. found out the motor was a 71 external balance motor, but it had the wrong harmonic balancer and flexplate on it. so I change to the correct external balance harmonic balancer and B&M flexplate with the notch cut in it. this did not solve the problem 100%......so I moved forward with number two below.
2. I had the transmission rebuilt by a very very reputable shop and a new "zero" or neutral balance torque converter with 3000 stall made for it. At the same time I also had a new driveshaft made from yoke back....new u joints, new yoke...and computer balanced. Also at the same time I had the third member rebuilt and went from a 3.23 to a 3.73....new bearings, new gears, new pinion, new pinion bearing, etc....same yoke though.....it was a good yoke....7260 u joints.
none of that has solved my vibration problem. not a dang thing I've done above. the motor is timed right...no misses. it runs pretty good around town. there is no vibration at low speed and around town....but when I get on the highway...3500-5500 rpm....it vibrates like crazy.
wth am I missing here? the motor has not been touched when my dad bought the car in 1989, it always ran fine. it is stock 340 only 71 block with cast crank (no idea what happened to the original) it has a crane purple shaft cam and that is it. nothing fancy. stock 340 headers.
could I have a slightly bent rod or maybe a piston that is way heavier then the others? I didn't think piston weight mattered too much on an external balance motor. I really appreciate any insight you guys may have.
for pictures...go here Henry's 70 Dart Swinger
Last edited: