Harmonic Vibration 3500-4000 RPM

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72Scamp318

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1972 Scamp 318 Auto, been sitting for ten years. It had this same vibration during the Bush years, but it went unsolved.

I just had the rearend rebuilt (3.55 Posi). It was howling, and I thought it might take care of the vibrations too. It took care of the howling and some of the vibrations, but...

The car thrums, vibrates, rhythmically at 3500 rpm in multiple gears. It's really pronounced in the dash for some reason.

My friend, who I bought the car from, thought it might be the torque converter, which he had pulled out of 1976 dart cop car; the transmission was rebuilt at that time, but not the TC. This was 15 years ago.

I have done a little research, and it could be:
Alternator out of balance
Powersteering out of balance
Harmonic dampener failed
Rotating assembly unlikely
Flywheel out of balance
Torque Converter fail?, out of balance?

Am I missing anything? There's not a lot of threads on the net, doesn't seem to be common problem.

He told me to wait for **** to blow up, which is what he was doing. I think this thrumming vibration is doing bad things to the rotating assembly, and even after the motor goes, the problem will still exist. I'd like to solve this. How do I figure out where it's coming from? What's the best order? I would rather diagnose than just start throwing money at it.

Scamp has urethane motor mounts. I doubt the transmission mount is also urethane.
 
Have you checked the driveshaft for straightness, dents, balance and correct length? What of the u joints? good and installed correctly?
 
If it vibrates around 3500 in each gear, it sounds like it is part of the engine. The drivetrain rpm would be different at 3500 going through the gears.

Did the cop car have a 318 or 360? If unknown, check converter for welded on weights. 360 was externally balanced.

The poly engine mounts could make any vibrations more noticeable, even if something isn't actually bad. But do check the other suggestions as well.
 
Neutral: I've had it give me bad vibes in neutral before, but I just took it around the block and revved it in neutral in my driveway, and there was no thrumming vibration. I'm going to take it out for spin this afternoon.

It's a 318.

I was looking at the pulleys, and I saw that the crank pulley had a slight wobble.

I'm wondering if there's something going on with the ignition.
 
That's the first thing to determine.....only out on the road and at what speed/RPM, or in neutral in your driveway and revved up.
 
the poly engine mount i had vibration at certain rpm as well as soon as i got rid of the poly mounts good bye vibrations . drove me insane dash and steering column would make a hell of a noise
 
So I went for a drive with my cousin. His opinion was that there were multiple vibrations. He believes that there may be problem with the pinion angle; the diff appears to point up a hair. He's a four wheelin guy. He wanted to measure the angle and weld new perches on the rear end. But I'm thinking it's out of 76 Dart going into a 72 Scamp, how could the angle be wrong?

The driveline isn't stock. It was an aftermarket one. Maybe there's something wrong with it. Maybe it's too long. Maybe it's unbalanced.

He also thinks that there's a separate vibration coming from the motor. We did feel extra vibrations from 3000 to 4000 rpm in neutal, but it wasn't rhythmic. He thought the slight wobble on crank pully was giving me vibrations, but not the intolerable thrumming I'm feeling at speed. And when I look at the harmonic balancer, I see a change at idle. Tough to say it it's a mark on the balancer or it's wobblin' too.

He also felt that the flex plate or torque converter could be at play, but less likely.
 
72Scamp318 said:
1972 Scamp 318 Auto, been sitting for ten years. It had this same vibration during the Bush years, but it went unsolved.

I just had the rearend rebuilt (3.55 Posi). It was howling, and I thought it might take care of the vibrations too. It took care of the howling and some of the vibrations, but...

The car thrums, vibrates, rhythmically at 3500 rpm in multiple gears. It's really pronounced in the dash for some reason.

My friend, who I bought the car from, thought it might be the torque converter, which he had pulled out of 1976 dart cop car; the transmission was rebuilt at that time, but not the TC. This was 15 years ago.

I have done a little research, and it could be:
Alternator out of balance
Powersteering out of balance
Harmonic dampener failed
Rotating assembly unlikely
Flywheel out of balance
Torque Converter fail?, out of balance?

Am I missing anything? There's not a lot of threads on the net, doesn't seem to be common problem.

He told me to wait for **** to blow up, which is what he was doing. I think this thrumming vibration is doing bad things to the rotating assembly, and even after the motor goes, the problem will still exist. I'd like to solve this. How do I figure out where it's coming from? What's the best order? I would rather diagnose than just start throwing money at it.

Scamp has urethane motor mounts. I doubt the transmission mount is also urethane.
Click to expand...

What engine was in the cop car the torque converter came from? If it came from and externally balanced engine the weights need to be knocked off the converter.
 
Remove the belts and run it without water pump or alt or ac or whatever. Rhythmic at 3500 rpm sounds like a torque convertor to me, not a Driveline issue. There should be no weights on a 318 neutral balanced convertor or very small ones not the honking 360 weights. also the balancer wobbling is not good. It won't throw it out of balance as there is no balance on a 318 damper. And the fit of a damper on a LA crank cannot be off enough to show a wobble. Pinion should be slightly down in reference to the angle of the output shift. If the output shaft is slightly down, the pinion could be slightly up but its usually 4-5 degrees negative of output shift angle as it will climb during leaf wrap up during acceleration.
 
360 convertor on a 318 engine...cop cars were 360's as I recall.
 
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