Have been working on various vibration issues with my 72 Roadrunner, 400, 727 car. I have one with the motor from around 1000 to 1200 rpm or so while the car sits. So pronounced that the steering wheel and dash vibrate quite a bit. Doesn’t do this at idle around 800 rpm or when you get in the 1300/1400 range. I unhooked the alternator, fan and power steering belts. Still have it. I noticed the crank pulley is pretty wobbly. Bad crank pulley cause this issue or does it sound more like a bad damper?

Thanks.

if you have the belts off, it doesn't take much to remove the lower pulley.
i would do that, then fire the engine to see if the vibration is still present.
while the engine is running, i would look at the damper to see if it's wobbling. if so, it needs replaced.

When did the vibration start?

Always there since the engine was built or did it start on it’s own or ?

If from day one realize that a normal 400 is externally balanced - meaning it takes a special damper and converter or flywheel.

Previous 30+ years owner isn’t sure when it started but that it’s been there for quite some time.

The damper on the motor does have a good size weight on it. Previous owner did have the motor rebuilt over 25 years ago but couldn’t recall if the vibration has been there that long or not.

What is special about the torque converter and flyweel?

Externally balanced converters will have large weights on them and flywheels usually have cutouts.

There are some externally balanced aftermarket flexplates which will have a weight added or a cutout.

Sounds like good questions for the fella I bought it from to see if he realized all that when the motor and tranny rebuilds were going on. I’m thinking this has been a long term issue with this car back when the original drive train was removed and replaced with a J400 (73). I haven’t checked the tranny yet.

First thing I would do is check to see if it has a weighted convertor or not. Years ago I had a 68 Dart 4spd car. Had a nasty vibration. Someone had swapped the 318 with a 360 and just bolted it back together with the internal balanced flywheel. The proper flywheel fixed it.

So it would be fare to say that with the weight on the damper, the motor is externally balanced and the converter and flywheel would need to comply. I would think the flywheel is external since the motor has the externally balanced damper weight, maybe. Pulling the tranny would be the only way to tell.

You don't have to pull the tranny to see the weighted convertor and flexplate. Automatics use a flexplate not a flywheel.

On the manual pull the inspection cover and look at the engine side of the flywheel for a cutout, bar the motor over 360 to check the whole thing.

On an auto same thing, pull the dust shield Andy see if there are large weights on the engine side of the converter. Not real small weights but a large one or two.

Sorry about the confusion. I have a 727 so didn’t mean flywheel but converter or flex plate.

Digging into to this after discussions here, I've found that the 73 400 externally balanced motor has a 68 727 transmission with no weights on the converter and a stock bow tie flex plate. Now sounds like I can either buy the correct weights and weld them to the converter or replace the bow tie job with a weighted flex plate? Would seem the flex plate change would be the better option if I can get a correct one somewhere. B&M 10237 flex plate appears to be a good choice.

Hopefully the previous owner running this combo for years hasn't messed up something else in the motor. Probably not good on bearings.

Thanks for all the great input !!

Got that B&M flex plate 10237 on order from Jegs, so hopefully that will do the trick.

Holler how it turns out, hopefully that works for you & I think it will.

Thanks. I think it will too. The plate should be here tomorrow from Jegs and I already have it setup with a buddy of mine to get it on a lift, pull the tranny back a little, and do the swap. Shouldn't take too long, I wouldn't think. This car has been shaking itself loose all over for well on 30 years now.

The 10237 B&M flex plate fixed my problem. Car rides like a new one now.

This may also help me fix the same kind of issue I've had with my 70 440+6 car. It is externally balanced as well and I would bet 35 years ago, someone put the wrong torque converter in it with no weights. Inspection cover coming off that this weekend to verify my suspicion.

Thanks everyone for the help !!

glad to hear you got it fixed !
as one who suffers from CRS, can you help refresh my memory a bit ?
did you say the vibration only came on at a low rpm, then disappeared at rpm slightly higher ?
sorry for the dumazz question, but i'm still happy it's fixed.

It was there at varying degrees all the time. The kicker was when the car was sitting still and Idling around 750/800 it was fine. You could rev up to about 1100/1200 RPM things would get real shaky then, just sitting still. Dash and steering wheel especially noticeable. Then would go away a bit when it got to around 1500/1600 RPM. Going down the road that same shake sequence would happen under load, plus some.

thanks for the clarification.
the vibration issues i have been involved personally with [360la, 400/440, all cast crank engines] got way worse the higher the rpm.
all were caused by torque converter balance issues and/or wrong flywheels being used.
thanks again for refreshing my memory on the way your problem acted.

I was pretty stoked when I fixed that problem because of the issue I've had with my 6 barrel Roadrunner for 35 years. I have done allot of stuff to that car trying to figure it out. Then low and behold I get another one. The 70 with the heavy rods needs balance as well and the motor has it's original damper with the weight. Getting the car out tomorrow to verify.

Thanks..