Balance Problems...Vibration......

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pastortom1

pastortom1

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I have a 340/727 combo right now that is definitely having a balance problem, to say the least......takes all the joy out of driving.

We've eliminated the rear-end and drive shaft, and the gearing system of the transmission, since the vibration is there even at an idle.

It's like a wheel out of balance.....usually out at a low speed, then it returns at a higher speed to drive you nuts. The engine vibrates at 1400 RPM, and at 2800 RPM.(no doubt, the frequency would show up again at an even higher RPM).

If you idle the car, at 1400 it begins to vibrate EVERYTHING.....doors, mirrors, seat, EVERYTHING.....very annoying.......again at 2800, it drives you crazy while you're driving.

Now SOMETHING has to be the culprit........either the crank, or the balance of the internals, OR could it be the flywheel OR the torque converter???

The previous owner installed a 2400 stall.......doesn't slam into gear....pretty smooth.......but miserable to put up with the vibrating.

I need some advice on how to further isolate the problem. ALSO, I'd like to know if there is a fluid filled harmonic balancer or dampener available for the motor, and where to get one if needed.........anyone have any experience here?

he engine has lots of power, BUT it just ain't right......I'm afraid to romp on it until this is straightened out............

HELP with any experience you guys have had with this problem.......

Thanks........................... Doc :blob:
 
340? cast or steel crank? internal or externally balanced? start with casting #'s on block ( date code ) i have seen the inner hub spin in the balancer. is it really strange when you check the timing? ok stupid question time have you doubled checked the plug wires to make sure you don't have one crossed or out of order. you can get a fluid dampner from jegs , summit , or whereever.
 
What year is the 340? And is it a cast crank or steel? If it's a cast crank then you should have a offset weight built into the dampner and a weighted converter, if not then you will need a B&M flywheel, as most aftermarket converters don't have the weights on them. And this is what it sounds like to me that the crank is cast and the converter needs to have a weight on it, but the simplest sloution is to put in the B&M flywheel as it's SFI approved and stronger than the stock one. They run about $70.00 from summit. If the B&M flywheel doesnt do it then the engine will have to come apart, it's the only way.
Also professional products sells the best dampner for the Mopars, as you can use it with either the cast or forged cranks as they have a removeable weight, and they are one of the cheapest ones out and look very good, you can put the weight in a couple of different spots if need be.
 
I'm guessing by idle, you mean the car is sitting still and not running down the road. Have you checked the motor mounts and the bellhousing bolts? Have you just put this engine together or did this vibration just occur in a running motor?
 
The 340 is date coded 1970.......don't know if it's steel or cast......all I can say without beginning a more thorough investigation is that it's bored .30 over, has a 480 Mopar cam, Holley , internals, Edelbrock Heads, all new internals....no expense spared...

You guys have spoken to a few possibilities that we haven't had a chance to check yet........ Tuesday was the cars maiden voyage..... 2 hours out and back........

We'll check the plugs and wires to see if there's anything obvious, check the motor mounts, and try to find out from the receipts we have just what's what in the flywheel department..............if I can't find anything, I'll crawl under and check it myself........... I was worried at first that a weight flew off the flywheel or something........

YES, by the way, "at idle" means at a standstill in park or neutral.......

One more interesting note....... The vibration is WORSE (while driving) when forward throttle pressure is applied......let off the gas, even slightly, and the vibration lessens in severity..........

If you guys have any more ideas, let me have 'em......... I'll get to the specifics in the morning.

Doc :blob:
 
Had you driven the car with the stall in it before?,I had a stall that was out of balance and it had many of the same symptoms,if you power brake it slightly it would really vibrate the steering wheel.Its just a thought,also my brother had a 360 built one time and it was balanced incorrectly and it vibrated as well but Im not sure how severe it was-could be many things but hopefully its simple.
 
If it's a '70, it should have a forged crank. If they used an aftermarket piston andd did not re-balance, it will vibrate just like you say. If it's a cast crank, without the right matching dampner and convertor, it will vibrate like you say. A cylinder misfire will not get better or worse...just speed up with rpm. In any case. Do Not run the engine any more. If you feel it, guess what it's doing to the engine's internals and block. I would start with pulling the oil pan and taking a good look-see at what you really have. Also, the balancers BJR speaks of are what I use too...And I just got one for an LA engine shipped from CA to CT for $70 new in the box.
 
A steel balancer won't fit on a cast crank (and vice-versa) will it? I, too, have a vibration, in the driveline, but at 70+ mph. My motor is also a '69/70 steel crank motor. How is balancing handled on these? I've been told a few times, but always seem to forget the difference between the two. I've heard that if you have the wrong balancer, or wrong converter and you would know without a doubt. I think my vibration may be from the holes the front u-joint caps fit into on my slip yoke...I think they are worn and there may be a millimeter of play in them, just enough to cause my driveline to vibrate at speed. Are slip yokes for a 727 available somewhere, or do I need to find a good used one? I remember a few months back when I put new u-joints in, the caps slid in VERY easily on that slip yoke. Sorry, don't mean to hi-jack the thread...just another example of vibration.
 
Also all Mopar dampners have the same ID and OD, once when we couldn't find one for a /6 so we use one off a big block 383 and reindicated the timing mark and it balanced out fine. It was for a steel crank and not cast as it didn't have the little offset weight on the front.
 
Like BJR said, I'm 99% sure they are all the same dimensions.

For vibrations, if you dont do a lot of them, here's a quick list of things to make note of...

If you feel a vibe:
Does it do it with the car not moving? What rpm does it do it at? Does it get worse (meaning harder shakes) as rpms are raised? Does it go away, then come back? Does it simply speed up with rpm? Are there any noises associated with it? Where do they tend towards? (front of engine, back of engine, left or right sides?)

If it does it while you are moving... What vehicle speeds does it start (and go away)? Does it happen in all forward gears? Does it shake in the steering wheel, or do you feel it in your seat (butt)? Does the car stop evenly, or does it pull to one side or the other? Same noise questions as above.

Answering those accurately will help nail things down for the person trying to help find it. I used to have to have the customer drive me around because they were doing something I dont, and I couldnt get the problem to occur. I just spent about an hour tracing a vibe in my wife's truck..I finally found a tire with a wheel weight mark and the weight gone...but her impression was "the truck's too dangerous to drive and I didnt do anything!"
 
moper said:
If it's a '70, it should have a forged crank. If they used an aftermarket piston andd did not re-balance, it will vibrate just like you say. If it's a cast crank, without the right matching dampner and convertor, it will vibrate like you say. A cylinder misfire will not get better or worse...just speed up with rpm. In any case. Do Not run the engine any more. If you feel it, guess what it's doing to the engine's internals and block. I would start with pulling the oil pan and taking a good look-see at what you really have. Also, the balancers BJR speaks of are what I use too...And I just got one for an LA engine shipped from CA to CT for $70 new in the box.
Click to expand...
just because the block is a '70 block doesn't mean a thing i've been to machine shops and professional motor building shops where they have racks of cranks. they buy cores, split them up and part them out. they could have got in the motor and swapped the crank with another instead of messing with the original to be turned and stored at a later date. a crank is a crank? right? just kidding! there is also a difference in 318 to 340 forged cranks, the 318 won't balance out to a 340 because two of the rod throws on a 340 are drilled to lighten the crank rotating mass weight to offset the heavier rod piston combo.just a bit of info i found out the hard way. hope you figure it out so you can enjoy your sweet ride!
 
I have to believe its a balance thing. did it start after puting it together? Does it vibrate bad out of gear reving it thru the RPM range or just in gear moving?

1.) Did it start after any part was chanced?
2.) need to make sure you have the right balancer on it along with...
3.) the right Torque/Stall Converter (for your forged/cast crank)

I had a 73, 340 with 4 speed that was built before I bought it. Had the vibration when I bought it (great price) and thought it would be just a matter of swapping out the wrong balancer. Turned out it had the right balancer, but a Flywheel for a "forged" crank. Had to find one for a late 72, or all of 73 340. It has a cut out on the face of the flywheel for balancing it.
 
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