340 -727 -3000 stall- Vibration under load - Trying to find source

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4mulas

4mulas

Fixem'up
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I’ve searched many threads on vibration etc before posting here.

Engine/trans combo info:
Steel crank internal balance early 72’ 340, balancer rubber in good shape and not slipped.
727 rebuilt with 3000 stall converter. Converter foot brakes to 3000 maybe even 3200 before it boils the tires off. Don’t know brand of converter, was told it had bearings from the seller of the car. I checked, No weights on the converter. Transgo shift kit. Shifts super crisp.

trani mount okay. New rubber motor mounts. Took fan belts off to eliminate fan or alter/water pump as source etc…. Checked for converter weights - none found.

the problem:
Have a droning vibration predominant at off idle to 3000/3200 rpm ish then smooths out when above that range (still very very slight above 3200). Vibration is in any gear under load and at constant throttle settings.

When rolling/driving in vibration rpm range (1000 - 3000) and click into neutral and engine idles down but car is still travelling fast the vibration essentially goes away. Tells me not tires, or driveshaft/u-joints etc. on pulls above 3200 ish iP to 6000 rpm fairly smooth, liveable anyway but you can still get ever so slight Light drone sensation.

when I say Droning vibration I mean the vibration is pulse like, comes in and out in a constant repeatable rhythm.

I’m thinking it’s the converter poorly made or not properly balanced. Want this gone… any help appreciated

Thoughts?
 
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I know this is a silly question but do you have number five and number seven spark plug wires switched?
 
Factory, I bought the car as is. Rebuilt trani, new converter. Yes it’s silly, the wires on right… :)
 
I’m just wondering how u know that the balancer hasn’t slipped? Possible be u joints or a bent drive shaft. Is the exhaust rubbing against anything? Tail shaft bushing could be worn. Is the yoke far enough into the tranny. Possible rear diff noise as it can follow the drive shaft. Kim
 
Personally I would be looking at that crankshaft to see if it's a cast iron instead of a forged Steel and if they put a neutral harmonic balancer on it.
 
Jadaharabi said:
Personally I would be looking at that crankshaft to see if it's a cast iron instead of a forged Steel and if they put a neutral harmonic balancer on it.
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That is actually possible. The fact is you don't know the history of the build and therefore you are on a witch hunt. I hope this would not be the case but you never know.... Unless someone gets lucky this type of problem could turn into going through the complete drivetrain....

JW
 
oldkimmer said:
I’m just wondering how u know that the balancer hasn’t slipped? Possible be u joints or a bent drive shaft. Is the exhaust rubbing against anything? Tail shaft bushing could be worn. Is the yoke far enough into the tranny. Possible rear diff noise as it can follow the drive shaft. Kim
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The timing is okay? If it slipped wouldn’t you get erroneous readings?

Slipping the car into neutral at speed and in the know vibration range almost eliminates the vibration so that eliminates the driveshaft as it’s is still spinning at the same rate, as are the u joints, granted the rear is a tiny tiny bit loose and will get changed right away anyway. I can’t see how a noisy diff would cause this and again is still spinning at the same rate when slipped into neutral?

tail shaft bushing is okay and the yoke engages the whole length of the bushing, although I would like to see about 1” more engagement on the slip yoke.

talking out loud is great and I hope to find a good idea I haven’t thought of that may lead towards a fix.
 
Jadaharabi said:
Personally I would be looking at that crankshaft to see if it's a cast iron instead of a forged Steel and if they put a neutral harmonic balancer on it.
Click to expand...

how can you tell without pulling the pan? They are outwardly appearing the same, no?
 
SLOPAR72 said:
That is actually possible. The fact is you don't know the history of the build and therefore you are on a witch hunt. I hope this would not be the case but you never know.... Unless someone gets lucky this type of problem could turn into going through the complete drivetrain....

JW
Click to expand...

i agree, it could be possible, however the seller of the car did reference an known builder who went through the engine and I am making the assumption that they wouldn’t have put the wrong balancer on it.

I agree, it could be a witch hunt and I really don’t want to go through the whole drivetrain but in the end may have too.

all of this already makes me want to scrap the headers in favour of manifolds as it will make removing starters and trans inspection covers to remove the trans and try converters etc….. also in case I have to remove a motor to disassemble and have balanced etc…..

but based on it the being a pulsating vibration (in and out at the same repeatable rhythm) it is pushing me towards transmission. If it were motor wouldn’t the vibration be constant, and all the time through the whole rev range??

oh yah @oldkimmer the exhaust isn’t touching anything either.
 
4mulas said:
i agree, it could be possible, however the seller of the car did reference an known builder who went through the engine and I am making the assumption that they wouldn’t have put the wrong balancer on it.

I agree, it could be a witch hunt and I really don’t want to go through the whole drivetrain but in the end may have too.

all of this already makes me want to scrap the headers in favour of manifolds as it will make removing starters and trans inspection covers to remove the trans and try converters etc….. also in case I have to remove a motor to disassemble and have balanced etc…..

but based on it the being a pulsating vibration (in and out at the same repeatable rhythm) it is pushing me towards transmission. If it were motor wouldn’t the vibration be constant, and all the time through the whole rev range??

I have dealt with an unbalanced Mopar setup (Cast crank motor that had an incorrect flexplate on it) and would shake the Glass out the car at idle. I am thinking this may not be your problem.... It does appear as though it is something past the Engine. You can disconnect the Driveshaft and spin the Rear axle assembly to see if there is a constant eb and flow. I have also seen a Driveshaft too long creating a Vibration issue but it's been too long ago to remember the details. I would try to eliminate the easiest and least costly to begin with.

JW

oh yah @oldkimmer the exhaust isn’t touching anything either.
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4mulas said:
The timing is okay? If it slipped wouldn’t you get erroneous readings?

Slipping the car into neutral at speed and in the know vibration range almost eliminates the vibration so that eliminates the driveshaft as it’s is still spinning at the same rate, as are the u joints, granted the rear is a tiny tiny bit loose and will get changed right away anyway. I can’t see how a noisy diff would cause this and again is still spinning at the same rate when slipped into neutral?

tail shaft bushing is okay and the yoke engages the whole length of the bushing, although I would like to see about 1” more engagement on the slip yoke.

talking out loud is great and I hope to find a good idea I haven’t thought of that may lead towards a fix.
Click to expand...
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I am disagreeing with your logic on the slipping it into neutral at the vibration speed. That would not eliminate the drive shaft that would eliminate the engine and transmission because your drive shaft and everything behind it is still spinning at the constant speed and your engine RPM would have dropped off. I may be wrong on that logic but I highly value my opinion. Lol
 
If you're sitting still in neutral and slowly rev the engine up do you get the vibration just sitting there?
 
Jadaharabi said:
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I am disagreeing with your logic on the slipping it into neutral at the vibration speed. That would not eliminate the drive shaft that would eliminate the engine and transmission because your drive shaft and everything behind it is still spinning at the constant speed and your engine RPM would have dropped off. I may be wrong on that logic but I highly value my opinion. Lol
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think you’re looking at this wrong. Picture this. You’re Travelling 60 mph, revving 3000 rpm it vibrates. Click to neutral vibrations drop as it goes to 1000rpm but the car is still travelling 60 mph. Which means the drive shaft is still going 60 mph too. If the driveshaft was the source of the problem it would still be vibrating.

to me this says the source of the vibration is motor or trans. It was vibrating at 60mph at 3000 rpm but has almost instantly gone away when revs dropped to 1000 in neutral and the driveshaft still spinning at 60mph rate.
 
The drive shaft may still be spinning at 60 mph but it is not under any load then. Neither is the differential. See where we’re going with this? Ok try this. Drive it at the same 3000 rpm in first and second and see if u have the same vibe in those gears. Kim
 
Very unusual that it smooths out at higher rpms. Shift to neutral while driving and hold it at 3k instead of letting drop to idle, no shakes? Shakes in neutral @ 3k rpm in the driveway the same as when driving? If it always shakes at 1-3k rpm, see Post #9-10, or pistons replaced with aftermarket slugs of significant wt diff and not rebalanced.
 
Get an 02 sensor welded into the exhaust and hook it to an air/fuel gauge. I bet the "imbalance" shows up when you hit the bottom of the lean scale.
 
Outsder said:
Very unusual that it smooths out at higher rpms. Shift to neutral while driving and hold it at 3k instead of letting drop to idle, no shakes? Shakes in neutral @ 3k rpm in the driveway the same as when driving? If it always shakes at 1-3k rpm, see Post #9-10, or pistons replaced with aftermarket slugs of significant wt diff and not rebalanced.
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OP needs to have a solid list of to-do's and get on a chassis dyno for an hour or so. Thinking back to when I had an imbalance issue on a motor it didn't really show up at idle but driving under load it was quickly evident something was wrong.

Do you have a local Driveshaft shop that can verify you don't have a jacked up Driveshaft?

JW
 
oldkimmer said:
The drive shaft may still be spinning at 60 mph but it is not under any load then. Neither is the differential. See where we’re going with this? Ok try this. Drive it at the same 3000 rpm in first and second and see if u have the same vibe in those gears. Kim
Click to expand...

I did start thinking about load/no load as you mentioned. It does vibrate equally in 1 2 and 3rd gear at the same rev. Also in park and neutral. Which is another reason I was leaning more towards engine in balance or converter. As it does vibrate when it’s not moving.

What about the stall converter being pulled to far forward to meet the flex plate? I haven’t checked that yet. I know many say to shim the flex plate to converter to achieve 3/16-1/8” spacing of the converter to pump from the fully seated position?

the diff is a tiny bit noisy, but was declared fully rebuilt. To me it doesn’t feel like that, but now, perhaps more than before I may be leaning towards driveshaft and rear unjoint that is a tad loose.
 
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If you rev it to 3000 in neutral sitting still and it does it, you should start at the torque converter. If it has a neutral balance torque converter on a cast crank which requires external balance. If the converter don’t have a big weight on it where flexplate bolts on then it is neutral balance. If that’s the case you can get a different flexplate from b&m that will correct that problem,, or change converter… Does the engine have a factory balancer on the front or an aftermarket? Some of the aftermarket’s will have a small bolt on weight for a cast crank engine…
 
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