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

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QKFSH67 said:
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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Factory balancer steel crank. Converter is neutral balance. i wish it was as easy as finding a weighted converter when I shouldn’t have one.
Going to check a few things tonight again and may end up yanking the trani to do a new converter anyway as I really don’t like this 3000-3200 rpm stall that’s in it now. It’s a mild built 340, should have around 2400 rpm converter at most imho.

May also remove the oil pan to double verify that’s it’s a forged crank too. As far as I know they both look the same externally. It’s and early 72 motor and came with a Steel crank from the factory (well prior to the serial number range where the cut of date was before changing to cast cranks) and I’m assuming the engine guy who was well know for mopar work back when it was done (23-24 years ago) knew enough to put the right balancer on the car
 
I assume this has been a issue the whole time you've owned the car?

Some of us are more sensitive to feel, noise, smell, etc. Trying to diagnose over the net can be a real challenge. Meaning it's difficult to tell how severe or not a certain symptom is.

I don't know if you have someone close, maybe even a member, that could come by and give you a second opinion before you start tearing things apart or spending $$$ that might not be necessary.

Crazy as it sounds, I once worked on a brand C for a guy and noticed a very similar feel to what your describing. The engine had a very mild hydraulic cam and when i went to install new VC gaskets i noticed it had polylocs for adjusters. So while i was in there i did a quick running valve adjustment and found the preload was multiple turns down, so i set them like i liked them with a fairly light preload. Gaskets on and buttoned up and out for a test drive and didn't feel that weirdness anymore.

Good luck finding your issue, i'll keep checking in. :thumbsup:
 
If you pull the pan I would check the rod bearings or at least one or two. If you've had this vibration as long as it sounds there's a chance it's hammering a rod bearing out.
 
How in the world can anybody think that it's an imbalance problem when it doesn't increase with rpm? REALLY?!?
 
Used to balance at work for industrial applications to 60k. Balance is a matter of tuning for a particular operating range. Every rotating assembly has a "critical" and a smooth range.

Usually automotive has the critical near idle and is not impactful during the higher power range. On higher rpm industrial equipment the critical is often in the mid point of the peak rpm which the equipment passes through as it seeks its full speed. Modern electronic drives often have features to lessen the effect of the critical range by insuring not dwelling or allowing running under load through the critical range.

That said, its pretty hard to offer more than guesses online regarding the OP's problem.
 
Every single vibration that I've seen that was caused by imbalance; wrong converter, wrong flexplate, spun harmonic balancer; INCREASED with rpm. If the vibration goes away at higher rpm; then it is NOT balance related.
 
CudaFactHackJob said:
Every single vibration that I've seen that was caused by imbalance; wrong converter, wrong flexplate, spun harmonic balancer; INCREASED with rpm. If the vibration goes away at higher rpm; then it is NOT balance related.
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Harmonics can make vibration come and go and be more predominant at specific ranges. My specific vibration is predominant between 1000-3200 ish (coincidentally the range of the converter) does in in neutral and park too when the car isn’t moving also, and does seem to be less after 3200-3500rpm but still faintly there in the same rhythm as the lower rpm vibration range.

I hope to find it soon. Likely going to change the converter regardless as I don’t really like the 3000 stall to begin with…. This will eliminate that piece once and for all, as even if a new converter were bad I doubt it would mimic the same vibration as I have now.
 
CudaFactHackJob said:
If it begins to decrease when rpm is increased, it is NOT balance related.
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What makes you say that? If your RPM increases your vibration becomes faster to the point where it doesn't feel like it did at lower RPM.

Well changing the converter didn't cure it but we now know it is in or on the engine.
 
I'm shocked at how many people don't know that the converter change wouldn't fix the problem. It's not balance related. Don't take my word for it; google "centrifugal imbalance"..or something related to that...I don't have time to find more sources for the obvious.
 
CudaFactHackJob said:
I'm shocked at how many people don't know that the converter change wouldn't fix the problem. It's not balance related. Don't take my word for it; google "centrifugal imbalance"..or something related to that...I don't have time to find more sources for the obvious.
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So you're telling me that an improperly balanced converter would not have an effect by changing the converter out? Where do you come up with this stuff?
One of the reasons for changing the converter was that he did not like the RPM that it engaged at.
I think he has a cast crank balance job with a neutral balancer or something strange like that.
 
CudaFactHackJob said:
I'm shocked at how many people don't know that the converter change wouldn't fix the problem. It's not balance related. Don't take my word for it; google "centrifugal imbalance"..or something related to that...I don't have time to find more sources for the obvious.
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You ain’t dealing with “centrifugal balance”.
 
My 64 poly318 PB '64 Belvedere developed a pretty good vibration if I put my foot in it some at takeoff. qt low of ATF!! ha
 
Jadaharabi said:
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So you're telling me that an improperly balanced converter would not have an effect by changing the converter out? Where do you come up with this stuff?
One of the reasons for changing the converter was that he did not like the RPM that it engaged at.
I think he has a cast crank balance job with a neutral balancer or something strange like that.
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My two cents: I think it all goes back to the rebuilder. I would first change the balancer, just in case it did slip. If not, then I would drop the oil pan and windage tray and check the crank, either forged or cast, then, with a flash light if possible check the connecting rods numbers if matching. Are all the bolts on the balancer the same size or missing one or two? Is a incorrect balancer pulley being used and not centered. Not sure of the reason for the "DOT" on the stock flexplate, but it should be opposite of the plug on the converter, is my understanding.
 
How can so many people not know that an actual imbalance caused by weight doesn't get worse with higher RPM?!?!?!?!
 
OMG fact is anybody who disagrees with me has NEVER fixed an actual imbalance. GOOGLE it people...but a word to the wise is sufficient....where are THEY?!? HAven't any of you actually replaced an incorrect converter before? I HAVE...the imbalance vibration doesn't GO AWAY the higher you rev it. Don't you people know ANYTHING about physics?? Did you even graduate HIGH SCHOOL?!?...And I don't mean PUBLIC SCHOOL....lol
 
CudaFactHackJob said:
OMG fact is anybody who disagrees with me has NEVER fixed an actual imbalance. GOOGLE it people...but a word to the wise is sufficient....where are THEY?!? HAven't any of you actually replaced an incorrect converter before? I HAVE...the imbalance vibration doesn't GO AWAY the higher you rev it. Don't you people know ANYTHING about physics?? Did you even graduate HIGH SCHOOL?!?...And I don't mean PUBLIC SCHOOL....lol
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Real work example. A 500 cc single cylinder two stroke crank is OVER balanced. It will literally shake at idle but as the RPM goes up it smooths right out.

Cranks go in and out of balance. V8 crank balancing is misunderstood by many.

I’m asking again. Have you ever balanced a crankshaft? A tire?
 
CudaFactHackJob said:
OMG fact is anybody who disagrees with me has NEVER fixed an actual imbalance. GOOGLE it people...but a word to the wise is sufficient....where are THEY?!? HAven't any of you actually replaced an incorrect converter before? I HAVE...the imbalance vibration doesn't GO AWAY the higher you rev it. Don't you people know ANYTHING about physics?? Did you even graduate HIGH SCHOOL?!?...And I don't mean PUBLIC SCHOOL....lol
Click to expand...
Is it not possible, that an imbalance at a lower speed would be more noticed, because the revolution would be slower. At a higher speed the vibration would be still there, but not noticed as much, because the vibration would be faster and would seem as to be leveling out as one. It may show up on a scope, with a read out. I'm sure the bars would level out.
 
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