Engine Vibration 3k RPM

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If the rotating assy wasn’t balanced, it really should be.[/QUOTE]
dadsbee said:
Sounds like a fuel delivery stumble and not an engine vibration to me, why I questioned it in my first post.
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512Stroker said:
Lets do the easy things first.
Check the torque converter bolts make sure they a tight and loctite has been applied.
Remove all the drive belts from the front of the engine, water pump, alternator, etc with the car in park check for your vibration you may have a fan or pump that's going south. Make dam sure you do not overheat the engine doing this, start with a cold engine.
Post some close up pictures of the spark plugs so we can see the timing mark on the ground strap.
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I removed and re loctited and torqued the converter bolts, no change. I will add taking the belt off to my list of things to check this weekend. As well as add pics of plugs to this post.

PRH said:
KB flat tops for a 340 are over 150g lighter than factory pistons...... each.

If the rotating assy wasn’t balanced, it really should be.
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That is the bummer part - it was a long time ago and my first build. I was/am young, and I though balancing was only really needed in a high hp build. I wish the machine shop would have suggested it. Looking back, I bought the motor complete and it was already .030 over. So I thought I could just re ring and bearing it, but there was too much clearance in the cylinders. So it had to moved to .040 and get new pistons, went ahead with a full rebuild.

512Stroker said:
"nonsense" hold on there partner, we/I are only giving the OP suggestions to check for it is up to the OP which path to follow. I agree that the rotating assembly could be the issue, but I would try everything else before I take the engine apart.
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I appreciate it. I want to be sure that I am not mis diagnosing it, either.
 
Dartswinger70 said:
if you can verify it isnt a misfire, Id say its your rotating mass. i had a similar problem and it was a defective pressure plate.yes I know yours is auto but I m just saying it sounds similar. Mine was so bad the dash rattles sitting still
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you may have to try a new converter etc. I had pulled my trans more than once figuring mine out...you could try a balancer first etc.
 
Dartswinger70 said:
if you can verify it isnt a misfire, Id say its your rotating mass. i had a similar problem and it was a defective pressure plate.yes I know yours is auto but I m just saying it sounds similar. Mine was so bad the dash rattles sitting still
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See that is the thing. Nothing in the car rattles. Its not that kind of vibration, I can feel it in my seat. It was interesting because I thought for sure it was pinion angle/driveline related so I took it my driveline guy and we drove it and he immediately went to engine vibration. He is the one who had me rev it in N to confirm.

He shares the lot with a racecar builder and he came over and watched the engine with the hood open, while I revved and he asked "whats vibrating?" I said sit in the passenger seat and you can feel it in your bottom.
 
Dartswinger70 said:
you may have to try a new converter etc. I had pulled my trans more than once figuring mine out...you could try a balancer first etc.
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I had a stock converter first, and then moved to a built one and the vibration is the same. I had thought about a new balancer, but want to rule out mis fire and timing jump first.
 
Yeah man try and find out what’s wrong before having to pull it out. It really sucks to have an engine in the shop for a problem but there isn’t a smoking gun during tear down.

this is what makes this unique and interesting - it was .030 over when you got it to freshen up. Then it was fucked so you bored it out. Did the previous guy balance his rotating assembly? And is it in your favor or does it actually make the vibration worse!?
 
T56MaxTorq said:
Yeah man try and find out what’s wrong before having to pull it out. It really sucks to have an engine in the shop for a problem but there isn’t a smoking gun during tear down.

this is what makes this unique and interesting - it was .030 over when you got it to freshen up. Then it was fucked so you bored it out. Did the previous guy balance his rotating assembly? And is it in your favor or does it actually make the vibration worse!?
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You make a good point that I hadn't thought about. He had TRW pistons, that I sold but I re used his crank and rods because they were in useable shape. I didn't know the history of the motor, I just bought it on CL intending on rebuilding anyway. It was only $300 for a complete 340, so I couldn't go wrong. I just wish the machine shop would have said something about the balance. They had all the original crap there, thats how we found out about the pistons being un usable for the block bore.
 
yeah, $300 for a complete 340 is still a deal even if its a worn out .030 bore. I had a 60 over 340 that I sold. Wonder how thin the walls were? I got $500 for the short block alone.
 
A trick I use that may help, I hook up an inductive timing light and run it on all wires, I point the light at a valve cover or inner fender well something similar the light will blink intermittent on any cylinder that is missing..it will look like an erratic flashing sequence. But back to my story I had also though I had a balance issue on my 340, I swapped in a 318 vibration still there..it drove me crazy...I did alot of wrenching and back wrenching on that car that was 2003 almost 20 years ago...

340 is back in car since 2008,and 318 sitting there collecting dust...:)
 
Even if the motor was balanced to perfection with the trw pistons....... once they got swapped for the much lighter KB’s, the rotating assy would no longer be balanced.
And it’s not just a little off either.

My old 340 .030 over TRW’s weighed 889g with pins.
The KB’s are 720g.

The reciprocating end of the rods counts one for one in the bobweight formula for a V8.

So the bobweight total with the new pistons is close to 170g lighter.

With the 170g lighter bobweights installed on the crank for balancing, you would then end up having to remove about 1/2-2/3 of that amount from the crank to put it back into normal balance.

The crank is trying to counter balance 170g of effective weight that isn’t there.

Whether or not this is the vibration the OP is feeling, I can’t say.
But one thing is for sure....... if the crank wasn’t balanced to the new pistons, the balance of the rotating assy is way way farther out of spec than any factory engine would have ever been.
 
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If the engine really was not balanced before assembly....
...with 8 KB-pistons roughly 150grams lighter than the stock pistons, there's some significant unbalanced weight flinging around in the engine now...

I think you should be able to feel that by just putting your hand on the engine while revving it up.
 
BigBlockMopar said:
If the engine really was not balanced before assembly....
...with 8 KB-pistons roughly 150grams lighter than the stock pistons, there's some significant unbalanced weight flinging around in the engine now...

I think you should be able to feel that by just putting your hand on the engine while revving it up.
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I agree BBM if it is that far out of balance it would shake all the time not just around 3K rpm. There is more to this story.
 
Ever notice how if you have a tire out of balance....... it’s worse at certain speeds....... and not as bad at other speeds?

Yet...... it’s out of balance the same amount all the time.

It’s the same with drive shafts.
When there’s a problem, they’re usually worse at certain speeds.
 
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