340 Engine Shake

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The crank looks like an old 318 or 273 forged type. I don't see any signs of rebalancing, it looks like a factory balance job.

The con rod nuts are not stock. General rule of thumb is to have one full thread sticking out past the nut.
 
I would be looking at the sensors of the FiTech. Can you bolt on a carb to try out?
 
mopar head said:
I think he said he had different temps on exhaust pipes, manifolds? headers?
something to ponder.
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I went back and checked those and they were all close with one being about 80* cooler. Can't remember which one as it didn't seem too significant.
 
pishta said:
I dont think its balance related as you look to have the stock rods and forged crank. Stock flat balancer? Get it shaking at idle and start pulling plug wires one at a time. See of anything changes. You could be losing a cylinder due to a wonky distributor bushing or something. Your O2 sensor will tell you if you lose a cylinder as it will go rich without combustion. Are you absolutely sure you got 5 and 7 plug wires correct? Bit me twice!
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Yes, stock rods and KB hypers. I've never pulled plug wires while the motor was running. Just pull from the spark plug boot? Do I reconnect if nothing changes? I've verified firing order multiple times.
 
Bewy said:
I would be looking at the sensors of the FiTech. Can you bolt on a carb to try out?
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I don't have a carb. I might be able to borrow one, but I don't have a fuel pressure regulator to bring the psi down for a carb.

There isn't anything noticable on the Fitech when it happens.
 
I went through a similar issue years ago, and it ended up being an out of balance pressure plate. I had always assumed the pressure plate threw a weight, or wasn't properly balanced from the factory. I installed a new one and issue was gone, but yes I had a bad vibration at idle in neutral or driving in gear, the dash would rattle if I revved it at idle that's how bad it was. I got good at pulling that 4 speed in less than an hour because I was trouble shooting it every night after work, I had the flywheel on a balancer, then replaced throwout bearing, etc. It was a PITA figuring it out.
 
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My only disagreement with a possible firing order mistake is that with two plug wires crossed, the engine WILL pop. You'll know it. But you've said you've verified the firing order several times, so that appears not to be it. Have you run a compression test? That would be high on my list of diagnostics.
 
Anyone know the weight difference between kb hypers and a stock piston? With a factory balance job and new pistons your bob weight will change. I bet your machinist either messed up or didn’t balance it.

edited to add;
Kb243 is advertised at 588 grams
Stock piston is 719 grams
 
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trapster said:
I wonder what would happen if you ran the motor without the harmonic balancer.
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Something real, real bad...sinister would happen...:) Just kidding , I would think the OP could remove it, fire the car up, and see if the vibration went away sitting in neutral, revving it in the garage for a minute or so...just for diagnostic purposes...
 
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Phreakish said:
With the car cold, and you key on the system. What does afr read? Don't start it, just key on.
Should start at 14.7 then climb to 20.1 and stay there..
A bad sensor will just stay at 14.7

I'd do as inertia suggested to check for leaks.
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Mine stays at 14.7, but responds quickly when started. I wonder if the Fitech ecu doesn't send voltage until it sees an rpm signal. I'll msg them and ask if this is normal, hate to replace for no reason.
 
plumkrazee70 said:
Yes, stock rods and KB hypers. I've never pulled plug wires while the motor was running. Just pull from the spark plug boot? Do I reconnect if nothing changes? I've verified firing order multiple times.
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Come to think of it. I attached my timing light to each wire and got the spark light. Wouldn't there be nothing if I had a dead cylinder?
 
RustyRatRod said:
My only disagreement with a possible firing order mistake is that with two plug wires crossed, the engine WILL pop. You'll know it. But you've said you've verified the firing order several times, so that appears not to be it. Have you run a compression test? That would be high on my list of diagnostics.
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I have not yet done a compression test. I will see if I can do that tonight.
 
Sounds like fuel/tune issue to me... can't see where much of what is being said above being intermittent.
 
Pressure plate, it vibrates at neutral or running down the street correct? Probably pretty bad but seems to go away at higher RPM? Is it unbearable to drive? Dash rattling?
 
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trapster said:
Check distributor for cracks.
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This is another excellent suggestion. Sometimes a cracked cap can cause all kinds of mayhem. Usually you get ignition pops, but not always.
 
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