Shake Shake Shake

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The way your engine shakes reminds me of when I tried using manifold vacuum advance, or put in too much initial... Where's your vacuum advance hooked to, and does the shaking change at all with less initial advance?
 
Ther's your problem! Your sticker on the air cleaner clearly says its a 273 not a 340!

Other than that, I've got nuthin.
 
Ther's your problem! Your sticker on the air cleaner clearly says its a 273 not a 340!

Other than that, I've got nuthin.
Air cleaner?
I think your on to something...slap a shaker air cleaner on there and call it good
 
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Intermittent shaking sure sounds like tuning. Interior balance issues will be constant. Can’t see how any valvetrain situations could create.
 
If the plugs look good I would look into the carb. Acts lean to me at idle. How did you set the mixture screws? With a vacuum gauge? Idle circuit could also have dirt in it or other problems
 
My 340 shakes at idle too. Check all the things that other members have brought up.
Make sure you keep everyone informed on your progress.
 
What is the idle rpm in the video?
Are you running a ported vacuum advance?
I have seen and heard that intermittent shake before, but I gotta thing a bit.
I think I would do a couple of easy things first, like;
take a look-see in the float bowl looking for fuel contamination. . And I would blow out the idle bleeds right away too, just cuz I'm there. I do this by revving the engine to ~2000, and slamming the choke shut, then releasing it before the engine stalls. I repeat up to three times.
Next would be, as already mentioned, the Strobe test on every wire and a glow-in-the-dark test at the coil. These are the easy things. Here are a few more;
Check your alternator for a fluctuating A/C output, which ain't supposed to be there.
Pull the regulator connector off, and let her run for a bit on battery alone.

This might get to be a toughie, so I agree to find a good baseline first.
As mentioned this starts with a compression test, and a cylinder balance test, possibly followed by a LeakDown test, but maybe not.And the exhaust outlet temp is an easy test too.

I asked about the rpm and timing, because I have seen more than a few times that if the idle is too high, the timing starts darting around, and or the low-speed circuit gets to acting funny. Too much Idle-timing will do that too, and is usually accompanied with a stinky exhaust.
I'm still thinking.

Ok in view of this is going on with multiple carbs I'm going with contaminated fuel, or a faulty ignition , system, possibly the amp.
So disconnect the Vcan and plug the carb -port.
Get the rpm down to something like 650/700.
Then check the timing; 15 will be fine if it stays there until about 1000 rpm, without doing any funny business. Slowly rev it up while watching the timing marks. Stop at 2000, and return to idle..
If the marks do not smoothly advance , but continue to advance only,with rpm, then check your timing chain. But if the sparks drop out or the timing jumps back and forth from advancing to retarding, then your magnetic pick up may be wired in reverse polarity....
or the rotor will need to be properly phased.
Your pick-up should have one Orange wire and NO purple one. The second could be black or gray., but NO purple. If you find a purple wire, get rid of it! or change the polarity. Any Orange wire pick-up off any Mopar engine will be correct. Then retry it. Make sure the ECU case is seeing the same ground that the battery is.
But if the strobe is working properly, stop the engine. Pull off the rocker gear. Take a plastic mallet and bop the heads of every valve, listening for the distintive popping noise they should make. You don't know where the pistons are so don't be wailing on them; a gentle bop is all you need. They should all sound the same. If you previously did a compression test or a cylinder balance test, then pay attention to any oddball cylinders. I do this test with air injected into each cylinder. This pushes the piston down out of the way, and magnifies the popping sound they make. And when you find one that ain't sounding right, it may require a workout to get rid of carbon on the seat. Also, since the piston is gone, you can push the valves down quite far and find a bad spring right away. If you find a spring that you can turn by hand it has to come outta there. At that point I would pull the heads for a rebuild.

I'm still thinking,
but it hasn't come to me yet, I'm old so that's how it goes sometimes. Sometimes I have to hunt around the keyboard to find a certain letter that I just punched maybe in the previous sentence or paragraph. Sometimes the remembrance comes in my sleep or the next day or several days later. Sometimes it never comes, or somebody else comes along with it. Life is grand, then you get old.
Got it! Dirty points is it. That's where I have heard that sound,lol. Mystery solved. Now how does that apply to you? IDK but the first thing I would do is go get a different ECU and sub it in there, with it's case properly grounded. Any Mopar Ecu that fits your plug will do, it don't matter what it's off. Just swap it in, verify it's grounded, and fire it up.
 
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What's your comparison? What are you using as the "standard" by which to compare? Compared to an engine equipped with EFI, for example, any carbureted engine will shake.
 
Crickets? Man I hate reading everyone's diagnostics and not getting closure.
 
What year of 340 is it?

I think it's a '68 motor but have to confirm. I hope to spend some more time on it today and tomorrow. The motor has Keith Black pistons.

I retract my suspicion it's a failing valve spring because I just put a fresh set of Edelbrock heads on it. The shake was present prior to the head swap.
 
KB pistons are not stock weight. That's your problem if you didn't balance the crank. On a 4 banger, as long as the pistons are the same weight your good (balance each other out), a 120 degree crank 6 is somewhat self balancing with identical pistons and a 90 degree 'V' 8 require a formula to balance prior to dynamic balancing the package.
 
Thanks everyone for jumping in and being patient as I mess with this.

I'm running ported vacuum, so I'm not into the advance when at idle.

Today I started over with the timing. Got it on TDC just to double check it. Then 15 base line advance, and with vacuum advance connected I get 40 total at about 2k rpm. I put the light on all the wires, and they are all firing. I have the idle set just a tad high in the video here. It's about 800 rpm.

The shake is a little better after resetting the timing. Take a look. It sounds healthy, although the video it does sound a little weird, but not in real life. I still have that tapping noise on the passenger side bank. I tried to get that on this video. Maybe I just have a flat lifter?

https://youtu.be/ecW_cBiR3Sc
 
Sounds fine to me. I'd tolerate that shake. How are your mounts? They get swollen and squishy after being oil soaked for 55 years. Ever tune the carb? Give both screws a 1/4 turn one way and see if it changes anything. Can always turn them back.
 
Sounds fine to me. I'd tolerate that shake. How are your mounts? They get swollen and squishy after being oil soaked for 55 years. Ever tune the carb? Give both screws a 1/4 turn one way and see if it changes anything. Can always turn them back.
Yeah I'm sure I'm stewing on it more than I should. The mounts are as new as the rebuild.... 20 years-ish. They could probably stand to be replaced. I've had a couple different carbs on there and same results. It's pretty dialed in. Thanks for the input!
 
lumpy cams are just lumpy. Whats your vacuum levels look like, steady?
 
I have the idle set just a tad high in the video here. It's about 800 rpm.
Why 800 in a 340?
I went to the Comp site, but trying to navigate thatchit just frustrated the dickens outta me.
I suspect a 270 H is a 270/270/110 sub 500lift cam with maybe as much as 224* at .050; I suspect.
If so;
You should be able to idle that down to 650 in gear, with the Transfer slot properly set. With a lil idle retard, I think it would go to 550.
I'm not saying that 800 is your problem, I'm asking why so high? As in; did you verify the TDC marks. It just sounds horribly timing advanced to me. And that would require a high idle so it doesn't stall.
on another note;
As I recall the factory forged pistons were something like 719 grams, compared to KB hypers at 543 or something. Seems to me if the crank was not balanced, the shake would be way worse, and it wouldn't come and go. A guy brought me a 360 once with an out of balance crank. It didn't act like yours. It shook continuously, and the faster it revved the worse it was. Had to take it apart and start over.

You know what I would do is; while it is idling,with the Vcan disabled, just give the Vcan a push back to the firewall to retard the timing;
If the engine speeds up before it slows down, it is over-advanced. Also; if it is horribly advanced, the exhaust will burn your eyes. Crank up the idle a half turn, then retard it some more;never mind about the timing lite;
just see if it speeds up first then slows down and smooths out.
If it does, then verify your timing marks with a piston stop..
 
Man that's weird, sounds like it's still "surging" a bit in that last video while you were standing at the back of the car. You said in the first post you've already messed around with the carb/tried others, did you tune the idle mixture for max manifold vacuum? The engine in my Duster has a more radical idle than yours but I was able to get it pretty smooth to where it doesn't want to surge up and down, I'd think that should be possible with your combo as well...

Or just ignore it lol, if you know the engine isn't hurt internally and you've said it runs well.
 
I've got the thing tuned pretty well, as I have had it before.

Sounds fine to me. I'd tolerate that shake. How are your mounts? They get swollen and squishy after being oil soaked for 55 years. Ever tune the carb? Give both screws a 1/4 turn one way and see if it changes anything. Can always turn them back.

I feel like if I had some more firm motor mounts the shake might go away all together. The mounts on the car now are the ones that came with the Schumacher 273 to 340 conversion brackets. I bought them about 15 years ago or more. They are rubber, not the cool urethane poly locks.

Check out the photos, the driver side mount is sagging quite a bit. Passenger side looks good. With one side failing and the other side firm.... I'd say that would create a bizarre shake. Agree?

Taken from the front of the car, driver side.
IMG_2146.JPG


Taken from the front of the car, passenger side.
IMG_2148.JPG
 
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I've got the thing tuned pretty well, as I have had it before.



I feel like if I had some more firm motor mounts the shake might go away all together. The mounts on the car now are the ones that came with the Schumacher 273 to 340 conversion brackets. I bought them about 15 years ago or more. They are rubber, not the cool urethane poly locks.

Check out the photo, the driver side mount is sagging quite a bit. Passenger side looks good. With one side failing and the other side firm.... I'd say that would create a bizarre shake like I have. Agree?

View attachment 1715444736

View attachment 1715444739
If you had 600 lbs on your back you would also sag. I think those mounts are normal. When they get squishy or mushy then replace them. They are made of rubber to keep from transferring motor vibrations to the car body/driver.
 
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