I would expect that the timing chain is pre-loaded at a constant rate when running.  This can be validated by simply watching the timing mark with a light and insuring it's steady.  If the timing mark jitters, then something between the chain and the internals of the distributor is moving around more than it should be.  Camshaft end play can also do this.

Lorena, Texas (South of Waco)

Nat.  Something that comes to mind is a worn valve guide but your vacuum check should have singled that out.  When you ran the vacuum check, did you check the vacuum at the rpm point that the engine had the vibration?

 

Beyond that, I’ll suggest backing up the ignition timing at least 10° and see if the ‘shutter’ diminishes or goes away.  If this makes a difference, then concentrate on the ignition advance curve as the source of the problem.  If no change, then it’s back to the carburetor assuming a cranking compression check on the engine has all the cylinders within 10% of each other.

Lorena, Texas (South of Waco)

This is just an addition to your conundrum. I have a 292 in my current truck that the dampener is separated on. I know it defies logic because i thought the engine was going to fly apart when it happened but i worried about it for the first month, then nothing really happened. I eventually took the weight off because nothing I did would keep it together, and since I'm in the process of building a new motor and am too cheep to get another one for this motor. I am still driving the dampener less motor in this truck with no adverse effects or vibration.... not saying that this wouldn't present a problem for someone else, I just got "lucky"... or my motor will grenade any day now... but no vibration from lack of dampener in my case. Since we're on the subject anyone care to chime in, in my case?

Spending my children's inheritance one Y Block at a time.

Harmonic dampers are designed specifically to counteract those shock waves (harmonics) being transmitted through the crankshaft.  That’s why they are called harmonic dampers and not harmonic balancers.  The dampers really don’t have anything to do with engine balance as long at the damper itself is undamaged and has the factory balance intact.  If the damper rings slips, then that could create an imbalance situation simply due to the outer damper being out of sync with the inner hub.

 

That leads into this.  The 1954 car 239 engines and the pickup and truck 239, 256, and 272 engines used solid hubs at the crankshaft without dampening capabilities.  Engine life is simply prolonged with an elastometer style of damper and especially when the engines get larger in displacement and have higher compression ratios.  The compression ratios of the first year Y's and the pickup engines was low enough that the factory engineers did not see the requirement for a damper on those particular engines.  As the compression ratio goes up, so does the need for a heavier harmonic damper.

Lorena, Texas (South of Waco)

I did another test today as per Ted’s recommendation. I checked the vacuum with distributor line disconnected. In drive at 450 rpm it was a solid 19". Then I revved the engine slowly. It held steady at about 20 1/2" right through the vibration and back down again. I do not have a timing light check to see if there are timing mark jitters. I am not 100% sure but I almost feel a very slight misfire during the vibration. I look at the tach and it seems I get really slight blips in the needle. I will look into this a bit closer. Thanks for all the help.

I did another test today as per Ted’s recommendation. I checked the vacuum with distributor line disconnected. In drive at 450 rpm it was a solid 19". Then I revved the engine slowly. It held steady at about 20 1/2" right through the vibration and back down again. I do not have a timing light check to see if there are timing mark jitters. I am not 100% sure but I almost feel a very slight misfire during the vibration. I look at the tach and it seems I get really slight blips in the needle. I will look into this a bit closer. BTW the distributor was professionally rebuilt about 6,000 miles ago. No play in the shaft. Spark advance plate moves smoothly and easily. I am using Autolite 46 Plugs. Wires are about 5 years old. 4160 Holley carb about 4 years old. Float bowls are set correctly. 1 1/2 turns out on drivers side screw- 1 1/4 turns out on the right side. (Car runs rough if they are out the same amount)

Thanks for all the help.

It may help to know when this shudder developed and what changes preceded it or if it has always been there. Also, did regapping lifters help any?

Some additional checks best done one at a time:

Check and record your cold cranking compression in each cylinder. Should not differ by more than 5 percent if all are healthy.

Check convertor flexplate for damage or loose convertor or flywheel bolts.

Install a brand new set of plugs and check for vibration.

If you have the stock Ford Y-block distributor, static time the engine using a 12 volt check light (a 12 Volt light socket, bulb and two arm length leads with alligator clips). Ground one and attach other to wire lead at the coil post common to the distributor lead. Pull distributor coil wire so engine can not fire for safety. Turn ignition on. Turn engine clockwise by hand at the crank damper bolt till #1 is coming up. Engine would have fired #1 cylinder when bulb lights. Note your timing mark on vibration damper.  Mark #1 spark post position on distributor base. Remove distributor cap. Check rotor movement between zero advance and full mechanical plus vacuum advance to insure the distributor post at rest falls midway between the full rotor tip movement range. This insures cap and rotor have been properly clocked by your favorite parts stores secret offshore supplier (and you thought they were usa made) and that spark will not jump to wrong post at full vacuum advance.

Check inside dome of distributor cap for any kind of dust or grease residue and any sign of electrical tracking between all nine posts. Replace at hint of a problem.

In total darkness. have someone hold engine at peak vibration rpm and check engine plug wires for any visible sign of electrical leakage to ground or adjacent wires. Y-Blocks hide plug wires very well. Get a new spark plug wire. Bypass each plug wire one at a time to see if the new wire eliminates the vibration.

Try a different condenser and check for vibration. They can go bad. I found the hold down screw loose on a condenser once. Drove me nuts finding it.

Try a different coil and check for vibration.

If power brake equipped, check for booster diaphram leak by plugging intake manifold tap for booster and check for vibration.

#8 cylinder, lighter in color, could mean burning leaner (intake manifold gasket leak) or coolant getting into that cylinder. Any radiator coolant loss over 6 months?

If none of the prior tests fix the problem, turn focus on carburetor. Let us know what carburetor list number you have. I ran my Holley list 1273 50 years with a slight hesitation off idle that I now attribute to insufficant transition slot exposure. Vanished when 650 cfm 4011 replaced it.  

Jerome

Adjusting the valves did not solve problem. However I believe vacuum is up a bit and a bit smoother idle. Some of the valves were too tight. I will try new plugs next. What do you recommend on a stock 312. I currently run Autolite 46 plugs. Are these hot or cold plug? Any advantages to the Platinum plugs?

Thanks

I would think that you would have a fluctuation on the vacuum gauge at the point of the vibration if there was a problem with the engine driveability. Sounds more like a mechanical vibration. My0.02

Dan      Kingman Az.      86409