Rpm related vibration 5.9 magnum

[pjc360]

I want to apologize in advance, this post is going to be long but in order to solve this there is a lot of information that needs to be shared to get an idea of what’s going on here.
I’ve got a real brain stumper on my hands here and I’m dumb founded and have no idea what’s going on, I need some serious help determing the cause of an rpm related vibration that I have.
Engine is a 300hp crate 360 magnum, mileage is just over 30k miles on engine, it was purchased brand new in 2003 by my father.
This vibration comes on at about 2300-2500 rpm.
Doesn’t matter what gear I’m in the vibration will be the same at 2500 rpm in every gear regaurdless of speed.
So I know it’s not drivetrain related, which for the longest time I thought it was.
This engine is in my truck, it’s a 91 w150 short bed 4x4.
I’m running a 2 pin non lock up A-518 auto, I have an LA style flex plate with a Magnum weighted torque converter.
I’ve verified the converter with other magnum weighted converters and they look identical.
Transmission was re-built a few years back, below 2500 rpm the truck runs nice and smooth, no vibrations what so ever.
I have the La style timing cover and v-belt set up which is what came on this engine as a crate.
I have brand new ps pump and ps pump brackets, brand new alternator and alternator brackets.
I’m running a double grooved crankshaft pulley and a single groove water pump pulley.
Belts are brand new and straight, everything is tight.
Fan is a brand new 6 blade flex fan bolted directly to water pump, no clutch fan.
Harmonic balancer is the balancer that came with the engine from Mopar as a crate, it’s never been swapped.
At first I thought this was an imbalance issue, but now I’m not so sure and I’m leaning towards some kind of issue with ether the valves or the valve springs in the heads.
Here is why, I ran the truck in park up to 2500 rpm and the vibration wasn’t as bad as I expected it to be if it were an imbalance problem.
There is a rough spot at 2500 and when I open the door and listen to the exhaust closely I can hear it popping and missing like it’s breaking up and not hitting evenly on all 8 cylinders.
I was fighting a miss fire issue a month or so ago and I thought I had narrowed it down to a faulty crane cams hi-6 cd ignition box.
But now I’m not so sure.
I did buy a brand new MSD Digital 6 box and wired that inn with all brand new wiring and I bought a brand new crane cams lx91 coil to go with the box.
I ran the trigger wires and the coil wires on opposite sides of the truck so they are as far away from each other as possible to prevent false triggering.
I’m running a brand new firecore 50 electronic vacuum advance distributor.
The mechanical advance is set at 16 degrees, initial timing is set at 18 degrees and total timing is set at 34 degrees all in at 2500 rpm with the 2 blue medium tension firecore advance springs.
I’m running brand new Ngk v-power bkr5e plugs gapped at .040.
Plug wires are brand new Taylor Thundervolt 8.2mm wires and all 8 wires ohm out at 40 ohms of resistance per foot.
I’ve verified my timing with 2 different lights, both are within 1/2 a degree of each other.
I have verified the timing mark on the balancer using a piston stop, the timing mark is accurate.
I’ve done a warm compression test on all 8 cylinders with the carb wide open.
I get 155psi on all 8 cylinders.
This vibration feels like it’s got a rythem to it, I can feel it in the steering wheel, in the floor boards and in the seat, and it seems to have a steady rythem to it like it rotates around with the engine, going 55 mph in 3rd gear down the highway puts me at about 2400-2500 rpm with 4.56 gears and 33 inch tires, while going 55 in third the vibration just goes womp, womp, womp.
Then when I shift the trans into overdrive and the rpm drops it ether clears up substantially or it completely goes away.
When I first got this firecore distributor is was suggested to me to slow down the advance rate with heavier springs.
So I swapped in one light silver spring and one heavy black spring which slowed the timing being all in to about 3400 rpm.
This didn’t help the issue and took away a lot of power, so after I wired the new Msd box inn I decided to switch back to the medium blue advance springs.
When I did that the vibration seemed to change, it got slightly better??? I don’t understand how or why that made it better.
I’ve also discovered that if I dis-connect the vacuum advance it will lessen the intensity of the vibration going down the highway but it doesnt eliminate it all together.
I am seriously at a complete loss as to what’s going on with this engine.
All I know is it’s an rpm dependant vibration, not speed dependent, and I don’t know if it’s imbalance or an engine miss, but I’m leaning towards engine miss because like I said I can hear the engine breaking up a little bit while in park at the same rpm and I have verified my converter and flex plate are right.
Since all the ignition parts are brand new and timing has been verified, I’m
Leaning towards something going on with a valve spring or a valve.
I just need to know how to confirm this.

 

[Marcohotrod]

a thing that makes this tricky is a ton of brand new parts-and many are not good old made in usa. remove valve covers and inspect valve springs and rockers. the compression test is good, valves are good. springs can be tested with a load tester while on the head. remove some belts and run it. how do the plugs look?

 

[Oldmanmopar]

I Can only come up with three things. Get a mechanical advance distributor and lock it to see what happens. A digital 6 has a 20 degree retard built in for start up for running a locked distributor so just set the timing at 34 for a test. you can always unlock the distributor and set it for 15 initial at Idle if you would like.

Second is the balance of the motor seams like the culprit. I had a crate motor that took a neutral balance converter . It was installed with a converter with weights and it vibrated.

Third Are you using a flex plate for a externally balanced motor and still adding the weights.

From my experience in what you are describing you have a balance issue. Unless you have had it balanced yourself you can never trust the parts you are applying. You need to take it to someone that has parts they can through at it with no cost for testing. Other than that you are just going to ruin the motor. It won't take long to destroy the bearings in that motor. especially the thrust bearing.

 

[mderoy340]

fatten up the primary jet.

 

[GeorgeH]

Neutral balanced flexplate?

 

[AJ/FormS]

If it's popping in the pipes, it's either
1) the timing is extremely unstable,or the plugs are sour.
2) engine is lean
3) sloppy spring pressure.
4) contaminated fuel
5) headers sucking air at the head flange.
Fix the popping first, then attack the vibration......which may or may not be related.
If the popping occurs at the same rpm whether loaded or in neutral, it's probably ignition related (could even be the tach)
If it's related to the engine load, it's probably an air/fuel issue.

1) unstable timing is easy to see on the timing lite.Sour plugs are not. But can be inferred by reading them. If they are white or yellowish, not new, and shiney, as in glazed, I would start by replacing them and fattening up the low speed circuit. If you have been tuning the low-speed circuit with an AFR gauge, STOP!
2) is an easy cure, unless the emulsion tubes are involved
3) I doubt this one, but you never know....
4) and 5) you can test for.

As to lean; This can be not enough fuel.... or....... too much air, so you gotta do your due diligence testing every device attached to manifold vacuum and all their plumbing.
I'm guessing 2500 is near cruise rpm, so the throttle valves are probably gonna be at the top of the transfers or slightly into the mains. So if you get into the carb, you are gonna have to check out the low-speed circuits.

Here is a test
Keep your eye on the temperature gauge, throughout the test. If it changes, the test is VOID.If the rpm changes with or without the air filter house installed, it may be the fan blowing across the bowl vent, or the vent too close to the top of the lid;so you have to keep that in mind.
Here we go;
Block the throttle open at the rough spot, using the fast-idle cam and it's speed screw, or whatever it takes. Grab the distributor, and change the timing in whatever direction it takes to reach a maximum rpm, then reduce the rpm back into the rough zone, with the speed screw. Now adjust the low-speed mixture screws to obtain the highest rpm, and again reduce the speed into the rough zone.
If it's still rough grab a shop towel and slowly cover the secondaries.If the rpm changes,stop the test and close the secondaries all the way. Then resume the test; readjust the mixture screws and retest the secondaries with the towel. After the secondaries stop messing up the transfers, bring that shop towel slowly over the primaries. If the rpm goes up, she's lean . If down, she's either right on or a bit rich. To find out which, clamp the fuel supply line, and wait .
Pretty soon one of three things is gonna happen;
1) the rpm will rise, level off, then fall to a stall.This started out as too rich, then perfect, then ran outta gas,or
2) the rpm will immediately begin to fall, and then stall. This is lean, or
3) The rpm will do nothing for 5 to 8, or more seconds, then begin to drop to a stall. This was perfect, then went lean and ran outta gas.
Awright kick it off the fast idle and let her cool for a bit, then fix whatever needs fixing.
But if you got a #3, there's more digging to do.And the first thing I would do is replace the plugs,and start the test over.
Good luck.

Oh yeah, put the timing back or recurve your D

done