"Breaking up" at 3k rpm (help)

My engine is breaking up at around 3k rpm.

I changed the plugs, wires, cap, rotor, and swapped out my msd 6al box with a buddys. Still having the same issue. Any advice?
The only thing I have not done is adjusted the rocker arms (1.5).

Has anyone else experienced this? Besides this issue it is running great.



440
727

MSD distributor/MSD 6AL
Holley red pump/-8 line/800 holley (6lb psi)

I was tracking a problem myself when i found this thread

https://board.moparts.org/ubbthreads/ubbthreads.php/topics/15704/1.html

Do you run a crank trigger?

No

Rotor phasing OK?

my problem manifested itself when upgrading to msd digital 6 plus. at the time I was triggering from the distributor, mopar. I could not get it to run clean . swapped pickup wires new cap rotor plugs, nothing helped. another distributor was no help either. I bit the bullet and went crank trigger. voila everything back to normal. msd digital stuff just doesn't seem to like mopar pickups. 470 15 to 1 race motor alcohol.

As the first reply from Dart451 suggested take a look at that thread. Good info on MSD electrical interference problems. I had the same issue with my digital 6AL box that went away with the installation is a shielded trigger wire from MSD. I personally think that is what MSD should supply with all of their ignition boxes as standard equipment.

Try rolling all the “knobs” back to zero.

Check to make sure you dont have the pick up wires swapped......

Just to be clear, the Violet wire on the MSD connects to the White/Orange on the mopar distributor and the green wire to black.




Joe

The cable is shielded. The wiring is correct.

Just as a matter of interest, I've recently hopefully solved a couple of issues in 2 different cars we have. 1 was the ign switch on our '68 sb Barracuda, it was so worn vibration would turn the main feed on and off, it pays to check the simple stuff.
The other was a crank trigger on the Challenger, This "went away" slowly getting worse after 6 passes or so. Closing the gap to .035" helped but that's way below MSDs min spec of 0.050 and it seemed reluctant to pull more than 6500 ish rpm. Hoping for good things now it's fixed as it ran a 9.59 @ 141 which is good for the 572 engine.

I had an issue with the engine, BB, breaking up around 6000 rpm. At first the engine was fine, ran good. Then I cleaned up the wiring routes, sorta put them close together to clean up the engine bay. Motor started missing, didnt realize until allot of Excedrine later that I had routed the pickup wires along side the alternator wires. Moved them back apart, added the shielded cable, miss went away. Now Im not sure if the shielded cable did it or just moving them away from each other did it. But, the miss was gone. You may just check the routing of your wires, trigger wire away from ANY wires. Wrap it in tin foil to jus check to see if that may be the problem. My 2 cents.

Don't rule out a soft valve spring.
I went through ignition and fuel systems , and finally that was the culprit.
It was a little higher rpm , but behaved just like you describe.
The car left and pulled to about 5100 and it sounded just about like it was banging off the chip.

Still fighting my issue I added the shielded trigger wire, did distributor mod and changed tach but still missing and having the shifter change gears to soon (elite rpm switch from digital display). Going to try replacing the digital 6 box with 6al that I ran trouble free last year and see if problem fixes itself.

I added nitrous this year and wanted the digital box to pull timing when spraying, only reason I changed from 6al box.

Double check your settings on the digital 6 box.
I'm sure you got one or more of the dials wrong.
Make sure you are absolutely sure you have the dial on 8 cylinder setting.
It can be confusing.
I bet the box is set wrong.
Also check your high speed rev limit on the dial.

I've had more than one digital box go bad, I keep my standby 6AL to plug in just to check if its the digital box that failed.
When my digital went bad i had no spark at all, It's Really quite frustrating when the digital boxes go bad.

I checked the settings last night it's confusing have it on 6 for 8cylinder starter delay active.

Now check where you have you high speed rev limiter at.

High speed set at 7100
shifter set at 6700 shift light set 6700
Some reason it would bang the shifter 2 then 3 almost instantly at wot and shift light never come on. Turned the shifter and shift light down to 6300 and almost instant 2 then 3 shift no light and everytime wot would start missing after a couple seconds

I just put the old 6AL box back in and everything is fine again. Shifts on cue with shift light at correct rpm
Need a way to pull timing when spraying now. Will see if I can return digital 6 with only 4 passes at track and 4 street miles.

Glad I live in sticks and can pull out of drive with open headers

I went thru 3 before i got a good one,(i'm on year 5 with this one) I purchased from Summit racing and their return policy is exceptional. All mine failed within 3 months one didn't work right out of the box. When you do get a good one you will be happy with the options it provides. Always keep that old 6AL for backup, especially at the track.

Sent box to MSD week and half ago. Never hear anything so called Tuesday, was told they just sending me a new one that is policy. Come home today and found they shipped my old box back (not in the original msd box I shipped it to them in), paper work said they testing it at 4000 rpm for 8 hours and everything was fine. Guess 4000 rpm is considered high rpm. Will call tomorrow see what they say. So far thumbs down on customer support.

Update after getting box back first time had same problem. Called up and they agreed to test on car, so I send the box back in. They said it tested fine on car, but was using the white trigger wire not magnetic. They recommended to add a ground wire to both heads, directly to battery. I add the ground wire and still same problem. Old 6al box works great ever time.

They are finally going to send me a new box.