340 breaking up @ 3500 rpm

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B

Big B

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long island ny
engine starts 2 brake up @ about 3500 rpm. have good fuel supply with a brand new 750 holley, 110 gph holley mech fuel pump, 6 1/2-7 1/2 psi at the carb. converted to electronic ign. have new mopar chrome module (breaks up at same rpm as orange one), accel super coil, new cap,rotor and wires. any ideas would b appreciated. thanks. big b
 
Sounds like low voltage at supply side of ballast resistor. Do you have a voltmeter or timing light? If the clamp-on timing light cuts out, it is an ignition problem.
 
How are the tracking marks on the underside of the distributor cap terminals. Are they centered ,or on the edge of terminals? Also are you running a vacuum advance? What is base , and total timing? Maybe throw old cap and rotor on and take a ride
 
If it breaks up at the same rpm,regardless of the load on the engine ( reving in park compared to climbing a hill) it could be valve train related. Broken spring or something.
 
Check your pickup to reluctor gap and rotor phasing. Is it better or worse with vac advance hooked up? Had similar issues and it ended up the little tri-prong clip under the advance plate was broken!
 
And following up on the comment from hangn0ut above, does the break up occur only under heavy throttle above 3500? I.e., can you rev it slowly past 3500 with light throttle while cruising or while sitting still and not have the breaking up problem? This difference breaking up or not with light versus heavy throttle) can help sort out if it is ignition or not. As hangn0ut notes, if this happens under all throttle conditions, it is less likely to be ignition.

Does the ignition have the ballast resistor that came with the coil?

How is the grounding of the ignition box (ECU)?

Have you checked the polarity of the pickup to be sure it is right?
 
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