440 breaking up mid rpm

[midnightrider1818]

Got my 73 charger back on the road, 440 with a mild cam, edelbrock performer rpm intake, holley 750 vaccum secondary carb and msd distributor with a street fire ignition box, runs great up to about 3400 rpm and then starts breaking up, anything under that it runs good and if I mash it to get the secondarys to kick in it runs great also and doesn't break up one bit. Almost like it runs out of fuel at the upper rpm range, 72 main jets and 6.5 power valve, I have about 36 degrees of timing in it but need to check that again, it has motorhome 452 heads on it so using the ur5 ngk plugs, i really dont know where to go from here, this is the cam im using and some pictures

 

[threewood]

I would start by hooking up a fuel pressure gauge you can see while driving to rule out fuel delivery.

 

[Bee1971]

So once the secondary throttle plates open , your saying under full throttle mash the throttle you are not having RPM issues

Little confused

“it runs good and if I mash it to get the secondarys to kick in it runs great also and doesn't break up one bit.”

If that’s the case sounds like a transition between the primary and secondary circuit

 

[midnightrider1818]

So once the secondary throttle plates open , your saying under full throttle mash the throttle you are not having RPM issues

Little confused

“it runs good and if I mash it to get the secondarys to kick in it runs great also and doesn't break up one bit.”

If that’s the case sounds like a transition between the primary and secondary circuit

Yes, thats the case wide open runs great, runs good all the way up runs good right up to about 3500 then starts missing, if I kick the secondaries in it will clear up

 

[Rishi]

Yes, thats the case wide open runs great, runs good all the way up runs good right up to about 3500 then starts missing, if I kick the secondaries in it will clear up

My first instinct is fuel delivery (or lack of it) but once you're past that 3500 RPM point, if it runs great above, then fuel may not be the issue.

Another idea is it can be an ignition issue.

I had a mid-rpm-range problem many years ago, that turned out to be leaky spark plug wires. I discovered the cause by accident, opening the hood one night while the engine was running (headlights off, pretty dark) and I could see the arcing of the wires at the spark plug brackets (looms)

The theory among my car buddies was at lower RPMS, the current wasn't enough to arc to ground, and at higher RPMS, there was sufficient current to overcome the loss do to arcing, but at mid-RPM ranges, conditions were just right to cause misfiring.

 

[RemCharger]

My first instinct is fuel delivery (or lack of it) but once you're past that 3500 RPM point, if it runs great above, then fuel may not be the issue.

Another idea is it can be an ignition issue.

I had a mid-rpm-range problem many years ago, that turned out to be leaky spark plug wires. I discovered the cause by accident, opening the hood one night while the engine was running (headlights off, pretty dark) and I could see the arcing of the wires at the spark plug brackets (looms)

The theory among my car buddies was at lower RPMS, the current wasn't enough to arc to ground, and at higher RPMS, there was sufficient current to overcome the loss do to arcing, but at mid-RPM ranges, conditions were just right to cause misfiring.

Kind of right,, leaking plug wires will show their true colors in a lean condition. So the lack of fuel is just that, and maybe the wires are marginal.. but could just be lean and miss firing. If there's enough current for the ignition at high rpm it rules out a lack of current theory