High RPM miss

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7

70net440

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I'll try to keep this as short as possible. 74 Dart bracket only car. 440/727 I am having popping through the exhaust at about 3,000 rpm that turns into severe backfire at 4,000. I have replaced all of the factory wiring except for the lights. I have a factory electronic dist., MSD model 6200 box, Autolite 86 plugs gapped at .035", GM one wire alt.
Things I have tried to fix the miss-fire:
New plugs and wires
New cap and rotor
Replaced dist. with a working one from my street car(Pertronix conversion)
Replace MSD coil with Pertronix from street car
Bypassed MSD box and only used Pertronix dist. and coil
Replaced Fram type fuel filter(old one looked clean)
Checked screen on inlet side of Holley Blue pump
Fuel pressure is at 7 psi at idle drops to about 6.5 with rpm increase
Replaced intermediate shaft bushing when I noticed the timing was bouncing about 2*
at idle and about 5* at 3,000 rpm. Initial is about 18* total bounces between 35* and 40*
at 3,000 rpm. None of the above changes have had any effect on this condition. Any help is greatly aprecciated.
 
You have money to spend on a trip to a dyno?

I'd almost say you have eliminated the ignition as a problem.

You might round up a different carb to try

At this point I think I'd be looking at valvetrain, sticking valve, bad/ weak spring, bad cam lobe, etc
 
Backfire with good ignition system is a symptom of raw fuel entering exhaust. I also suspect a sluggish/sticking exhaust valve at higher rpm.
 
Does this engine pop through all the exhaust ports? just one bank? Just one port? I would listen with a stethoscope and try to find the culprit that way. Already a few guesses so I'll throw my $0.02. Cracked or burned exhaust valve.
 
Thanks for the replies guys. An engine builder friend of mine suggested looking for weak valve springs. Time for more detective work.
 
I would do a leak down test as well. You could have a spring issue, very common.
 
check and make sure the plug wires are correctly installed. if your msd box has a rev limit plug in, use a higher rpm one. what valve springs are you using?
 
No idea on the internals of the motor. Guy I bought it from didn't have it long and he new nothing about the motor. MSD has no rev limiter. I have triple checked that the plug wires are in the right order. Hopefully, I will get some time tommorrow to check things out some more. Thanks for the replies.
 
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