Engine Stumbling Under Load (Long)

[Cooter_76]

I've been chasing a gremlin in my engine for the last couple of months, and I'm hoping someone here can point me in the right direction.
Setup:
Edelbrock 1405 manual choke 4-barrel w/offroad needle & seat
Primary Jet: 400
Rods: 7047
Springs: Silver
Secondary Jet: 395
Ignition: Duraspark
Cam: ?????
Timing: 13 deg BTDC
Vacuum @ 800 rpm: 16"
Spark tips are white, suggesting a lean condition.

It started with the engine losing power under load at highway speed. Trying to maintain speed up hills, the engine would sorta stumble. If I gave it more gas it would accelerate out of it. A friend suggested the springs were too weak to overcome engine vacuum and the carb was not going into power mode. Changed springs to silver (strongest.) A test drive revealed that the spring change took care of the stumbling at highway speed. But then thee engine would backfire under acceleration, above 2500 rpm. I took the carb apart and cleaned it thoroughly. It didn't appear to be dirty and the inline fuel filter is clean. No change. It's still stumbling under moderate acceleration and backfires occasionally under hard accel.
I've tried various rod/jet/spring combinations over the years and have never been satisfied with the way the engine ran, but it's never run this bad. Should I go back to the original Edelbrock factory setup, which ran too rich? ?:?

 

[bax]

I would say check your float heights first. Then go up a few sizes on the jets. Backfire under load at high rpm could be a lean indication. make one change at a time.

 

[basher13]

Had something close to what your descibing, just fixed it today. After messing with the carb and timing I bought a set of wires, solved the problem. Wires didn't look bad but hadn't been changed in awhile.
Good luck

 

[broncnaz]

back the timing off a little especailly if its back firing through the carb. Step up to a little richer jetting the white plugs are saying way to lean.

 

[67broncorebuild]

check all plugs and wires! and then check again.
I had no power off the line, but ran at higher speeds, plug wires crossed. Would have a hard cough through carb.

 

[Cooter_76]

The wires are only 2 years old and have less than 10K miles on them. I do have another set I can throw on. Plugs are cheap enough, maybe I'll throw a new set in to eliminate those variables.
After that I'll try larger primary jets or thinner rods next.