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I have a 390fe that's completely stock minus an HEI distributor and a 4 barrel intake and holley carb. With my basic 600cfm carburetor, a problem arose where the acceleration from idling around 700 rpm to about 1500 rpm was real fluttery and loud, and really weak. After 1500 rpm, it would instantly smooth out and cruise just fine. The problem gets worse as the car warms up to normal temperature. It also happens with hard acceleration at any rpm. I recently upgraded to a 670 street avenger, I got it used but completely rebuilt it and replaced the main body. The problem's still there. I've messed around with float level, power valve, and jetting. Where should I go from there? I'll look into my accelerator nozzle and pump cams next, and maybe plug wires because it sounds like it could be missing sometimes.
 

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Bump it up to 14 and if that helps then hook your vac to manifold vacuum.
You’ll need to readjust your idle rpm.
 

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Besides checking your timing what is the rest of your engine specs and transmission and rear gearing etc. Also when you changed from the 600 holley to the street avenger 670 you really did not change anything at all as the 600 main body is the same exact one used for the 670 street avenger.

The only difference is the part number stamped on the tower. The regular zinc and aluminum holley 670 street avenger carbs use the same 1 9/16 base plate as the 600 and also the same primary metering block. The only difference between the 600 and the regular 670 street avenger is the street avenger has center hung fuel bowls and a rear metering block and a quick change for the vacuum change.The holley ultra 670 street avenger with billet base plate and metering blocks uses the bigger 1 11/16 base plate like all holley 650 to 800 size range cfm carbs use. Why holley does that to customers who buy them I don't know. I read that they over state the cfm flow rate on the avengers when they first came out cause to many people like to over carb there engines and that was from a sales rep themselves.

That has nothing to do with your issues just a little history lesson there and for the most part why your still having the same problem as its in essence the same carb with different fuel bowls and a rear metering block on it. Bump your timing like stated above and also make sure your vacuum advance timing is not giving you more then about 12 to 14 degrees of timing on that part so you don't end up with too much timing while cruising.

After re doing your timing let me know what size shooter is your using and what color pump cam. 90 percent of the time the street avenger comes with a 28 or 31 shooter and most always an orange pump cam. There are issues at times with the accelerator pump arm not making very good contact with the pump cam profile and can cause some pump shot issues but there is a lot more usually then that causing the sputtering your having. Been through lots of them but don't get disgruntled there good carbs once dialed in.

Read that post about the street avenger and you will learn all kinds of tips. I just recently built my self one again for my dart shp 377 and it runs quite well after I did some mods to it.
 

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Found the problem, I wiggled around all the spark plug wires to see if the idle rpm would change and one of them shocked me when I grabbed it. I covered it in electrical tape and it fixed the problem completely.
 

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Found the problem, I wiggled around all the spark plug wires to see if the idle rpm would change and one of them shocked me when I grabbed it. I covered it in electrical tape and it fixed the problem completely.
Start saving for a better set of wires...the electrical tape won't hold there for long.
 
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