70 454 backfire through carb problem - Impala Tech
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post #1 of 2 (permalink) Old 03-03-2012, 10:21 AM Thread Starter
Join Date: Apr 2010
Location: omaha
Posts: 55
70 454 backfire through carb problem

First i want to say thanks it seems like every time i post a problem i get an answer that fixes it within an hour or two. I have been a member of several other car forums and it usually takes days and i have weed through answer of people who just want to talk trash... This community is amazing.

Above is a link of what is happening.

Basically the car idle pretty well once its warmed up and will crank right up at any temp above 30F. Once it's warmed up if i give it gradual gas it will work and sound just fine.

If you tap the gas a little like as if you were at a light and it turned green, it will hesitate, sputter and sometimes backfire up through the crab.

This only happens if the engine is at idle and you gas it, if on the highway cruising i gas it or floor it it just goes with no problems. seems to be a low RPM problem. I looked around for vac leaks and i cant find anything.

Thanks for the help!
aaronacj is offline  
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post #2 of 2 (permalink) Old 03-03-2012, 01:09 PM
Join Date: Jan 2006
Posts: 12,458
Not a vacuum leak issue (you can tell by the idle RPM). Back firing is due to a lean spot in the accelerator pump curve or a timing issue. I see you have a vacuum acuated secondary with a vacuum can spring kit modification. If you have a stiffer spring then go up one or two colors to incease the vaccum differential before the back barrels start to open as they are coming in too soon.

Spring colors are by increasing stiffness.
Short Yellow
Silver (plain)

You should have a silver spring in there now from the factory. If so try the brown one as that will shift the opening RPM from 1,960 RPM up to 2,380 RPM.

If you do not have a spring kit you can buy a HOLLEY SECONDARY DIAPHRAGM SPRING KIT P/N 20-13 from Summit, Jeggs, Speedway, Honest Charlies, or any other mail order speed shop.

Big Dave
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