Backfire Through Carburetor

VasiliBrown

New Member
Jul 22, 2014
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Yesterday I got my 302 running with f303 cam. It had a holley 750 dp and would driwn the spark plugs. Switched to a holley 600 street avenger today and it won’t stop throwing fore out of the carburetor. Nothing changed exept the carburetor. Upon inspection it looks like my distributo cap has a crack in it. Can a crack in the cap cause a backfire out of the carb?
 
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7991LXnSHO

wanna catch the space herp
10 Year Member
Sep 1, 2010
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Kearney, NE
A misfire can cause a backfire from bad spark timing.
Fix that, check ignition timing, then check the floats levels, needles, idle screws and power valves in the carbs. Vac lines are also on the list.
The 600 street avenger should be a much better fit, but a 700 DP in proper tune should still run.
 

VasiliBrown

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Jul 22, 2014
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So I verified timing by taking the distributor out and putting it back in. I put the 750 back on and it fired up fine. What could be wrong with the 600 to cause the backfire?
 

7991LXnSHO

wanna catch the space herp
10 Year Member
Sep 1, 2010
5,583
1,851
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Kearney, NE
So I verified timing by taking the distributor out and putting it back in. I put the 750 back on and it fired up fine. What could be wrong with the 600 to cause the backfire?
Answered in post #2. Check all the adjustable and moving things. A lean backfire can be from a fuel distribution problem. A Holley SW vac secondary carb is likely to have power valve protection, but that could be leaking and causing the wrong mix, too.

How did you check the timing with pulling the distributor, and not with a timing light? You can sometimes get it close while installing it, but the light is needed for verification and tuning in cases like this.