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Discussion Starter · #1 ·
I have a carbed 347 that has been running well however, the last couple of times I've had it out I'm getting some load pops out the exhaust if I get at all aggressive on the gas. It seems to be only under load not if I rev the engine in idle. I have a vacuum advance distributor but it's capped off and been running well that way up until now.
The timing was set properly (as I said the motor did not have this problem until the last 2 times I ran it), and all the plug wires are new and look good, the motor only has 500 miles on it.
Any insight would be appreciated.

Thanks
 

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maybe running a bit rich w/ unused fuel detonating. maybe pull a few plugs to check if that's the case. I kinda like the sound, a bit sporty.
 

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Backfire under load we call shooting ducks and it comes from timing that is way off or a way too lean condition.
 

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Discussion Starter · #5 ·
Gordon,
I'll recheck the timing, It was running well up until the last couple short rides as I said. I checked the distributor to make sure it wasn't loose and it was good so I don't think it's the timing but it doesn't hurt to check. In the short amount of time I had yesterday I pulled one of the plugs and it looks like it has some carbon buildup, which I thought was from a too rich condition.
I'll post here when I get to the root of the problem. Thanks for the help.
 

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If it's only under load it's probably an ignition issue. If the timing hasn't changed since it was running well, I would look for a crossfire. Check the inside of the distributor cap for carbon tracks and check the plug wires for burn marks anywhere they are close to one another. Also, if it uses the 302 firing order you need to make sure the 5 and 8 wires are not parallel. they had an issue with induction which was firing the #8 along with the 5 and breaking pistons. The fix was to make the wires cross each other at a 90* angle.
 

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Discussion Starter · #7 ·
Thanks for all the suggestions.
So I leaned out the mixture about a half turn on the carb and rechecked the timing (it was around 14 at idle a little more advanced than I thought I had set it, moved it back to 10). Took it out for a test drive and the first couple of accelerations at very low speed I heard a couple of soft pops. However after a few minutes of driving I could not get any more popping even when I tried hitting the gas pretty aggressively under high load.
Fingers crossed that it was either the mixture, the timing, or a combo of both. As I said the plug that I took out had some carbon buildup on it, not sure if that would cause the issue.
 

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Discussion Starter · #8 ·
As an update I am still facing this issue but am now almost sure it is something in the carb. The problem persists when the engine is cold but after about 10-15 minutes of driving it just completely goes away and never comes back. I'm thinking maybe my choke isn't closing when it should but open to any other ideas. I'm running an Edelbrock AVS2
 

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I had this same problem twice
First was a cracked plug
Second was a cracked distributor

things expand when they get hot, maybe a crack is closing up once it’s warm and making the issue go away?
 

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Discussion Starter · #11 ·
Hey guys, per your advise I pulled all the plugs yesterday and sure enough number 6 had hair line cracks on either side of the ceramic. Swapped it out and took it for a test drive, no more popping!
Really appreciate the advise, it probably would have taken forever for me to find that plug issue without it. Thanks again!!!
 
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