Backfire when cold

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ozmoparjoe

ozmoparjoe

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Hi everyone, I'm after some advice as to why my 360 would start having the issue of backfiring through the exhaust when cold. Was fine for quite a while and developed this problem seemingly out of the blue, once it has run for a few minutes and starts to warm up it's fine. The other day i pulled the intake (tunnel ram) because I was convinced it had a vacuum leak, but it all looked good so I put it all back together with new gaskets and sealant. Did a compression test and that's fine, timing is fine, wires and plugs are fine. Distributor is a MSD ready to run, carbs are Edelbrock AVS 500 cfm (each). All parts have about 2000 miles on them. I'm starting to think its a carburetor issue, what do you guys think?
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Under what conditions does this happen? And this IS? out the exhaust and not the carbs?

Timing or idle mixture, off hand, and what do you call "cold?" Outdoor temp in F?
 
67Dart273 said:
Under what conditions does this happen? And this IS? out the exhaust and not the carbs?

Timing or idle mixture, off hand, and what do you call "cold?" Outdoor temp in F?
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If I haven't started it for a couple of days, and the engine is cold it backfires through the exhaust if i give it any gas at all. It did it this morning and it's summer here in Australia, temp at the time would have been around 77F.
 
Judging from your location........I would keep me Cockatoo cool and don't go acting like a fool, Curl!!:poke::poke:
 
Popping in the exhaust is NOT a backfire.
Popping in the exhaust is an afterfire or a missfire.
A backfire is when the fire in the cylinder goes BACK up into the intake, and usually out the carb making a nasty fireball.
 
Cold pops are usually lean, but could be TOO rich and unburned gas in the headers. Could be contaminated or old gas, and rarely plugs unless they get wet. I've never traced it to timing.
Worst case is probably a sticking exhaust valve.When an engine misfires, that A/F charge moves down the pipe but it's ready to burn. So if/when it sees a different ignition source, it catches fire and pops. You can feel/hear it in the tailpipe .
If it pops up near the header flange, then the header is probably sucking fresh air, tighten the screws and see what happens. In this case, as the engine warms up the popping changes to ticking
 
AJ/FormS said:
Cold pops are usually lean, but could be TOO rich and unburned gas in the headers. Could be contaminated or old gas, and rarely plugs unless they get wet. I've never traced it to timing.
Worst case is probably a sticking exhaust valve.When an engine misfires, that A/F charge moves down the pipe but it's ready to burn. So if/when it sees a different ignition source, it catches fire and pops. You can feel/hear it in the tailpipe .
If it pops up near the header flange, then the header is probably sucking fresh air, tighten the screws and see what happens. In this case, as the engine warms up the popping changes to ticking
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Thanks I will check it out
 
AJ/FormS said:
Cold pops are usually lean, but could be TOO rich and unburned gas in the headers. Could be contaminated or old gas, and rarely plugs unless they get wet. I've never traced it to timing.
Worst case is probably a sticking exhaust valve.When an engine misfires, that A/F charge moves down the pipe but it's ready to burn. So if/when it sees a different ignition source, it catches fire and pops. You can feel/hear it in the tailpipe .
If it pops up near the header flange, then the header is probably sucking fresh air, tighten the screws and see what happens. In this case, as the engine warms up the popping changes to ticking
Click to expand...
^^^^THIS^^^^
Afterfires are frequently caused by a valve hanging up when the engine is cold. As the engine warms up and guide clearance opens up, it goes away. In the aviation industry this is known as "morning sickness" and usually (eventually) ends up with a valve kissing the piston top.
 
Captainkirk said:
^^^^THIS^^^^
Afterfires are frequently caused by a valve hanging up when the engine is cold. As the engine warms up and guide clearance opens up, it goes away. In the aviation industry this is known as "morning sickness" and usually (eventually) ends up with a valve kissing the piston top.
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Thanks for that info, i certainly hope it isn't that, but in saying that, after i re sealed the intake, it seems to be running quite well now.....
 
My 340 has recently been backfiring
Out the exhaust at slow speed when cold,
And running rough all the time.
Wound up being the brake booster leaking
Vacuum.
 
Mopar69gts said:
My 340 has recently been backfiring
Out the exhaust at slow speed when cold,
And running rough all the time.
Wound up being the brake booster leaking
Vacuum.
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A rhythmic miss,once every other revolution,is typical when the vacuum line is plumbed to just one runner, as opposed to the plenum under the carb. I mean it's normal to route it that way, and it works fine as long as the diaphragm is good.
 
AJ/FormS said:
A rhythmic miss,once every other revolution,is typical when the vacuum line is plumbed to just one runner, as opposed to the plenum under the carb. I mean it's normal to route it that way, and it works fine as long as the diaphragm is good.
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That makes sense
And it is plumbed to one runner.
Never really thought about it before.
I've been driving it with the booster hose plugged, it takes both feet on the brake pedal to stop!...lol
Thank you for the reply.
I hope the op. Gets his problem figured out
 
Mopar69gts said:
t takes both feet on the brake pedal to stop!...lol
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the M/C for the boosted application is IIRC ; 1 and 1/32, versus about 15/16 for manual; so yes it takes a lot more legpower.
Especially on a disc-brake car, that does not have the self-energizing feature of drum brakes.
 
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