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PostPosted: Wed Jan 19, 2022 6:06 am 
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EFI Slant 6

Joined: Mon May 26, 2014 4:02 pm
Posts: 334
Location: Vermont
Car Model: Slant Six powered M37
I have been rushing the warm up phase recently on the cold start, and will often get a pop and stumble on the one- two upshift with a manual transmission. I push the choke in before I begin driving, and I am sure the intake manifold is not yet hot enough to keep the fuel vaporized. No questions about that but a general curiosity and awareness that I do not understand how a lean backfire towards the carb can even occur.

If the intake valve is closed upon ignition, and there is no waste spark happening in other cylinders, how does the pop occur?


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PostPosted: Wed Jan 19, 2022 7:44 am 
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Supercharged

Joined: Wed Sep 17, 2008 6:48 pm
Posts: 3545
Location: Indianapolis
Car Model:
I think one cause of a lean backfire out the exhaust is the mixture in the cylinder is not dense enough to 100% burn when the spark initiates the flame front that burns across the cylinder so there is non combusted air and fuel still in the chamber. Then on the exhaust stroke as that un burnt fuel hits the hot exhaust valve the fuel ignites and causes a 'pop' out the exhaust.
Are you sure it is backfiring out the carb? Because if it is just the intake and not the exhaust and the ignition timing is within operating norms, my guess is you have a valve or two hanging open.

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PostPosted: Wed Jan 19, 2022 8:02 am 
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EFI Slant 6

Joined: Mon May 26, 2014 4:02 pm
Posts: 334
Location: Vermont
Car Model: Slant Six powered M37
DadTruck wrote:
Are you sure it is backfiring out the carb? Because if it is just the intake and not the exhaust and the ignition timing is within operating norms, my guess is you have a valve or two hanging open.


Good question.. I got the sense that the pop sound comes from the engine room, and not from the tailpipe... and my muffler is blown out, so I am assuming it would be somewhat noticeable in tone or sound if the pop was exhaust based.. BUT I could be wrong.


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PostPosted: Sat Jan 29, 2022 11:22 am 
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EFI Slant 6

Joined: Sat Mar 19, 2016 7:25 pm
Posts: 285
Location: SW PA
Car Model:
When the unburned charge exits & meet the fire exiting another, it ignites in the exhaust & goes to a source of low pressure, an exhaust valve on overlap with the intake valve opening would be a low pressure target. The erratic/intermittant nature being dependent on timing & proximity of the events to succeed in causing the intake backfire.


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