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Loud backfire POP when shut down?

Whenever I shut down the car, I get a LOUD explosion pop right from inside the muffler (a custom "non-blocking" i.e. I can see thru it from 1 end to the other).

Any suggestion on what can I do to get rid of that? (For background - my idle AFR is around 14.8 so she's not running rich there).

thanks...
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87 930 K27HFS/B&B/Twin-Plug... Megasquirted
Old 08-19-2008, 01:30 PM
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GJF GJF is offline
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possibly a wasted spark discharging a spark on a cylinder when the exhaust valve open igniting the unspent gasses in the exhaust?
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87 911 Carrera platform
Twin Garrett's, Big Valve, Xtreme Cylinder Heads, JB Racing cylinders and machine work, custom JE pistons, GT2 EVO cams, GT3 oil pump, modified 3.2L intake, 80mm TB, custom Intercooler, equal length headers, DFI, KEP Stage 2 custom twin disc, G50 custom gear set, LSD, Tilton trans pump, Fluidyne cooler
Old 08-19-2008, 01:35 PM
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umm... you could be right! But if that's the case, then it means there's nothing I can do to stop that then?
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87 930 K27HFS/B&B/Twin-Plug... Megasquirted
Old 08-19-2008, 01:47 PM
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Are you twinplug and what coils are you running?
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87 911 Carrera platform
Twin Garrett's, Big Valve, Xtreme Cylinder Heads, JB Racing cylinders and machine work, custom JE pistons, GT2 EVO cams, GT3 oil pump, modified 3.2L intake, 80mm TB, custom Intercooler, equal length headers, DFI, KEP Stage 2 custom twin disc, G50 custom gear set, LSD, Tilton trans pump, Fluidyne cooler

Last edited by GJF; 08-19-2008 at 01:55 PM.. Reason: spelling
Old 08-19-2008, 01:55 PM
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Yes, I'm twin plug; wasted spark. Coils are Mitsubishi ones (for 3000GT).
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87 930 K27HFS/B&B/Twin-Plug... Megasquirted
Old 08-19-2008, 02:02 PM
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Are you firing the coils to these respected cylinders?
coil A 1&4 coil B 3&6 coil C 2&5
And the same for the second set of coils?
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87 911 Carrera platform
Twin Garrett's, Big Valve, Xtreme Cylinder Heads, JB Racing cylinders and machine work, custom JE pistons, GT2 EVO cams, GT3 oil pump, modified 3.2L intake, 80mm TB, custom Intercooler, equal length headers, DFI, KEP Stage 2 custom twin disc, G50 custom gear set, LSD, Tilton trans pump, Fluidyne cooler
Old 08-19-2008, 02:23 PM
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Quote:
Originally Posted by GJF View Post
Are you firing the coils to these respected cylinders?
coil A 1&4 coil B 3&6 coil C 2&5
And the same for the second set of coils?
Yes...
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87 930 K27HFS/B&B/Twin-Plug... Megasquirted
Old 08-19-2008, 05:12 PM
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What are your timing values from idle to 0" of vacuum? (Or 101kPa) Or let's see your timing map. Not sure but maybe too advanced from idle to 0"?
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87 911 Carrera platform
Twin Garrett's, Big Valve, Xtreme Cylinder Heads, JB Racing cylinders and machine work, custom JE pistons, GT2 EVO cams, GT3 oil pump, modified 3.2L intake, 80mm TB, custom Intercooler, equal length headers, DFI, KEP Stage 2 custom twin disc, G50 custom gear set, LSD, Tilton trans pump, Fluidyne cooler
Old 08-19-2008, 05:43 PM
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Going from memory, my idle timing is around 8 deg.
I don't think other timing could cause this as I'm nowhere near off-idle vacuum?
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87 930 K27HFS/B&B/Twin-Plug... Megasquirted
Old 08-19-2008, 07:10 PM
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