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Join Date: Aug 2005
Posts: 34
backfiring during deceleration

I just put 83 motor back in car after repairing broken head studs and went for test drive. During downshifting or rapid decline in RPM's engine rapidly backfires. This does not occur during acceleration only during downshifting and not at idle.

Any suggestions or ideas would be appreciated.
Thanks

Mike
1974 911S
Old 02-08-2006, 11:12 AM
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Lean mixture? Air leaks in the intake?
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John
'69 911E

"It's a poor craftsman who blames their tools" -- Unknown
"Any suspension -- no matter how poorly designed -- can be made to work reasonably well if you just stop it from moving." -- Colin Chapman
Old 02-08-2006, 12:28 PM
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Timing? Vac leak, mixture.
Old 02-08-2006, 12:53 PM
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mixture check

On the CIS systems how do you check the mixture or change the mixture.
Thanks
Old 02-08-2006, 12:56 PM
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CIS needs a few special tools, Wayne has 'em.
Old 02-08-2006, 12:59 PM
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My experience is that this condition is caused by a leak in the exhaust (hole in muffler or leak in the heat exchange to muffler connections). Lifting off the throttle rapidly causes a rich mixture leading to unburnt fuel in the exhaust stream that gets ignited when exposed to outside air at the exhaust system leak.

Check you muffler connections for carbon trails.
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Tom Butler
1973 RSR Clone
1970 911E Restoration in Progress
Old 02-08-2006, 07:16 PM
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more then likely lean mixture...not necessarily a bad thing on deceleration......just too lean to fire in the c chamber....backing off is an other way to put it
Old 02-09-2006, 10:03 PM
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