Someone was asking about backfiring from a '71 Midget [sorry-I deleted the
message after writing the email address and missed your name]
I dug out some of my old notes on emissions systems.
Emissions come in four flavors: spark retard, exhaust manifold air pump,
exhaust gas recirculation (EGR), and gasoline vapor recoup (charcoal
cannister)
The spark retard reduces NOx emissions by reducing peak combusion temperature
during low speed acceleration. This is done by reducing the timing, but
should only operate after the engine has warmed up.
The exhaust manifold air pump introduces air into the exhaust to burn off any
unburned fuel. This system works except during a sudden throttle closing.
When the throttle is closed suddenly the mixture runs rich.
*** If air is pumped in at this point there will be backfiring in the
exhaust.***
A diverter valve [is supposed to] senses the change in vacuum at sudden
throttle close and diverts the air pump supplied air into the atmosphere.
The EGR diverts up to 20% of exhaust gases back into the intake manifold to
reduce NOx emissions by reducing combustion temperature. A malfunctioning
EGR causes rough idle and reduced power at full throttle [and half throttle
and quarter throttle and....] It should function mostly at part throttle
with a warm engine. It should be off completely with a cold engine.
The only thing about the charcoal cannister is that it can get clogged.
Removing fuel from the tank then causes a partial vacuum, making the pump
work harder.
Hope this helps. If the above info isn't accurate, blame the book I got it
from, but I'll accept corrections cheerfully.
Regards,
Sean Johnson
'75 Spitfire
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