>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.
>
I believe the diverter valve in the case of an MG (I only have experience
with the 73 B emission stuff)
is the gulp valve, but it dosn't divert, it leans out the mixture. After
reading Sean's message the purpose came clear.
The Gulp Valve OPENS when the throttles close thus dumping extra air into
the intake manifold and leaning out the closed throttle mixture (I knew
this part, but why? If you lean the mixture the car should backfire). If
what Sean says is true, about the car backfiring if the airpump puts air
into the exhaust with a rich mixture, then the gulp valve is leaning out
the mixture to prevent the backfire caused by adding extra air to a rich
exhaust mixture.
IF this is all true (PLEASE let me know if it ain't) then the midget may
have a bad gulp valve.
Also is the restrictor in the line leading to the gulp valve? I'm not sure
about the midget but the B is supposed to have a restrictor to decrease the
airflow to the gulp valve (often not there).
Somewhat along the same lines, re: the charcoal canister.
A friend of mine bought a spiffy new aluminum valve cover for his '73BGT,
installed it and immediatly had problems getting the mixture settings
correct on the carbs. (HIF4). After chasing this for a couple of hours, we
discovered that there was NO restrictor in the port to the charcoal
canister in the Al valve cover. This allowed the canister to see a higher
vacume than 'normal' which in turn pulled vacume in the Carb. float bowls
(through the anti-run on circuit) thus leaning the mixture.
Two things came clear:
1) The line from the charcoal canister to the valve cover MUST have a
restrictor in it (very small, some thing like 1/16 in or so)
2) The valve cover oil fill cap is supposed to be vented, this should be
cleaned periodicly (something I never did) or replaced.
******************************************************************************
Geoffrey Hargreaves
Curator,
National Ice Core Laboratory (NICL)
USGS, M.S. 975
Box 25046, DFC
Denver, CO 80225
(303) 202-4830 wk
(303) 202-4845 Fax
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