hi everyone,
just got the 79 'b pre-smog checked today, and it scored too high on CO at
25mph (but was okay at 15mph). This bemuses me because i thought i had it
set pretty lean (any leaner and it backfires occasionally). Hydrocarbon
emissions were very low, oxygen levels were very low (0.0 at 25mph) and so
were NO emissions. Anyone have any experience with this kind of thing?
My current reasoning is that if the HC levels and O2 levels are both very
low, then there is almost complete combustion of the fuel going on, the high
CO levels would seem to indicate that with a little more oxygen, the CO
would be burnt to carbon dioxide. It seems that if the air pump or check
valve are misbehaving then not enough oxygen would be pumped into the
exhaust manifold to burn off the remaining CO, so i'm going to check this
tomorrow. The only other culprit is the carb. like i said, both the
colourtune and the engine's behaviour would seem to indicate that the
mixture is set pretty much right. I can see two other possibilities. Firstly
there's the funny little 'CO adjustment screw' on the side of the carb near
the petrol intake. it's also called the idle mixture adjustment screw in the
shop manual. would this affect the engine at higher revs and under load?
alternatively, would the choke adjusting screw (aka fast idle screw) have
any effect if the choke is off?
any advice very much appreciated
cheers
Joe
for the technically minded here's the readout:
Speed RPM %co2 %02 HC(ppm) %co NO(ppm)
15 2021 14.3 0.2 4 1.14 115
25 2104 14.2 0.0 3 1.67 128
_________________
Dr. Joseph Garner
Department of Animal Science,
University of California,
One Shields Avenue,
Davis,
CA 95616
USA
tel: + 530 754 5291
fax: + 530 752 0175
_________________
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