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1981 Datsun 720 Failed Smog CO2 Measured Passed Max Allowed

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I just took my Datsun 720 to get smogged.

It measured 1.67 CO2 @ 15MPH when the max allowed is 1.58 CO2. 

It measured 1.48 CO2 @ 25MPH when the max allowed is 1.38 CO2.


It also failed the fuel cap test, so I ordered a new one. 


I warmed it up for about fifteen minutes and drove it an additional ten minutes on the freeway before bringing it in. Am I failing by a lot? What can I do to get it to pass?

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  • 2 weeks later...

CO is caused by over rich fuel/air (carb) or incomplete combustion. (ignition)  Either way, gas isn't being burned and is sent out the exhaust.


Carb too rich

1/ choke on or part on (maybe air filter, but never seen one so dirty it would do this)

2/ float set too high, inlet needle stuck, fuel pressure too high and flooding

3/ jet size too large (off idle only)

4/ idle mixture set too rich (idle only)



1/ Make sure both sides of plugs are firing, gaped and NGK BPR6ES and BPR5ES (exhaust side)

2/ Check ignition timing



1/ Check valve lash.... tight exhaust would leak compression

2/ Check compression.... could be a badly sealing valve.

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  • 3 weeks later...

The following is not only good for smog but also for a reliable daily driver.


Full tune up (plugs, cables, cap, rotor, vacuum hoses, filters), change motor oil before test, carb in tune, compression would be good to know, fresh gas fill up before test is good. Catalytic works best when heated up so drive it for a good 1hr and keep er hot before test.


If it fails again, better keep your pockets full of cash.

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CO is unburned gas. So either it wasn't lit properly (ignition) or there was too much for the amount of oxygen in the air. (carb)


The catalytic converter simply cleans up what isn't burned in the engine but it can't work miracles if you are dumping fuel in from a bad carb or poor ignition. In addition the cat needs an oxygen supply to combine the excess fuel with, in order to work. This extra 'air' is provided by the AIS (air injection system) using negative exhaust pulses to suck in air from the air filter through a one way valve. You should have a single pipe from the exhaust manifold to a small box on the side of the air filter where the one way valve is.

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High CO is "underburned" fuel.  Unburned fuel is high HC.


High CO can be too rich (carb leak, closed choke, plugged air filter, sunken float), or burning oil vapor from an open PCV.


High HC can be too lean (lean misfire), cracked/leaking vacuum hoses, or ignition failures (no spark in a cylinder, wrong timing, weak spark from bad wires/cap/rotor, etc), or a very worn out engine (oil fouling and blowby).



That's what the books say, at least. 

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You're right. HC is unburnt fuel. CO isn't just unburned fuel but fuel burned without enough oxygen present. I'm only partly right. Properly burned, carbon bonds with two oxygen atoms (dioxide) Carbon monoxide (one) is carbon bonded to only one oxygen atom and is extremely toxic..

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  • 4 weeks later...

Run non-resistor plugs such as the BP6ES not BPR6ES.  = hotter spark.

Run a GOOD set of plug wires that Ohm less than 5000 Ohms per wire.  = hotter spark.  

Retard the ignition timing so it runs poorly = hotter combustion chamber & converter = less CO at the tailpipe..  

That should get you to pass smog.  

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