MGB Smog test fail

[Jerry]

1978 MGB with a rebuilt engine and about 400 miles on it will not pass smog after 3 tests. The carb is pretty lean but the CO2 still does not pass. We can tell it is lean based on the lack of acceleration and pinging. IE: the acceleration was there prior to adjusting and taking it to smog. The smog pump worked when we rebuilt the engine but since its job is pumping outside air into the exhaust prior to the catalytic convertor, how do I know it is pumping enough air? Is there a way to increase the air that is pumped into the exhaust, ie, use the dilutional effect?
Last question, the car has its original catalytic converter. We know it works, because it gets really hot. But would there be an advantage in adding another one to the car to get the smog test passed?

Any other ideas?
Jerry

PS: California car

 

[Sarastro]

Back in that era, as I remember, it was almost impossible to pass a smog test unless the carb mixture was set up with the car hooked to the emission tester. You adjusted the mixture for minimum emissions. It might be just that simple.

For one thing, having the mixture as lean as possible does not mean you will minimize emissions. The catalytic converter works best in a narrow range close to the stoichiometric air/fuel ratio (14.7); one way, you get more NOx and the other, more CO and I forget what else. The sweet spot is rather narrow.

As for the catalytic converter, I'm not sure that its getting hot means it's necessarily working, although I realize it has to be hot to work. I'm no expert on this, but I always assumed it gets hot just because of the heat of the exhaust. If there are other phenomena that heat it, I'm not aware of that.

People who know more about this than me can tell from the test results if the problem is simply adjustment or the cat converter, air pump, or whatever. So, I think that if you take it to a shop that specializes in this, they should be able to diagnose it.

 

[Deleted member 8987]

https://www.aircare.ca/repinfo-ere-causes-co.php
Above gives some pointers.
When I used to have to get cars through this process, I'd start with several things.
First, remove the air cleaner element and place it in the boot (unless they look in the air cleaner housing).
This STOPS any restrictions causing high CO2.
Next, unless they do a visual, I'd remove the PCV hose from the valve cover and let it suck clean air.
If that isn't possible, due to peeking, change the oil and filter. Contaminated oil will do that with a PCV system.
Then, if this is an idle test, lean out the idle mixture.
If high speed, drop the float level a bit or if your car can limit load mixture, set that lower.

Do you have the importation paperwork?
I do remember the readings were hard and fast as far as meeting specific levels...except British cars, which IF you had the original paperwork that stated what your car's readings were, that it was accepted for sale with, THOSE were the limits, not the book.

Dave

 

[Jerry]

Car has been passed through 2-4 owners so the original paperwork is gone. The guy that did the test did inspect, but really has not much idea what is stock. I know the removal of the air cleaner makes it lean out because we took the car for a test drive without the air cleaner. It was lean, no accelleration at all. In California, the test is run at 15 mph and 25mph.

I keep thinking the CAT is no longer working as it should . An expensive part.

 

[Deleted member 8987]

Air cleaner better not lean it out that much if removed. Means you've adjusted the carb(s) with it on and it's very restricted.
But...we have a problem.
Bear with me, as it's been a LONG time since I did this in a shop.
But:
CO2 SHOULD be high, the higher the better. Higher COP2 readings tell you you have good combustion.
Low C02 shows possible misfires. Higher the CO2, the more efficiently the engine is operating.

So, CO2 is LOW? Only way it won't "pass". All sorts of things, including dilution from exhaust leaks.

Confirm.
Dave

 

[Deleted member 8987]

What are your 02 readings?

 

[Jerry]

15mph o2 3.1
25mph O2 3.6

I will correct my fail above, it failed the CO measurement. IE: at 15mph max is 1.41 and the car was 2.03 same fail at 25mph.

Jerry

 

[Deleted member 8987]

Ah. Okay, CO, not CO2.
Read up on the link I provided earlier: https://www.aircare.ca/repinfo-ere-causes-co.php

Fuel system too rich. Fuel pressure too high. Contaminated oil. Charcoal cannister saturated. Those kind of things.

Zenith carb, right?
Diaphragm leaking, causing slow lift and rich mixture?

If your HC's are good, really need to look at carb internals, diaphragm, cannister, polluted crankcase (and that happens from short trips).
Dave

 

[JPSmit]

BTW, I have a Harley Davidson take off muffler - I am 95% certain it has a cat built in - cost me $25, often they are free.

 

[smaceng]

Without having a history on the CAT, I would replace that, and as suggested above, need to tune the mixture and timing with the exhaust being tested. You are not that far from what you need. Scott in CA

 

[PAUL161]

1978 MGB with a rebuilt engine and about 400 miles on it will not pass smog after 3 tests. The carb is pretty lean but the CO2 still does not pass. We can tell it is lean based on the lack of acceleration and pinging. IE: the acceleration was there prior to adjusting and taking it to smog. The smog pump worked when we rebuilt the engine but since its job is pumping outside air into the exhaust prior to the catalytic convertor, how do I know it is pumping enough air? Is there a way to increase the air that is pumped into the exhaust, ie, use the dilutional effect?
Last question, the car has its original catalytic converter. We know it works, because it gets really hot. But would there be an advantage in adding another one to the car to get the smog test passed?

Any other ideas?
Jerry

PS: California car

Move to Oklahoma, we don't have any smog tests! Actually, we don't have ANY vehicle inspections. PJ

 

[AngliaGT]

In California,you have to have a California only cat.
I was surprised to hear that when our '94 Dakota needed a new one.

- Doug