Emissions and putting things up your exhaust

What did it fail on?

You should have 2 readings %CO and ppmHC
Limit is 4% CO and 1200ppmHC

CO is a measure of how rich it's running, bigger = richer, I dunno about the HC...

Stuffing things up the exhaust will change the emmissions but only because the engine will struggle! Also plastic in exhausts isn't a good mix.

I'd guess it's runnign a bit rich, a good old school tune up should fix it.

forgot to say, it failed on CO level. It's around 6.2 (almost double) The oil probably could do with changing, air filter is flowing well and other than that the exhaust is the only difference since it last passed. When it had the standard backbox the CO was around 0.9

That is way too high. Changing the back box shouldn't effect your emissions to that extent, the only exhaust modification that may do that is a de cat pipe. Even so the figure seems to high. The purpose of a catylitic converter is to convert CO into CO2.

First thing to do is to check the mixture but before going near the mixture screw check first that the choke auto or manual is sticking, then go for the mixture setting on the carb, ideally do it with emissions probe in and lean off the mixture untill it 1 sounds better and is running smoother and 2 CO goes below pass value. Also take the car and absolutly thrash it up through all the gears and represent it, this often is enough to make it pass as it blows soot out of the exhaust which can be picked up by the flow affecting your emissions result. Also make sure the engine is properly warmed up and completly off choke.

Sounds good does it drive any better now?