Well, read the manual that's in your glove box!!
A quick google search reveals quite a bit of info.
"Naw, Kar-Kit was a NOX reduction scheme used in CA in the 70's. (I used to install them back then). Anyway, a Kar-Kit plugged the vacum advance lines to the carb and distributor and retarded the timing. It also installed the sticker on the inst panel. The 60 MPH came from the oil embargo of the 70's and the national speed limit was 55. There was also a chance of overheating since the vac adv was deactivated and the static timing was retarded.
There is no requirement to use the kar-kit anymore so I would suggest you remove it, add the vac adv hose back to the dist, and adjust the timing back to factory specs.
Read more: http://forums.mustangandfords.com/mustang-monthly/70/7246628/vintage-mustangs/vintage-emission-systems/#ixzz4ZqoSRdpK"
"Back in the early 1970s here in SoCal a law was passed requiring all cars to have additional NOx devices installed, regardless of any other requirement / non-requirement for smog equipment. The whole thing was a fiasco, a total failure, and the requirement was repealed shortly afterwards.
My 1955 Chev had to have a device installed on the upper radiator hose and was linked to the vac advance on the distributor to control Oxides of Nitrogen produced at various temp/advance situations the engine would see.
I removed it and threw it away as soon as I registered the car (the same day). So did everyone else I knew; the devices made the cars run poorly and wiped gas mileage.
Of course, since my '55 had a 13:1 genuine 430" short stroke injected ZL-1 engine from a McLaren Can Am car, high 9s at Lions and OCIR, I didn't really care about mileage. These engines were suddenly cheap in the early 70s when there were a bunch around (if you knew where to find them).
It was funny to see the gas station mechanic install a smog add-on to a full-race car. You weren't allowed to install the device yourself. The mechanic followed the State of California guidelines to install it and used black electrical tape to fasten the sensor to the upper radiator hose. Of course as soon as the engine warmed up the tape failed and the stupid thing fell into the fan and was ripped away. Later the mechanic complained that he tried everything he could to get a smooth idle for me, but the engine was "really rough running... maybe you should rebuild it". When I explained the engine was a 700 hp McLaren motor (it even had the McLaren SCCA-spec valve covers) he told me I was wrong, since McLaren stopped making cars before 1927.
This was an "Official Smog Station" certified by the State of California."
