My 85 F150 4.9 4x4 4spd factory carter 1 bbl. failed the goverment emmisions test yesterday .
It looks like the carb set to rich. Does anybody know how to properly set the idle mixture screw. I don't want to start turning screws left and right and #^&% everything up by running to lean.

Below are the test results:

40km/h ( 25mph)
HC ppm limit= 144 ,Test result=45, Passed
CO% limit= 1.2 ,Test result= 0.10 Passed

Curb idle
HC ppm limit= 300 ,Test result=149, Passed
CO% limit= 1.5 ,Test result= 4.24 FAILED

 

turn the mixture screw until the engine runs smoothest then a 1/4-1/2 more turn. that's what i do

 

With the way most emission checks are, the leaner, the better. I'd try Vampiro's suggestion -just 1/4 to 1/2 turn leaner, not richer. A lot of shops that do this test will adjust this accordingly for a "minor" charge. Summit has a device you can connect to your exhaust to adjust your mixture, but you'd need an O2 bung to fit it into...probably a little to pricey too.

 

I just put my 85-300 through the emisions test and had no problems.
I set the mixture screw at 1 3/4 turns out from seated, leaned out the electric assist choke slightly while hot and used a cheater vaccum line from the manifold vaccum to the valve in the thermactor system (air pump) to keep the valve open at all times. This allows air from the pump to flow into the exhaust system quicker. There are two valves in the thermactor system, use the lower of the two. This helped me. Here are my test results:
@40km/hr @ curb idle
HC ppm...20 19
CO% ...0 0
NO ppm ...999

Good luck