I have a new to me F150 with a I6 and a 4 speed. It has lived it's whole life in Idaho and never had to pass an emissions test. I brought it into Utah and it fail. It hadn't been driven much the last few years but I've been using it as a daily driver for the last few months. It has 117K miles on it and I've been getting 13-14 mpg out of it right now.

I'm needing some help, ideas, checklist on what I can repair, replace, and tune up to help her pass the test and stay on the road. The failure was high CO levels. We did check the gas caps and they both passed. Thanks in Advance,

 

Well, an '81 300 did not have much in the way of emissions equipment. I am not sure about the '81, but my '82 300 only has the following emissions systems.

Air pump
Catalytic converter
EGR
PCV

High CO emissions are usually caused by running on the rich side. This would also increase your hydrocarbon emissions, but maybe the Cat and air pump take care of that. Also, 13 to 14 MPG is not bad for your truck, so it is not running super rich.

How are your emissions tested? Treadmill, or just at-idle test? If it is tested at-idle, you could tweak the idle mixture screw pretty easily to lean it out a bit and see if that solves the problem. I am not 100% sure, but I think on the YF carb, the idle mixture screw controls air through the idle slot, not fuel, so backing the screw out makes it leaner. Someone can correct me if I am wrong.

 

Should also add - retarded ignition timing can also cause high CO emissions. Check your timing and make sure it is where it should be.

 

Hey RodSUT,
Been there, done that. I live in the only play in Idaho that does do emissions testing.

The 81 is very simple, as far as emissions equipment goes. I actually got mine to pass the sniffer test without any of it. Just a good working 3-way cat, and a well tuned carb was all it needed. The new cats can pretty much do all that the 81's emissions equipment was capable of.

So, be sure your catalytic converter is good. New 3-way cats are highly recommended.

Next, there's the EGR. The EGR spacer sticks out right under the carb and the EGR valve is on the end of it with a vacuum line going to it. Replace the EGR valve if it's bad, and use a vacuum gauge to be sure the vacuum line only pulls vacuum on the valve when it's revved (should be 0 at idle) and only after the truck's warmed up. If not, re-check your vacuum line routing. There's also a pipe that goes from the rear of the exhaust manifold and up into the EGR spacer.

There's also the smog (or air) pump. It has a rubber hose that exits the pump, goes to a valve (with a vacuum line that pulls it open under certain conditions) and then goes into the EGR spacer. It gives fresh air into the system.

That's pretty much it. If those things are in good shape, then start checking the little things:

Replace the PCV valve.
Make sure the air cleaner is new or clean.
Make sure the little breather filter inside the air cleaner housing is new.
Be sure the charcoal canister is hooked up correctly (check vacuum hose routing).
Be sure your timing is spot on. Too much advance or too much retardation can screw it up.
Be sure the engine is HOT before doing an emissions test. Take it on the freeway and drive for about 10 miles @ 70/75mph and pull right off and into the emissions testing station.

Lastly, be sure the carb doesn't have any leaks, check the throttle shaft for play. That's the one that killed me, and I couldn't get mine to pass for anything until I replaced the carburetor. My throttle shafts were completely worn and were sucking air. Failed with around a 1200 / 220, was able to tune my old carb until I finally got it down to around 500 / 220 and that was as close as I could get it. Replaced the carb and passed with an 80 / 220.


With mine, I had EFI exhaust manifolds on at the time, so my EGR and smog pump were NOT hooked up. I got it to pass easily once I put on a good carburetor. The 3-way cat took care of the rest.

 

Also, if possible, can you post up your exact numbers from the test? Those help a lot.