I have a 1977 4.9 (300) inline 6 that I can't get to pass emissions. The truck came from California where it just passed emissions but I can't get it to pass here in Utah. I've checked everything. Vacuum, ignition, etc. Rebuilt the carb which is a 1 brl Carter YF 1_V. I finally got it to pass on everything else (HC and High speed Co2) by getting the coolant temp up but it still won't pass the idle CO. It fails by -3.37 %. I've retarted the timing to about 4 degrees. Suppose to be a 10 BTC. I have the idle mixture screw turned out at 1 1/2 turns have put a vacuum gauge where the PCV valve would go and have tried turning it out further but the vacuum doesn't seem to increase any. If I turn the idle mixture screw in it will start to stall. If I disconnect the vacuum gauge and put the PCV valve back on and in the valve cover and try to turn the mixture screw in, it doesn't seem to phase it at all. I'm at a loss as to what I should try next to get this thing to pass emissions. I've had it tested 3 times. Each time was better as I made various adjustments but have run out of adjustments to try unless I turn the idle mixture screw way out to try and lean out the idle mixture. Any ideas if this would help. Any ideas as what I should try next?
Here are the readings for the idle test.
Standard HC(ppm) 500 CO% 3.00
Reading HC (ppm) 221 CO% 6.37
Deviation HC(ppm) 279 Co% -3.37
Result HC (ppm) Pass Co% Fail
CO2% 10.5 RPM 664

 

Obviously the choke is wide open? You could try going into the test with very little premium fuel in the tank and adding a bunch of methal hydrate (gas line antifreeze) to the tank just before it goes in, like at least a gallon. After you test then fill with regual fuel. The methal hydrate is alcohol so it burns pretty clean and hot. Don't drive it like this for long without diluting it with gas though.

Also, have you checked your trans modulator valve to ensure it's not leaking and allowing tranny fluid to be pulled into the engine? Did you change your oil before the test? Make sure she's got a nice hot exhaust before it tests too.

 

The choke is wide open. It's a standard transmission. Inline 6 engine with headers. I'll have to pay them to tweek it. I've tried everything.

 

Might try taking it to another testing facility. If the one you took it to also does repairs, they could be just trying to gouge you. Just my 2 cents worth............

 

CO is carbon monoxide. That's what the cat turns into CO2, carbon dioxide.

Lowering HC will lower CO, as you have seen. But it would seem maybe your cat isn't working as well as it should.

You have headers--any chance the cat isn't getting or staying hot enough?

Also, did you have a smog pump? If so, is that hooked up and running right?

 

This truck doesn't require a Cat converter or a smog pump. It passed on those items. It just won't pass Co2 on idle.