CJ7 barely failed emissions

That sucks. I just replaced it two years ago.

I have no cat and got 0.16% CO.

Am watching Dune, just in between episodes, will think on it.

Mike

KurtS wrote:

High CO = too rich. Lean out the idle mixture a bit. You didn't fail by much, so it shouldn't take a big adjustment.

That's what I thought. It has been running a little rich lately (black tail pipe recently). Since the canister is new from the dealer a few months ago, I'll give the tubes a ream. It's been about 9 months since the last time. Tnx guys!

It doesn't work that way. The computer will correct the mixture (lean or rich) to achieve the "proper" amount of O2 in the exhaust.

Plugged idle tubes can cause it to run too lean(not enough fuel). Yours is too rich(too much fuel). You could probably give each of the idle mixture screws about 1/2 to 1 turn in (clockwise) and be fine. If the tubes are plugged, cleaning them will only add more fuel. Not the direction you need to go.

Other way around bllsht.

The computer sees a lean condition because of blocked tubes and goes into full rich mode.

When anything gets out of the computer's range, like blocked tubes will do, it goes into 'limp home' mode and sets the carb rich and clamps down on the timing advance.

The mix screws on a computer controlled Carter BBD are blocked by a 1/4" deep and 1/4" round metal plug that has to be drilled or cut out to adjust them.

Mike

OOPS!

The one line should read:

Clean tubes and the 'idle' screw spring is about 5/8" long....

Mike

Mike Roma>

Blocked idle tubes cause a lean condition. The computer will try to correct. If the mix is so lean it's out of the stepper motor's ability to correct the mix, it's still too lean.

In the post I replied to, you said "Too rich can be caused from partially blocked idle tubes." That's not true.

The idle mixture screws are set correctly when the stepper motor is moving in and out near the middle of it's range.

That's because it's so lean you have to open the throttle to keep it running.

Yes, because it's too lean and they're trying to keep it running.

There aren't many Jeeps with carbs running around that still have sealed mixture screws. By now they've all been cut, drilled or whatever. It's probably too rich because somebody's been f*ing with it and don't know what they're doing.

It goes as rich as it needs to go, to correct the mixture. Remember that it's looking for oxygen content in the exhaust, as reported by the O2 sensor. If O2 content gets too low, it will back out the stepper motor to lean it out.

Nope. When the mixture gets out of range, it moves the stepper as far as it can, in the desired direction, in an attempt to correct the situation.

I bet there aren't many around that still have the plugs in them. I haven't seen one with the plugs in years. That means there's a lot of mixture screws out there that anyone who knows which end of a screwdriver to hold can, and will, turn. Usually in the rich direction.

Could a Jeep possibly have been that close to passing smog in "limp home" mode?

Before this gets ugly, bllsht is right on the money. Thinking back to the last rebuild, I remember backing the idle screws out a little mit - not much. I had heard it's best to get them out of the way as far as possible when the computer is still in charge. Apparently my source was sadly mistaken.

I guess as bllsht says, there aren't many if any perfect' setups left so the mix screws are now in play on most...

Mike

KurtS wrote:

Not likely, more likely his air filter is dirty or something.

Mike

I don't think the computer is that smart.

I think when it sees a lean it can overcompensate.

The OP was only slightly over on CO.

The stepper motor only has so many steps. The proper mix could be a half a step leaner than the computer sets it.

Mike

bllsht wrote:

I hope you're kidding...

The computer monitors O2 content of the exhaust. It alters the mix to achieve it's desired O2 level. That's what it's designed to do, and yes, it is that smart.

Right. Too rich. Too much fuel. Not enough O2. That's what I said.

Which is why I said the mixture screws are probably too rich. If the stepper motor has reached it's lean limit, then the mixture screws are set too rich, or something else has gone wrong in the carb to cause a rich condition. The computer will not cause it to be too rich.

Not kidding, really....

Here is how I see it working. If I am mistaken, I would appreciate the feedback.

The OP stated he has a 'stock everything' 85.

I therefore do not assume that the carb has been drilled or cut up to access the idle mix screws.

The stepper motor has (for easy numbers) 'say' 5 steps from full rich to full lean.

The carb if manually set up has 'say' 5 turns on the mix screws from full rich to full lean.

With a perfectly working system, either the stepper or the mix screws if a manual BBD operate approximately in the middle of the range or at 2.5 turns out for the screws or jumping from step 2 to step 3 to get the O2 sensor to read correctly with the computer.

Now we toss in a partially plugged idle tube or something else gummed up forcing a lean to 'some' extent.

If I am manually setting the mix, I then need to open the mix screws to 'say' 3 1/4 turns out to get it stable.

In comes the stepper motor on the computer BBD. It has a choice of either the 3rd step or the 4th step.

It tries step 3, nope, too lean. 1/4 turn too lean if manual. It then tries step 4, nope again, too rich. 3/4 a turn if manual too rich.

It keeps jumping back and forth between step 3 and 4. Every time it hits step 4, it goes rich, a lot more rich than it goes lean when on step 3.

Bingo, a carb that 'just' fails emissions by being slightly rich.

I also believe this is one of the main reasons you see the mix screws cut open. Forcing a mix change via the manual mix screws can make the O2 tell the stepper motor to 'see' it as 'say' 2.5 steps exact like it was from the factory so when it samples it is an even 2 step to 3 step pulse which puts the mix back into emissions range.

This is just fixing the symptoms, that blocked idle tube is still there...

Mike

bllsht wrote:

OK. So Factory Settings should be in the neighborhood of 2.5 turns out from full travel in, correct?