Oh woe is BBD me - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 08-23-2000, 08:28 PM
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Oh woe is BBD me

A little history first:
- 87 4.2l., BBD carb.
- using the mopar orange ignition module with stepper motor disconnected.
- MSD Coil and wires
- about 30k km on a rebuild.
- government insurance is mandatory in Canada, and is illegal to drive without it.
- to get insurance, you must pass an emissions test every year.

I had my annual emission due so I gave my YJ a tune up. And being the smart a$$ that I am, I leaned out the carb a little. I have only an idle test, because they think YJ's are all wheel drive. I don't correct them.
I fail, high CO.
In my mind I am running to lean (to much air). So I screw the screws all the way in and back out about 2 turns. then with an expanded scale tach, I do my best to set the best lean idle. I turn the screws out slowly and evenly until the idle speed drops. Then back in until the rpm comes back.

Test number 2 and 3:
My CO is off the scale!. I know my mix is richer than it was, but here I am. the testing facility closes in 10 minutes. I quickly re-adjust the screws out (more air), run the test again, no change.

My insurance runs out tomorrow, and I can't renew without passing.

What am I missing?

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post #2 of (permalink) Old 08-23-2000, 09:52 PM
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Re: Oh woe is BBD me

I'm sure there are lots of other people here who can tell you about the in's and out's of the bbd, plugged air tubes and setting the stepper before you disconnect it. But there is one thing I can suggest if you really want to know if you're lean or rich. If your O2 sensor is still intact connect a good digital voltmeter to the sensor wire. If the voltage is less then 0.5 volts you're lean if its greater then 0.5 its rich. I think I got that the right way around. This is the poor man's AF gauge. The O2 sensor produces its own voltage so it doesn't have to be connected to the computer or anything.
BTW, I've always had to run the idle rich to get it smooth, but then I don't have to pass emissions here on Vancouver Island.

post #3 of (permalink) Old 08-24-2000, 07:50 AM
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Re: Oh woe is BBD me

Just a thought,but are you giving the cat. time to warm up?If you have to sit in line idling for more than 10 min. it may not be fired up either,try holding rpm up at 1500 just before they test.Check your o2 as max said,by the way,lean increases hc,rich increase both co and hc.Set your carb to factory spec's and try to determine what else is missing.You can check your cat with a temp probe,the rear of it should be 30%hotter than the inlet.hope this helps

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post #4 of (permalink) Old 08-24-2000, 09:59 PM
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Re: Oh woe is BBD me

Try turning the mixture screws in and out until you get the highest r.p.m. Then set your curb idle 10% over spec. i.e. if your idle spec is 800 rpm, set it to 880 rpm. Now turn your mixture screws in (lean) until your idle comes down to 800. This is called lean drop, and should get you into the ball park.

If you failed for high CO%, you are too rich. Backing the screws out until the rpm drops is incorrect, there is also a built in limiter in the idle fuel mixture circuit, so you get to a certain point, and the mixture screws won't do anything.

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post #5 of (permalink) Old 08-27-2000, 02:55 PM
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Re: Oh woe is BBD me

What does your "Idle" test consist of? If it has a 2 speed idle test, like that we had here in CA, it also tests at 2500 rpm. You may have the metering rods adjusted wrong if it is failing at a higher RPM.

Dorfs is right about the lean drop method of adjusting idle mixture, it is the way to do it properly. What is the limit of idle RPM? Set it to the max allowable. Also, reset your timing to the lowest setting allowed on the emissions label, if it says 9*BTDC +/- 2*, set it to 7*.

It may pay, in the long run, to pay a tune-up mechanic to set your Jeep on a gas analyser, that is the best way to do it.

Good luck!

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