How to lean out a Rochester 2bbl?

[JohnS]

Re:

Yes, find a port with full manifold vacuum, which will be around 20" at idle. Turning the screws in will lean it out which will raise the vacuum slightly, then it will drop and stumble. Turning them out richens it. You want to first richen it up, then slowly turn them in alternately to get the highest vacuum before it stumbles. So say you can get a smooth idle at 21", but leaning it anymore makes it stumble, leave it at 21". If you cannot get a smooth idle by this method, you have internal carb problems. A leaky float will cause flooding that no amount of adjusting can fix, it needs replacing. Also first check all hoses for vacuum leaks, another reason why you can't get a smooth idle. If this is a computer carb, it is supposed to self-adjust. But it will still respond to manual mixture screw adjustment within reason.

Wow, thanks for all the info! I'll let you know how it goes when I get around to working on it this week.

Thanks from me too

 

[Bonnewagon]

What kind of emissions test do you face? A simple tailpipe sniffer can be fooled by leaning out the idle mixture screws, but the idle quality usually suffers badly, and as I said before NoX goes up with lean misfire. But then you can adjust for better drive-ability later on. Do they let you adjust it on site until it passes? Or do you have to drive up as is and pray? An IM-240 test ( on the rollers) is impossible to pass unless everything is right on.

 

[Mr. 81]

What kind of emissions test do you face? A simple tailpipe sniffer can be fooled by leaning out the idle mixture screws, but the idle quality usually suffers badly, and as I said before NoX goes up with lean misfire. But then you can adjust for better drive-ability later on. Do they let you adjust it on site until it passes? Or do you have to drive up as is and pray? An IM-240 test ( on the rollers) is impossible to pass unless everything is right on.

Unfortunately, here in CA the test is on a dyno with all kinds of testing involved -- tailpipe sniffer, timing, visual inspection, gas cap pressure test, etc.

So today I finally got around to looking at the carb, here is what I saw:



The above two photos show the holes containing the idle mixture screws, right? The driver side one is below the connector, but hard to see in the pic. How the hell do I get to those things? There's not even enough room for a stubby screwdriver. Is there a special tool I need to get?

Here's a pic of the carb in general. Is this one computer controlled?

 

[Bonnewagon]

Yup, that's a CCC computer carb. Not a lot you can do besides a rebuild. For the IM-240 dyno test you'd be better off getting EVERYTHING up to snuff emissions wise.By that I mean a full tune-up, with new plugs etc, because they test you under the full range of driving conditions. Since you failed on HC, which is unburned fuel, the tune up will cover all except the carb. Leaning the idle screws will do nothing for 25mph running. It sounds like there is a flooding/float problem as you say you smell gas and a rebuild would fix that. But at 25mph the mixture is adjusted by the computer so there may be more trouble inside. So now you must decide do you rebuild it yourself or get one already done? This depends on your skill level, and as always, the best carb is the original that came with the car so my advice is to get the original rebuilt, not an off-the-shelf replacement. As for the adjusting screws, this is what I use, it is a Snap-On tool with the "double D" socket that fits Rochester carbs..

 

[Mr. 81]

Yup, that's a CCC computer carb. Not a lot you can do besides a rebuild. For the IM-240 dyno test you'd be better off getting EVERYTHING up to snuff emissions wise.By that I mean a full tune-up, with new plugs etc, because they test you under the full range of driving conditions. Since you failed on HC, which is unburned fuel, the tune up will cover all except the carb. Leaning the idle screws will do nothing for 25mph running. It sounds like there is a flooding/float problem as you say you smell gas and a rebuild would fix that. But at 25mph the mixture is adjusted by the computer so there may be more trouble inside. So now you must decide do you rebuild it yourself or get one already done? This depends on your skill level, and as always, the best carb is the original that came with the car so my advice is to get the original rebuilt, not an off-the-shelf replacement. As for the adjusting screws, this is what I use, it is a Snap-On tool with the "double D" socket that fits Rochester carbs..

Wow, really? I was worried that these "idle" adjustment screws wouldn't help my 25mph HC emissions. In that case, I'll look for a cheap rebuilt carb or get mine rebuilt. By the way, When I start my car I need to keep my foot lightly on the gas until the temp is almost out of the blue or the car will stall. Is that a choke problem? or could the idle screws actually help with that issue?

BTW, Thanks again for all this info!!

 

[AlanB]

My reply is specific to Mr. 81. It would be very helpful if you listed all of your emissions numbers, both at idle and at speed. You should have readings for CO, HC, CO2, O2, and NOx. These other readings are really important in determining the root cause of high HC. Running too lean can cause high HC (lean misfire) but should have relatively low CO. A very rich engine will have high HC along with high CO. Also, low CO2 and high O2 would help indicate if your AIR pump is feeding air to the cat during the test, which would help lower HC. If you have high CO2 and low O2 then you AIR pump is probably not working.

I would not guess on a carb until you are certain that your emissions devices are working. Is the AIR pump feeding the cat? How old is your O2 sensor? There are ways to test them, but new ones are $18. How old is your cat? How about secondary ignition, plugs, wires, cap , rotor? Even a slight misfire under load will raise HC. Your total emissions numbers will help to eliminate or draw attention to certain components.

Before replacing the carb, you should check M/C dwell using an inexpensive dwell meter. I don't have time to go into the procedure now, but M/C dwell indicates how rich or lean the ECM is adjusting the carb. You would check dwell at idle and also at 3000 RPM and look for around 30 deg M/C dwell. Lower means the car is lean and the ECM is adding fuel to adjust, and higher means the car is rich and the ECM is reducing fuel, or leaning it out to adjust.

I few tricks that might help HC? Use regular gas, use a 195 deg stat to get the engine as hot as possible. Retarded timing lowers HC, but CA checks that. Is there a range, or do they set it exactly? If there is a range, try setting it to the low (retarded)side of the range and see if that is enough to get you through.

 

[Bonnewagon]

AlanB is correct, you have to know what is the big picture here. Yes, and needing to hold your foot on the gas is a choke/fast idle problem. Mixture adjustment does nothing here. I'm thinking this motor needs to be properly inspected and diagnosed by someone who really knows the CCC system and set up correctly from scratch.

 

[Mr. 81]

My reply is specific to Mr. 81. It would be very helpful if you listed all of your emissions numbers, both at idle and at speed. You should have readings for CO, HC, CO2, O2, and NOx. These other readings are really important in determining the root cause of high HC. Running too lean can cause high HC (lean misfire) but should have relatively low CO. A very rich engine will have high HC along with high CO. Also, low CO2 and high O2 would help indicate if your AIR pump is feeding air to the cat during the test, which would help lower HC. If you have high CO2 and low O2 then you AIR pump is probably not working.

I would not guess on a carb until you are certain that your emissions devices are working. Is the AIR pump feeding the cat? How old is your O2 sensor? There are ways to test them, but new ones are $18. How old is your cat? How about secondary ignition, plugs, wires, cap , rotor? Even a slight misfire under load will raise HC. Your total emissions numbers will help to eliminate or draw attention to certain components.

Before replacing the carb, you should check M/C dwell using an inexpensive dwell meter. I don't have time to go into the procedure now, but M/C dwell indicates how rich or lean the ECM is adjusting the carb. You would check dwell at idle and also at 3000 RPM and look for around 30 deg M/C dwell. Lower means the car is lean and the ECM is adding fuel to adjust, and higher means the car is rich and the ECM is reducing fuel, or leaning it out to adjust.

I few tricks that might help HC? Use regular gas, use a 195 deg stat to get the engine as hot as possible. Retarded timing lowers HC, but CA checks that. Is there a range, or do they set it exactly? If there is a range, try setting it to the low (retarded)side of the range and see if that is enough to get you through.

Man, I had no clue how much there was to this! I can definitely post all the numbers and what i've done to the car so far. I'll do it as soon as I find the paperwork. Thanks!

 

[Mr. 81]

So I finally dug up the paperwork for the failed smog. As you can see, the ONLY thing I failed was HCs at 25mph! So far I have changed the air filter, fuel filter, oil/oil filter, spark plugs, wires, pcv valve, and I've made sure there is no old gas left in the car. I've been running it on 87 octane.
Your help is much appreciated!

 

[Bonnewagon]

You are right, all is well besides that 25 mph reading. All I can think of is that at 25 mph you've just upshifted and the carb is too rich with that particular load on the motor. I believe the MAP sensor (vacuum) and tach should be telling the computer that the load/rpm has changed and should adjust the mixture accordingly. Hopefully AlanB will check in and lend his knowledge here, he seems to be fluent in emissions testing.