How to lean out a Rochester 2bbl?

[Mr. 81]

I've had to deal with a few hurdles in registering my car here in California. One of which is smog, of course. I just barely failed smog the last time around, with excess hydrocarbons at 25 mph. A couple of people have told me to lean out the mixture, but i've never worked on a carb before. So my question is: How do I lean out the miture on a Rochester 2bbl carb and Is this even the proper course of action? It does smell strongly of gasoline...

 

[JohnS]

Hopefully some who know for sure will comment because I am in the same boat with not passing emissions but these are 2 things that were suggested to me to do. I turned in the 2 mixure screws at the base of the carb 1/2 turn. I believed this helped as the idle increased a bit which if I rememeber correctly = lean. Also upon the recommendtion of the same person he advised to turn in the Idle bleed screw (which is at the top of the carb under the air cleaner) then back it out 1 1/2 turns.

I really don't know if this worked as I just did it yesterday and have not been back to the inspection station but I have noticed the exhaust does not smell as bad and I do not see any black smoke. Worst case I can always put it back to the way it was.

 

[JohnS]

oh yea, check to make sure the flap in the air cleaner housing tube is open.

 

[Mr. 81]

Hopefully some who know for sure will comment because I am in the same boat with not passing emissions but these are 2 things that were suggested to me to do. I turned in the 2 mixure screws at the base of the carb 1/2 turn. I believed this helped as the idle increased a bit which if I rememeber correctly = lean. Also upon the recommendtion of the same person he advised to turn in the Idle bleed screw (which is at the top of the carb under the air cleaner) then back it out 1 1/2 turns.

I really don't know if this worked as I just did it yesterday and have not been back to the inspection station but I have noticed the exhaust does not smell as bad and I do not see any black smoke. Worst case I can always put it back to the way it was.

Thanks! I'll try these when i get a chance.

 

[Bonnewagon]

You can only lean it out so much before you go too far and raise the NoX. You may have a leaky bowl float that is causing the richness. On stubborn cars I used to pass emissions by changing to leaner jets and then swapping back the rich ones after the test

 

[Mr. 81]

You can only lean it out so much before you go too far and raise the NoX. You may have a leaky bowl float that is causing the richness. On stubborn cars I used to pass emissions by changing to leaner jets and then swapping back the rich ones after the test

Thanks for the advice, i can afford to raise the nox a little bit at this point, so i'll try the mixture screws before anything else.

 

[Bonnewagon]

Best way to adjust idle screws is with a vacuum gauge. That way you see what affect you are having and you can adjust for the highest vacuum, and leanest run without causing a lean misfire. But failure to respond usually means it's time for a carb rebuild.

 

[Mr. 81]

Best way to adjust idle screws is with a vacuum gauge. That way you see what affect you are having and you can adjust for the highest vacuum, and leanest run without causing a lean misfire. But failure to respond usually means it's time for a carb rebuild.

Thanks. So I attach the gauge to a vacuum port (on the front of the carb?) and set the screws one at a time for the highest vacuum?

 

[Bonnewagon]

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.

 

[Mr. 81]

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.