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it is normally the HO2S ( heated oxygen sensor) in the exhaust manifold before the cat converters
That unit reports the exhaust gas composition to the ECM which adjusts the mixture according to the MAF/VAF sensor readings
Add to all of those the ambient temp sensor and the BARO sensor and you have the system that adjusts the air/fuel mixture
how to fix
have the fault codes read and replace the faulty sensors
? 5:54
https://www.youtube.com/watch?v=39Wcbp6_yNQ
Feb 23, 2014 - Uploaded by TheWhatisright Hopcorvetteman
It is a standard 350 Chevy with no real modification. .... missing something or can they be in a different location on my carb ( 81vette rochester quadrojet ). ... There are two fuel mixture adjustments on the rochester Carburetor.
The mixture screw is located in a resessed hole just above the therma start plate at the rear of the carby. The rear of the carby faces away from the inlet manifold and you require a special tool to access this adjustment. The other screw adjustments on the carby and linkages are for idle, fast idle, and choke limits including those models with automatic (thermal type) choke mechanisms.
I would advise that if you make any adjustments to the mixture that you have the mixture checked using an exhaust gas analyser.However, as a general rule of thumb, screw the mixture adjustment all the way in until you feel it tighten but DO NOT FORCE IT !! the screw has a delicate needle valve on the end which will be damaged if the screw is forced beyond it's normal length of travel.
Unscrew the mixture screw a total of one and a quarter turns back out and then have the setting checked with a exhaust gas analyser. Too rich and not only is this expensive but will cause damage to spark plugs etc. Too lean a setting will cause a misfire which will damage valves and pistons.
the idle adjustment is located at the throttle plates, but consider having a mechanic look at it, as you may need mixture adjustment, or have vac leaks....
Here are some images that might help to assist you, and let me know if you require any further assistance.
Adjust the curb idle with the engine warmed up and the choke off.
The air/fuel mixture screws should be adjusted with the engine warmed up and at normal idle speed, then turn each air/fuel mixture screw in one at a time until the engine just begins to mis-fire, then back the air/fuel mixture screw out 1-1/2 to 2 turns out a half a turn at a time until the engine idle is the smoothest. (your vehicle might have restrictor caps over the air/fuel mixture screws to limit the amount of adjustment)
The choke adjustments;
To adjust the vacuum tension pull on the choke pull-off:
Adjust the fast idle screw to the proper RPM with the engine cold and the choke on.
You need to adjust the automatic choke. Your choke may not be opening fully. There is a fast idle cam linked to the choke to keep the throttle open more until the engine is warm. The idle mixture screw is part 37 in the picture, but get the choke adjusted right first, and you may not have to adjust the mixture screw. See adjustment procedures at link beloe.
many of the late model cars & trucks can't be
adjusted some of the carbs are preset some you can force and some don't have an adjusting screw for the jets if the carb is in good working order but needs a bit more fuel you u must adjust the float for more or less fuel check to make sure the float valve is clean free and not sticking ..
That engine runs at low speed, starts and idles on the center carb only. first make sure the carbs are synchronized so the front and rear ones are fully closed at idle, then adjust idle speed and mixture on the center carb. Sweet ride by the way, if you get tired of working on it and want to get rid of it I will take it. LOL
On any carburator,one screw is for the idle speed, the other the air/fuel mix.Start the engine and let it get plenty warmed up. Adjust the idle screw to make the engine run slow. Turn the air/fuel screw in until the engine starts to die then out till it starts running loady.You want the screw between these two locations. Adjust it till the engine runs the fastest. Now adjust the idle speed screw to about 1000 RPM.
I hope this helps you.
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