how do i adjust the fuel air mixture?

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i have a 74 chevy nova with a v8 4bbl rochester, i think its called a quadrajet (sp?) i know where the two screws are to adjust. one day i was trying to find how to adjust the idle, and i messed with the driverside screw of the air fuel mixture. i didnt hear or feel anything change so i tried to put the screw back to where it was before i touched it. but i dont think i did it right. after that i found out how to adjust my idle. about a month ago, i noticed that a few of my sparkplugs were black and the're pretty new (less than a thousand miles) so i believe my mixture is too rich. i have the hayne's rebuild book on my car and it briefly tells me how to adjust the mixture but i dont understand it. thats about it.

posted by  fAt2Rak005

Is your Rochester a 4MV? /80/11/b7/c0/0900823d8011b7c0.jsp

posted by  tbaxleyjr

i think it is.

posted by  fAt2Rak005

Ok since I'm really old, I know this carb pretty well so I'll try to help.

The idle adjust screws probably are not the cause of the sooted plugs. I will get to that later.

The Idle adjust screws on the carb only adjusts the idle fuel/air mixture at idle. Here is an old trick. get a plastic glass half full of water or a hand held engine tackometer. After you start the car place the glass on the block where you can see it. I know you don't have the ear to do this. The ripples in the water will tell you how smooth the car is idleing. The carb has three fuel circuits in it, one for Idle, one for cruise and one for full wide open bore. Idle is set with the screws. Each mixture screw adjusts one bank of the engine. That is a cross flow manifold so the screw on the right feeds the bank on the left. The other circuits are set by sizing jets.

To adjust the idle mixture screws, first set the idle screw to the lowest idle you can get and the car still run. This is the screw on the linkage. The screw that prevents the throttle plate from closing fully. Now adjust the idle screws for the smoothest idle you can get. Both of these screws should start at about 1 and 1/2 turns from full in, and will probaly adjust less a 1/4 turn either way. As you adjust you will notice the engine increase in smoothness and speed. Adjust for best idle. You may need to re adjust the idle screw on the linkage again to slow the engine down some more. If the idle screw is to high, the mixture screws will appear to not work because fuel is being pulled through the cruise circuit instead of the idle circuit. The ripples in the glass of water will show how smooth the engine is running. ( a tach is best though, adjust for highest speed) After the the mixture screws are set adjust the idle screw fo the best idle, ( about 800/900 rpm) and your done.

Now a couple of things about the carb. Since you don't drive arround at idle all the time, then the idle screws aren't responsible for the blackened plugs. This carb is famous for running rich, and there are two things that can cause it.

1. The accelerator pump ckt has two channels about 1 and 1/2 inches long which reach down in the fuel bowl. The bottom of the ports set down in the fuel and the little plugs located at the very bottom, leak which allows the accelerator pump nozzles to siphon fuel while running down the road. There was a mod in the rebuild kit which included a large rubber block which provides a seal for these drilling ports.

If you look down in the carb (with the engine off) You will see two little pipes sticking out from the middle of the carb. These are the output ports from the accelerator pump. Grab the throttle (without the engine running) and open it wide open, you should see two small jets of fuel shooting into the throats of the carb. This says the acc pump is ok. The acc pump enriches the fuel mixture when you romp on the gas (need because of the sudden inrush of air). If the car bogs on hard acceleration this is usually the problem.

2. I don't think this car has an electric choke, I think they came later. I think this carb has a stove located on the choke heated by a tube running down to the manifold. If this plugs up the choke doesn't operate properly and doesn't open all the way. You will find the manifold hole plugged, the pipe will will be plugged, and the choke housing will be plugged. Clean em. Be sure to put the choke back it the exact same place, Make a snall mark on the plastic and the metal housing. There is a gage with marks on the housing. Just like the timing marks. Use them.

Now if you really want that thing to run, I have a Holly 850 Double Pumper that will fit that engine. Course you'll need a 3/4 race cam, some Solid lifters, and and and. Oh heck, forget it.

Hope this helps. I have the manual on that carb if you get in trouble.

posted by  jcutsh

i fixed the idle a while back, all three of them. but i do think something is wrong with my accelerator jets because i've noticed that sometimes after coming to a complete stop, when i put on the gas my car will almost stall. it doesnt do it all the time. so how can i fix that?

posted by  fAt2Rak005

Ok fat
The acc pump is located on the side of the carb where all the linkage is located. It is a small piston device operated by the linkage. several thinks can cause that engine to bogg down.
1 engine timing
2 acc pump
3 linkage adjust on the acc pump
4 low fuel level in the carb bowl.
5 improper idle mixture screws

Pull the breather. Look into the throat of the carb. Open the throttle whith your hand. Watch the two jets, they should put out two strong squirts of fuel as you move the linkage. This will continue to happen untill you empty the bowl of fuel. Then you'll have to restart the car to fill the carb again. If they don't squirt there is something wrong with the acc pump. The pump should start pumping at about 10 percent throttle open.
The pump consist of a round leather piston about the size of a nickel. The leather gets cracked with age. Another reason for the pump to not work is if the float in the carb is not set. If the fuel level in the bowl is low it hurts pump performance. The pump only comes into play when you try to accelarate quickly. If you drive like a little old lady then you may not notice that its bad.
You can find the acc pump by looking at the linkage and finding a small rod or sometimes a flat rod which goes into the carb body itself. I think you can by just the acc pump kit for that carb. Check Napa.

posted by  jcutsh

hey i have a quadra jet carb and my plugs r black so its running to rich and it backfires wen i step on the gas and hesitates so can u guys held me out

posted by  71 chevy c-10

any way you can start a new thread?

posted by  tbaxleyjr

i have a quadajet from a 75 blazer with a 350. i cant get it to idle down and is runs wide opened and wont kick down. know any ideas? thanks

posted by  cowboyrr85

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