Thread: Need some basic information about adjusting my carb

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My car is running entirely too rich. It svery strong. My dad says he was gonna adjust for me but hasnt yet (that was a month ago) I know how to adjust it. Theres three screws. Air,Fuel, and idle.

Its idles at about 1100 rpms but i need to know how you know when a carb is adjusted right. I want to learn to do this as it has had to be done to the car periodically

You may know all of this, but just in case....your carb has three basic circuits - idle, cruise/mid-range, and wide-open. Your accelerator pump bridges the transition from idle to mid-range, or from idle to WOT giving a shot of raw fuel into the venturi to prevent stumble (going lean on the change in air flow). For the three screws you mention, idle speed is easy - adjust it to the speed you want or as specified for your engine. The other two are idle jets, which meter the amount of fuel into the airstream during idle. At idle speed, the old school adjustment is to turn in a screw until you note an rpm decrease (lean falloff point), then turn that screw out, counting the turns until you reach a rpm decrease (rich falloff point), and set the screw at the mid-point between the two. If you don't see rich/lean falloff points it probably indicates that you're leaking internally, and you need to put a kit in it. If you're running rich in mid-range and/or WOT those fuel circuits are adjusted with changing out main jets and/or metering rods, depending on the carb. Some can be done easier than others. What type of carb are you working with, and where are you seeing the it too rich?

My answer is very basic stuff, and I'm not trying to infer you're not way more of a carb guy than I am.

I use a vac. gauge setting the idle mix getting the highest reading with idle speed set where it needs to be. do 1 screw at a time going back and forth. till you get highest vac and idle right.

Originally Posted by cffisher

I use a vac. gauge setting the idle mix getting the highest reading with idle speed set where it needs to be. do 1 screw at a time going back and forth. till you get highest vac and idle right.

And this is how I do mine too. Before I owned a vacuum gauge I did it the way RSpears describes. I have found the vacuum gauge to be very reliable and accurate. A vacuum gauge isn't real expensive either and it can tell you alot about your engines condition inside!

I think we are all saying the same thing.....I turn the two mixture screws in all the way (do not tighten or you will damage the needle valves). This will clean out any trash that may be lodged around the needle valves. Open both needle valves 1.5 turns and then with engine warmed up and choke completely open, set idle screw at 600RPM. Using a vacuum guage, adjust each needle valve for maximum vacuum. After that, close each needle valve (detune) about a quarter of a turn, then readjust idle speed to recommended setting (around 550-600rpm).

I do not turn the needle valves in (detune) and I keep an eye on RPM the whole time. I also use a higher rpm setting than 600.

What kind of carb are we talking about???

One with 3 screws....

That makes it a tripod right?

Thank you for all the information @rspears I dont know anything about how a carb works internally I only know what they do so thank you for the information, @dave severson Its a q-jet not sure the exact specs its one that came off a 76 chevy van we had laying around and i had the carb rebuilt before installing it.

You'll have to let us know how it works out.You may want to check for cracked hoses

@cffisher I will let ya know , thanks for your help im gonna try and adjust it after i get home from school today

I agree, that is how I adjusted my 2300 Holley. Works very well.

So now we know Q-Jet, and Charlie suggested wisely that you check all of your hoses for cracks (replace any that are suspect). Can you tell us where in your load range you're running rich? Are you seeing black, sooty plug deposits after cruising, black smoke under heavy acceleration, or is it just idling crappy?

its not smoking at all exept for when you first crank it and thats just my valve guides, You can just smell it like the exhaust is much to strong.