You need to connect the vac. gage to full manifold vacuum. That is,any fitting that's below the carb throttle plates or a fitting on the intake manifold. Once you're hooked up there and the eng. warmed up, the procedure I use is to lower the idle about 150 rpm or so and adjust one mixture screw cw and then ccw till you get the highest reading,then do the same with the other screw, Then raise the idle rpm to normal and repeat the steps. This helps to balance out the carb to obtain good final readings. At the end the screws will not have the same amount of turns. Hope this helps. Below is a link that'll help troubleshoot for various gage readings.
http://www.secondchancegarage.com/public/186.cfm Bob:bowtieb:

 

I hooked up the vac gauge at the intake manifold where the brake booster is. There is a spot where I could tap in without removing any hoses/lines. I started with 2 turns out, and the highest vacuum I could get was 14. It is better than it was when I started.

I hope this is better. I will go for a drive tomorrow.

 

Idle screws are for idle.

I doubt a rich idle is going to make your plugs black unless you let it idle more than drive it. What was the Q-jet on previously? You may need to start playing with different primary jets and/or rods to lean it out. Then you have your power piston spring to check also. If the spring is too strong for the vacuum it has, the power piston will make it run very rich. You can run the same q-jet on a six cylinder or a BB 454. It's all in the jets and rods and springs. The adjusting screws are for idle, and they are different than in other carbs, they meter the air not the fuel. Plan on getting real familiar with all the internals of it. They are great carbs but take some time setting them up correctly. Search the net and read as much as you can about them. There is a lot of info out there.

 

I stand corrected!

I doubt a rich idle is going to make your plugs black unless you let it idle more than drive it. What was the Q-jet on previously? You may need to start playing with different primary jets and/or rods to lean it out. Then you have your power piston spring to check also. If the spring is too strong for the vacuum it has, the power piston will make it run very rich. You can run the same q-jet on a six cylinder or a BB 454. It's all in the jets and rods and springs. The adjusting screws are for idle, and they are different than in other carbs, they meter the air not the fuel. Plan on getting real familiar with all the internals of it. They are great carbs but take some time setting them up correctly. Search the net and read as much as you can about them. There is a lot of info out there.

I have been told by a good source that I am wrong saying the idle adjustment screws meter air not fuel. (here and in other posts) Also found that the number of places I've read that is overwhelmed by the number of places that say they meter fuel. So does anyone know this for fact? I really am surprised no one here corrected me. Has anyone else been mistaken by the same incorrect web pages?

 

A gas soaked float will make a Q-jet run rich, they are known for it.

 

They also have well plugs that leak into the intake.

 

Yes and yes to above. I always think of the hard things first!?

 

The two screws on the lower front of the carb are idle mixture screws only. Once past an idle speed the primary mixture needles come into play, and then progressive secondary needles in the back. The primary needles may have been swapped out at some time. There are many different models, but some of the needles are tapered and some are stepped. You'll need to pull the top off the carb to swap then out. Thicker diameter ones for leaner mixture. Size is stamped on the needles.

With unleaded gas your exhaust will burn a gray or charcoal color normally, the same with plugs. Does your exhaust smell rich and burn your eyes if revved above an idle speed when standing behind your car? That would be an observation of running rich. Does the car stumble when accelerating?

I'm not sure where you initially plugged your vacuum gauge, but it should read something, not zero.

Idle mixture adjustment: Turn the screws all the way in gently, then back them out about 2 turns. Start the car, with vacuum attached, and adjust the screws to get the highest vacuum, and then turn them back in a quarter of a turn. Again, these are only for idle.

 

Glad to see someone backing me up on the IDLE screws. Although my exhaust pipes stay sooty, my plugs burn tan to white. Is the O/P here yet? Was the carb taken off a larger motor? There should be a number on the carb, I think starting with 170 . Get that number and you may be able to tell what it came off of.

 

I worked for a Q-jet guru when I was a kid. In fact I got a job there when Dad took his 73 Rally STX van because it was getting 9 mpg and a trail of black smoke. He did 3 or 4 Q-jets a week and that was 35 years ago. A good cleaning and kit with a new float and tweaking the choke, and the van was all the way up to 14 mpg!

I'm with you on the idle mixture screws, but I'd never suggest diving into changing metering rods and jets before trying a new float and correct choke adjustment/operation. No metering rods or jets are going to help if the float is sunk and/or the choke is sticking.

 

I don't have a ton of experience with Q-jets, but I've got a factory remanufactured piece on my Caddy. So I can help from a standpoint of knowing how these carbs react when they are in good working order. I was not comfortable overhauling my Q-jet, so I got an remanufactured carb for $190 (after core exchage) from Rock Auto. This way, I knew it would be right. First things first, you need to determine what size the carb is. If you have the Q-jet off of something like my 472, it's not going to be optimum on a small block.
My tailpipe is also sooty, but the plugs show a good mixture. I wouldn't worry about what you see in the pipe, worry about what your plugs are telling you. With the screws as far out as they were, it was way too rich on that one side. That tells me someone tried compensating for a mechanical issue in the carb. You want to start with 2.5 turns out as a baseline, then tweak from there. Keep in mind modern fuel does not pack the same energy older fuel did because of the ethanol content, so you'll likely find yourself fattening up the idle mixture a hair beyond what factory settings would have been. My engine reacts to pretty small changes in the idle mixture screws, so if yours doesn't, there's likely internal problems with the carb. This is another place where the ethanol blends hurt old cars. Any rubber or plastic in an old carb is not ethanol safe, and it will deteriorate relatively quickly because of it. So definitely give it a good overhaul, or just replace it altogether. Q-jets can be great performance carbs, but you need the right size and a little tuning know-how to get the most out of them on non-stock applications.

 

I With the screws as far out as they were, it was way too rich on that one side.

Like I mentioned before, the screws on a q-jet meter the air, not the fuel, so the screw that was out further would make it run leaner not richer. Also those screws are only for idle, not for driving conditions.

 

The way I read one carb site is that the idle adjustment screws do not change the air/fuel ratio, that's pre-determined by the jetting in the carb. What they do control in the VOLUME of air/fuel going into the card at idle. So going out is more air/fuel and going in is less air/fuel but the ratio remains the same.
I found my old wheatstone air/ratio mixture reader in the garage. Had used it to set up my 30 Chevy stock engine several years ago and have never had to readjust. So after achieving high vacuum, high RPM on my WCFB 2505S, I hooked up my wheatstone bridge air/fuel analyzer with the hose into the exhaust pipe and found my ratio was 11/1. No wonder my car didn't idle well. It took 7/8's turns of both adjusting screws inward to get me to between 13 & 14:1 on the guage. My car now idles near perfect. Just my findings and .02.