lean /rich adjustment

Just got a rebuilt Q-Jet(750) from Summit a couple weeks ago. Installed and all is working fine but I think the lean /rich mixture needs adjusting. When I went to do this I found that there are no adjustment screws but rather a squarish looking head sticking out. Don't understand this , but while in a local salvage yard looking for a steering box I ran across several other Q jets sitting on one of the shelves that had the same 'head' sticking out of where the air adjustment screws ought to be. What's up? Is there some type of 'tool' to use more turning them?
Thanks,
Barry

 

What "air adjustment" screws are you talking about?

Are you adjusting full throttle, part throttle, or idle mixture?

 

The two screws(that's my question they are not screws there as far as I can tell but rather two square headed screws(?)) at the front base of the carb. Theses are for fuel mixture correct? Sorry no pics to help show where I'm talking about.
Barry

 

Those are your idle mixture screws. Would suggest leaving them as is unless you have a substantially modified engine, a vacuum guage and/or an O2 sensor.

 

Thanks Brad , that's what I thought. Now back to my original question -- these are not phillip type screw heads or straight screw heads , they are almost retangular screw heads... Is there a special type tool needed to turn them if I do decide to adjust them. It does smell like it's running to lean and idle as the exhaust fumes are really acidy smelling... Anyone else have these type screws as fuel mixture screws?
Thanks,
Barry

 

Guys,

These screws are the classic Double D screws, for mixture adjustments. If the engine is running well, leave them alone, however, it is likely that the screws are not properly set, as they are intended to be adjusted when the carb is installed. There are two ways to adjust them, either using a tach, adjusting for max idle speed, or to be adjusted using a vacuum gauge, adjusting for max idle vacuum. There are more emissions friendly ways, using an emissions analyzer, and I can cover that as well. Let me know which provcedure you want to use. and I'll cover the method. Ray

 

I would love to know the "listen to the idle, look and smell the exhaust" method. You know, like the old days. o No:

All these analyzers and scientific methods make my head spin. I can't afford another dime on something that doesn't bolt to my car. I have no money left. :laugh:

I've looked in old tuning books and all and there is no fuel air mixture adjustment procedure. It tells you what the screws are, but that's it. :error:

 

Ray, I can see where it could be called a double "D" set screw, but the question is -is there a tool /socket/ attachment the is used to turn it?
Barry
btw Yes I would also like to know about the idle lean adjustment- Thanks

 

Yes, there is a tool, I think mine may be a KD tool. I've had it so long I don't remember.

 

End view

 

Far friggin' out!!!! Where can a guy get a tool like that? For that matter what is it called? Does it have two different sized ends?
Barry

 

Try KD Tools website for part number 2776.

Yes mine is double d on one end, and a hex head on the other.

 

Snap-On also has a double d socket, part #S6139.

 

To cover the two methods of adjustment, they are relatively simple, based on the tools available. If using the tach method, or vacuum gauge the method is the same for both.
1. Set the ignition dwell, and timing, since the timing will affect the mixture. Typically, advanced timing will require slightly richer metering.
2. Once the ignition is optimized and set, set the idle speed relatively close to the desired speed. Bear in mind that on the Q-jet, the idle transfer slots is used to deliver fuel off-idle, but before the main system begins to feed. It is essential that the throttle settings be very close to correct, regardless of the idle speed. Additional idle speeds will be covered later.
3. Once the idle speed is close, adjust the idle mixture for maximum vacuum on the gauge, or maximum idle on the tach. The place to start is typically three turns out on both sides. If the direction the screw is turned causes a loss of rpm, turn it the other way. Make the adjustments in small increments, and adjust both screws equally. Typically, on the Qjet, the limiter is the orifice in the throttle body, and the screw will reach a point where additional backing out will not add fuel.
4. If using either a tach, or a vacuum gauge, the idea is to watch carefully for even the slightest drop, then backing the screw out to regain the lost rpm. Make the adjustment several times, assuring that you are at the maximum, doing the same to both sides.
5. If the idle was very lean, or rich, it is possible that the idle speed will rise more than 100 rpm, which will require the readjustment of the idle speed before making the final mixture adjustment. Adjust the speed to 20 rpm higher than desired.
6. Once the highest idle speed is obtained, turn each screw IN, just enough to make a 10 rpm drop with each screw. This will finalize the mixture adjustments with a tach, or vacuum gauge. If using a vacuum gauge for the adjustments, turn each screw IN to create a 1/2" drop in vacuum. The tach setting method is actually better since it requires only one tool.

Using the emissions analyzer, follow the same procedure up to step four. Using the analyzer, carefully adjust each screw for the minimum HC readings displayed. One thing to note here. It is possible for the analyzer to occasionally "spike" to a very much higher number for HC's indicating a lean misfire. Example. On a '71 Olds or Buick, with the 350, the HC readings may drop to 100 ppm, then spike to 250. If that happens, the mixture is too lean. Reset the screw to 1/8 turn out, and observe the reading. Though it may be slightly higher, say 125 ppm, it will not spike to a higher reading. Do the same to both screws, making very slight adjustments to create the mimimum HC readings, with no spikes. This can get tedious, but the resulting idle quality is worth the trouble, and it minimizes fuel usage, creating good mileage as well. It will also create a crisp throttle response, and make the exhaust smell less like fumes. If necessary to create a higher idle speed, small holes can be drilled into the throttle blades to accommodate cams or other add-ons. The throttle position with respect to the off-idle slot is essential for correct fuel delivery. Please ask if I need to elaborate on this. Questions? Please ask either by private message or on the forum. I will answer any and all posed. Ray