I have a an issue with the idle, on my car. I have a new carb, manifold,and distributor set up. Everything works fine, except when I sit at idle in gear for a period of time. I put a wide band o2 sensor on the set up, and got some readings. I am a bit lean I feel at idle. Readings are 15:1, then after a while, it goes to 16:1. That is when it starts to get a bit rough. I played with it a bit, and now, it surges at that time. I think it is on the edge of being too lean? The o2 sensor set up is a friends, so I do not have easy access to it. I would like to set it up from scratch, to see if I can get rid of the idle fluctuation. I would then, when convenient, put the o2 sensor on it to see where it is. My long winded question is this: Step by step, what is the proceedure to set ones idle mixture screws? I have a vacuum guage, if I need one. Any help would be appreciated. Regards, John. o No:
Install vacuum gauge on manifold vacuum port. Adjust idle mix for highest possible vacuum at idle. (I sometimes do this with the auto trans in "D".) Re-adjust idle speed if needed. Assure both mixture screws are adjusted for maximum vacuum at proper idle speed. Turn screws LEAN just enough to REDUCE the vacuum about 1/4--1/2 inch. It shouldn't take much; adjust both screws evenly; the point is to get the mixture just shy of best idle. Makes very little difference in the idle speed or quality, but removes a lot of pollution from the tailpipe.
Qj The power piston spring controls the power piston. Manifold vacuum will try to pull that piston downward. The power piston spring is constantly trying to push the piston upward. And since the primary rods hang off the power piston the rods are being lifted out or pushed down into the primary jets. The spring needs to be matched to the manifold vacuum. There is a variety of springs available depending on your manifold vacuum. In addition, at the base and part of the power piston is a pin. This pin is a mechanical control as to how high or low the piston sits in its bore. And since the rods hang off the piston it determines how far down in the jets the rods rest. The rods are tapered (except for the tip). So if the piston is adjusted too low you'll have too much taper in the jets and a lean condition, conversely if its adjusted too high it can result in not enough taper in the jet and a rich condition. This is all assuming its metered (jet/rod combo) correctly to begin with. You can adjust this height by taking out the sealed plug directly between the two idle mixture screws in the base. This will allow access to a slotted screw that will raise or lower a lever arrangement in the base that the pin (of the power piston) rests on. Very small adjustments make a big difference. Access to your friends O2 sensor and a vacuum gauge will be a huge bonus in these adjustments. When the spring can't control the movement of the piston because of a mismatch between tension and vacuum you'll have a fluctuation in mixture control and the problem just compounds itself as the rods lift and lower in the jets. The power valve on some of the Holley carbs has the same basic function as the power piston on the QJ. It is set to open or close at a predetermined vacuum reading depending on spring tension built into the unit. To adjust the idle mixture screws: Lightly seat both screws with the engine off. Turn them both out 1 1/2 turns. Start the engine with the vacuum gauge hooked to a manifold vacuum port. Now adjust them in or out til you get the highest vacuum reading then in an 1/8 of a turn or so. You need to make sure the primary throttle blades are closed or mostly so, otherwise air will enter past the blades and bypass the idle circuit. A stock type camshaft will develop vacuum readings in the high teens to 20". The more overlap the camshaft has the less available idle manifold vacuum.
Thank you Schurkey. I hope you have help when you adjust the car in "D". My problem happens in "D", not in Park. When you say adjust both screws evenly, do you mean turn one than the other during all proceedures, untill there is a change? Regards, John.
Dave, I hope I do not have to deal with the first part of your response. It may be a bit above my comfort level. Your second part is where I will start. once I have seated them, and backed them off, do I adjust one side at a time for best vacuum, or do I do i adjust them both at the same time. I would think, I should adjust one side at a time for best vacuum, then move on to the other. I am not sure how to close the primary throttle blades. Is that my idle speed adjustment screw? Do you mean to say that i should get the idle down to as low as it will idle, to set the mixture screws? I have a Postons GS113 cam, 72' Quadrajet, Performer manifold, long tube headers, 3 inch exhaust, pertronix conversion, taylor wires, recurved dist, 34* total, all in by 1300rpm. Stock block & pistons, factory stage 1 heads, that have been lightly cleaned up. I have had 13lbs of vacuum in the past. I will check it when I fool with it again. Thanks for your help. I can tell you what has been done to the Quad if you think that will help. Regards, John. o No:
As to making sure the throttle blades are closed, yes just turn the idle screw outward just a little. It may already be set, its just something to check. If you get little to no response on the idle mixture screws then you'll know the incoming air is running passed those throttle blades and bypassing the idle circuit. With a camshaft that is other than stock, it may be a question as to having the correct power piston spring. With 13" of vacuum the spring issue surely needs to be addressed, if it hasn't already, not knowing who set your carb up. You should turn each idle mixture screw a little at a time alternating from side to side to attain the highest reading. You may find that one side will be 2 turns out (for instance) and the other 2 1/4 turns. You may also find that you'll get a reading you think is the highest for that screw and when you turn the other screw to its highest reading you can go back to the 1st screw and turn it in or out and it'll rise a little more. Its a back and forth deal several times over. Just remember to turn each screw back in slightly when you're done....not too much. With 13" of vacuum you probably find marginal power assist at times for your brakes. I found that to be true with an aftermarket cammed engine I ran for years. Installed a set of Rhoades lifters and the vacuum at idle increased enough to allow use of the power assist at low cruising speeds. Very slight solid lifter type noise resulted but with my application it wasn't objectionable.
Thanks Dave. John Osborne set up my carb and distributor. I do not see any power piston spring notations on my invoice, just "spring" #7036019. My carb is a #7042240. Invoice is as follows: Rebuild Q-jet, #7035134 needle and seat. #76 jets, #7036019 spring,(could this be it?) G secondary hanger. Vacuum plugs and air cleaner gaskets. Secondary rods are "CE". I have since changed them to "AY", because according to the wide band, I was running 9.2:1 to 9.3:1 at wot. A bit too rich for power. I have not run the car since I swapped them out. My cruising was 12:1 to 13:1. That seemed o.k.. Hopefully, the spring is correct, and all I need to do is to play with the idle screws a touch. What do you think? Thanks for the help, I appreciate it!!! Regards, John.
John I'ld also check out if the timing is fluxuating at idle wich may be adding to the problem. With all the advance in so soon the weights may be bouncing around when the engine is idling. Especially when the engine is not warmed up. Ray
Thanks Ray, I will double check timing for fluctuation. My problem however, seems to be more noticable when all warmed up. Regards, John.
That is the 8-4 power piston spring. Which means it begins to open at 8" vacuum and is fully open at 4". Good spring. And the one I would have used also, if not the 10-6 spring. But you're safe with that spring. And the slightly leaner sec. rods are a good with closed exhaust. Its really getting into finite tuning with small increments such as your talking in sec. rods, but thats whats its all about.....fine tuning for best results. Takes time but is well worth the effort. Its typically real difficult to get the last little bit out of each carb when the builder has to install the parts and then send it out to be tuned for each individual package. There's almost always something left on the table as unforseen variables can creep onto the scene. Most likely John installed some 46 rods too. It sounds like after you get it adjusted with the idle mixture screws you may need to get behind the sealed plug and tweak the mechanical stop for the power piston. Easy to do and with a wideband sensor available you can get it dead on. Good Luck and let me know if I can help any further.
Yes Dave, 46B primary rods. I have 1 7/8 long tube headers, I am not sure what you mean by closed exhaust? I think you mean mufflers? Right? Please explain. Thanks again for the help. have a great fourth! Regards, John. :beer
Yeah, I just meant running through the full exhaust. You could richen it up some on the secondary side with open headers but that's easy to do at the track if you ever decide to try it. I'll be working through the weekend and the 4th, but then its a week off, so no complaints on this end. Good Luck with your project :TU:
It appears that the correct idle mixture adjusting procedure was lost in the plethora of other issues discussed. The factory uses this procedure. Adjust the idle mixture to obtain maximum idle speed at about 800 rpm. Adjust the idle again to obtain the idle speed +20 rpm. Readjust the idle mixtire LEAN to drop the speed 10 rpm on each screw. If it becomes necessary to readjust the idle speed, do so, expecting the readjust the mixture. This see-saw procedure will yield a mixture of about 13:1, but not much leaner. Tachs were the tool of preference without a gas analyzer. Using a gas analyzer, the procedure is slightly different, but the outcome is the same. An analyzer uses the lowest HC readings, at the correct speed, and the jockeying procedure was used to obtain the lowest HC readings at the correct speed. 15-16:1 is very lean for an emissions engine in stock form, and one with a cam is typically run slightly richer. You might try advancing the timing slightly to make up for the loss in efficiency, then adjust the carb using one of the suggested procedures. Ray
