When tuning a Qjet after installing, how do you really know if it is right? Do you only have the idle and characteristics of the exhaust to go by? Such as if the exhaust is black it is rich, if it sputters it is either too rich or too lean... Any input would be appreciated. Also there is a vacuum line that sticks stright out twards the passenger side wheel, right near the choke, where sould the other end of that line go to?
I can't help you with an answer but you might want to try posting this in "The Mixing Shop" section under "The Vulnerable Quadrajet" heading.:TU: Might be a quicker way to a knowledgable person with the answers you seek. Matt
How do you know when your carb is tuned? When tuning a Qjet after installing, how do you really know if it is right? Do you only have the idle and characteristics of the exhaust to go by? Such as if the exhaust is black it is rich, if it sputters it is either too rich or too lean... Any input would be appreciated. Also there is a vacuum line that sticks stright out twards the passenger side wheel, right near the choke, where sould the other end of that line go to?
Re: How do you know when your carb is tuned? First thing to do is remove one of your spark plugs and see what color it is. Black and sooty would be rich. A bone white type of color would be lean. A tan color would be a happy medium. Be careful though, fuel additives can make it hard to read your plugs. Pulling a plug would be a good place to start. John B. Good luck.
Re: How do you know when your carb is tuned? A pic of the vacuum hookup would help, but it sounds like youre talking about the one that sort of points to the passenger front tire. That one should have a short hose connected to the choke pulloff right next to it. A plastic (usually white or tan) thing on the front of the choke bracket on the passenger side of the carb. As for tuning, Warm the car up all the way then turn it off Hook up a vacuum gage to manifold vacuum on the motor. Turn the idle mix screws all the way in, then 1.5 turns out for starters. start the car set your desired idle rpm with the throttle screw check your vacuum - stock motors pull about 20 in Hg when properly adjusted. adjustment: turn your idle mix screws each 1/2 turn counter clockwise (richer) and note the effect on vacuum and idle rpm. should run smoother and more vacuum since youre adding more fuel readjust your throttle screw to get your idle rpms where you want it. repeat this adjustment until you get max vacuum at the idle rpm your motor wants. As you tinker with it, you'll get a feel for where your motor wants to run. too rich will stumble, too lean will stumble. Look for smoothness and vacuum. Some ppl back off the final setting of the idle mix screws 1/2 turn for a lean-best setting. Some ppl do the adjustments with Ebrake on, wheels chocked, and in D with an auto trans. Not me. I set mine in idle, and make small adjustments after test drive. Another idea is to do it in D, but ease the car up against a wall or tree to keep it from running away. Lean conditions smell waaay like gas. Rich condition smells some, and has the black smoke from the tailpipes. good luck EDIT This assumes that all other conditions of the motor are in good repair and functioning properly. Be sure youre not in the advance curve at idle speeds, timing wise....
Re: How do you know when your carb is tuned? Thanks guys, (now don't shoot me for this), but here is a pic of a carb like the one I am running, it is a back up carb while my Buick one is out for rebuild, I got it cheeep but came off a running 396. The vacuum port I drew an arrow by, should this connect to the choke "pulloff" like a Buick would?
Re: How do you know when your carb is tuned? holey smokes that shouldnt be there i think? It looks to me like one of the three choke bracket pedestals might have broken off of the carb main body and someone put that pipe in there as a replacement. Of course I could be wrong too. Does it even draw vacuum? If it does I would just cap it.
Its tuned right when there is no hesitation when the pedal is floored.No bogs or pops just quick revs:TU:
ditto, what GS says,,,, and the engine will run effortlessly,,,, that is it will not stumble, have flat spots, or run rough.... the thing that affects an engine the most is the ign. system and timing sequence,,,, make sure the ign. is right and then and only then, start to work on the carb..... and make changes one at a time if you decide to modify it.....
Not sure where i read it but for the idle circuit slightly choke off engine and watch rpms if they go up ur to lean.
Re: How do you know when your carb is tuned? You were right, it seems it is just a mount! Ok I got it running smooth by turning the mixture screws left until all was well, accelerating without hesitation. However even with the screw that adjusts the accelerator lever, (the idle) loosened back so far that it does not even touch the lever anymore, I have a high idle. I don't have a tach in this 72 Skylark, but it is enough to shift gears though town without me touching the gas pedal... How come my idle is so high?
Re: How do you know when your carb is tuned? look and see if there are some adjustment screws on the butterflies that holds them open...if there are, try that adjustment. If the butterflies are completely closed and your car is idling high, look for a huge vacuum leak, possibly at the carb base. This should be addressed immediately if it's the case cause that could be a very lean condition
Look at the bottom of the base of the carb. See if your carb has two idle bypass air holes there. There would be one each, a little smaller than 1/8" diameter, in the "channels" by the primary butterflies. If you see them, plug them up and try the carb. These holes are there as a programmed vacuum leak to help big cube, heavy vcammed motors idle with less throttle angle, and thus avoid nozzle drip. Dont know if thats your problem, but its worth checking. Edit: if you find and plug them, youll have to tune the carb again remember. Another thought: have you checked the choke idle screw on the passenger side? Make sure its not holding the throttle open once the car is warm.
I think it was a sticking throttle linkage, I lightly turned counter clockwise on the secondary linkage and the idle went down. It then stuck once more, then went down and seems to be staying down now. I am somewhat concerned about being too lean. I might have pre ignition while idling. How much run time is needed to be able to diagnose lean or rich by visually inspecting the plugs? Those holes.. "idle bypass air holes. There would be one each, a little smaller than 1/8" diameter, in the "channels" by the primary butterflies" Where are these channels? Sorry I'm not real familiar. THanks for your help!
if they were there you would see them attached to the primary bores, looking at the bottom of the carburetor. Couldnt miss them. If you can get your carb to rpm down when you manually turn the secondart throttle shaft counterclockwise - which is what i think you said- then theres a problem there. They should close completely without assistance. If they hang open at all you will have hi rpm. sometimes they can hang up on a base gasket. Check for interference.
I think it was an external linkage, I put a slightly tighter bend in the rod between the primary and secondary and that fixed it. The primary seemed to be coming down all the way but the linkage was keepin the secondary from coming down all the way. Just a slight tweak fixed it. The idle is now good and low. *QUESTION, if I am at around 3 turns out on the mixture screws and still no rich symptoms (such as dark exhaust) yet the engine idles "good" should I keep turning them counter clockwise until I do see a rich condition, just to know the carb is capable of running rich, than go back? I got this carb used and from a 396 chevy but I don't know if that chevy was stock or whatnot. Can the mixture scres fall out if turned too far?
sure they can fall out if turned too far. As for adjusting, I would only richen so far as it helps smooth the engine and bring rpms up. once the motor stops responding to the added fuel youve gone far enough. by tinkering with it rich and lean back and forth you should get a good feel for the "sweet spot" where your engine runs best. PS at that many turns out and considering it came from a 396 I bet you do have the idle bypass air holes.
