My 396 nova has a the original quadrajet ,I sometimes run the pure stock musclecar races.the car was running a little lean in high gear changed from a 71 to 74 jets ,from a.126 seat to a .142 and changed power piston spring.the car runs alot better now;but will not idle;adjust the mixture screws has no effect.Installed the stock power piston spring back in the car;no luck.It will idle at 1000 rpm ,and then eventually idles down on its own and shuts off.I hate to but the car back the way it was;any help appreciated.Chris
What was it's "normal" idle before you changed things? Do you have a copy of Doug Roe's book "Rochester Carbs"? Sounds like you've got some trash in the idle circuits. I wouldn't think that upping the main jet size would affect idle that much, unless it's just so over-rich that it's loading up at idle and dying. If that IS the case, then try going back down on the jet size and going to a smaller (richer) rod. Make sure the power piston and spring aren't binding up, clean it out thoroughly, then see what you've got.
Chris, Did you re adjust the float level? You should do that when you go to a bigger needle/seat. When you go up 3 jet sizes, you should go up 1 size on the primary rods. This will give you richer WOT mixture, but keep your cruise mixture the same.
quadajet idle problems Larry,when i raised the jet size ,i did also raise the primaries one.the car performed amazing well with the .142 seat and 74jets ,but when the car would go to idle mode it would load up alittle and shut off.Larry,do you think I should adjust the float even lower.I put the car back where it was with the .126 seat and 71 jets and it idle ok ;but definetley doesn;t perform as welll as the other set-up. one other thing i had the carb rebuilt awhile back when the carb came backthe vacuum diaphragm break was broke.i installed another one I had ,how do you check this or could this have any thing to do with the problems now.Thanks for any help.Chris Teed
Chris, I would try lowering the float level. Sounds like it's flooding. The biggest seat I've seen is the .135. If you can't solve the problem with float level adjustments, go to a smaller inlet. I doubt you need one that large, and using one too big is counter productive, it results in flooding.
There is the possibility that the larger seat is leaking, causing the bowl to continue to fill, even though the float is topped out. One other thing to check is whether the main system is "dripping" causing a rich condition. This could be caused by a higher than normal idle. Since you mentioned the change of the power piston spring, make sure that the power piston allows the power piston to bottom out at high vacuum, idle. A stronger spring will cause the power piston to hang off idle enriching the misture. Try using the largerr jets, and metering rods with the original power piston spring. Since the idle and off idle orifices are original, it is likely that the main system is feeding at idle, causing your problems. Check the power piston spring as follows: with the engine idling, try pushing down gently on the power piston. If it allows some downward motion, the idle should stabilize, depending on the idle mixture settings. Onde other thing to note is the timing, vacuum advances and the idle settings. A throttle opening wider than normal will cause the main system to feed as well. Let me know... Ray
Another thing to keep in mind: Only make ONE change at a time. Otherwise, you never know which/change or combination of changes is causing what. I lived there for awhile, it's no fun going around in circles with carbs. :rant:
Thanks guys,that is exactly what I am going to do ;make one change at a time and see the results.When these quadrajet are right they run good;but when they are off;it will take you for a loop;I have the Doug roe book on carbs ;which help.Interesting thing; everybody seems to have a different opinion on the problem;I will keeps ya posted ;thanks for the help.Chris Teed
Keep something in mind, everyone. We are dealing with several factors here. We are having to listen to the customer's complaint. In his mind, it is accurate, but it is all we have to go on. If we had the car in our hands, with the hood open, the problem would be easy to fix. Qjets are not complicated as long as we remember how the different circuits overlap, producing either perfect performance, or a problem. I believe that the differing potential locations of the problem are the result of everyone hearing differing descriptions of the dilemma. If everything is checked as described, we will have a better idea of the problem, or eliminate it. Qjets were my bread and butter since their inception. I repaired, rebuilt and modified them for street, emissions or performance either on the street or track. I welcome the questions, but hope everyone understands that if we are going to help, differing ideas will flow. It is part of the problem solution process. Ray
I have noticed under an idle condition,that as i look down into the carb gas appears to be dripping from the venturis down to the throttle blades.This is at 1000 rpm .I have placed the carb to its original state of 71 jets 43 primaries rod and .126 seat;the float level is at a quarter inch,and the piston pistons is down and not pulling up ;at idle.Should i lower the float a 1/16 more?Thanks for any help;I think I am getting closer.It will idle at a 1000 rpm ,but put it in gear at a stop light ;to where it idles down to 700 rpm and it has difficulities running;also ,if you shut the car off sometimes;it is diffucult to restart ,unless you remove the air breather;appears to be rich.thanks for any help.Chris Teed
Chris, Sounds like nozzle drip. If your initial timing is not high enough, the throttle blades will be too far open, actually causing you to activate the main metering system. This can also result from trying to use a small block Q-jet on a big block. The big block carbs have what's called a fixed idle air bypass. It allows extra air to bypass the venturis, so that the throttle blades can be nearly closed at idle. What carb # are you using?? (it should be stamped on the main body of the carb behind the throttle linkage, drivers side). What is your initial timing?? If you have too much mechanical advance in the distributor, resulting in a low amount of initial used, you may need to recurve the distributor. Another way to get more advance at idle, is to use vacuum advance from a full manifold source. This will allow you to idle at the same RPM, with a smaller throttle opening.
Chris, Ray is talking about the same thing here. Check the carburetor part #, and your timing at idle. You aren't on the idle circuit of the carb, the main circuit is starting to feed at idle.
Chris, I went back to read your original post. If it is the original carb(are you sure??), it should be right for your engine. If you have a hotter cam, the idle circuit may not be up to the task. I know on the Buick big blocks, the Q-jets have the fixed idle air bypass. Not sure on the Chevy carbs (Ray, do you know??) If not, you may be able to cure the problem with more advance at idle, maybe not. It's certainly worth a try. Good luck.
Any engine application requiring the larger 830 cfm carb had the idle air bypass, which allowed the necessary additional air for idle without the excessive throttle opening. My question is simple. Did the carb operate correctly before the mods? There are ways to increase the idle without opening the throttle. They include advancing the camshaft, advancing the ignition timing, connecting the vacuum advance to manifold vacuum, etc. My concern is whether this is a wild goose chase. What initially caused the problem. Were you trying to make changes and lost the base line, or was this a situation where the problem was there at the time of the initial purchase. A different cam, for example, may require additional fuel at idle, or a different torque convertor. First things first. Will the engine idle at 750, in park? Does the idle stabilize if the throttle is closed slightly? It is essential to eliminate the nozzle drip, which is typically the result of excessive throttle openings. Also, make sure the timing is stable. I strongly suggest checking the timing advances for sticking, for which older Delco distributors are known. It is essential to keep the basics in sight here. I cannot stress this enough. Check the advances, install a new set of plugs, and points, if used, and go from there. Check the advances, including the vacuum advance, and set the ignition timing. Set the idle at 800 in park, and adjust the idle mixture carefully to best idle, readjusting the speed to maintain the idle speed. Let's start there, and let's stop guessing. Start with the basics and go from there. It would be easy for me to fix the problem, but I would still start with the basics. Since the plugs have not been mentioned, let's replace them and go from there. After that, please be prepared to do some testing, getting hard answers to some hard questions. Ray
quadajet idle problems Ray,Larry,etc.here we go.the car has always had a little trouble idling from day one.Installed new ngk pugs,timing advances are not sticking,and the car has a stable timing;;has a pertronix ignitor II installed in stock distributor.Set idle at 800 rpm and tried to set mixture screws;no change in idle.When the idle is slightly closed;the car eventually shuts off.the camshaft is alittle over stock with a 276/283 duration 112 lobe center.I didnt check the vacuum at the lower rpm, before,but here they are:12 inches at 900 rpm,14 at 1200 rpm and 16 at 1500 rpm.If the car came equipped with the idle by-pass as the 455 buicks and pontiac quadrajet would i not have this problem.the car is an original matching # car ,and I would like to keep the original carb on the car,surely there are some mod i can perform to make this carb work.once again thanks guys for all your help;I like the sound of these quadrajet;any help appreciated.Chris
quadajet idle problems One other thing;the float level is set a 1/4 inch ;would lower it alittle more help the problem at all.Chris
Chris, The low manifold vacuum at idle is the tip off for me. Even the 396 had manifold vacuum readings close to 16". It appears that the cam is the issue we need to address. What is the initial timing? The problem with the larger jets and the low manifold vacuum is causing too much fuel at idle. The 12" of vacuum is creating a situation where the power piston is rising, feeding additional fuel at idle. The larger jets are aggravating the situation by the additional fuel bypassing the metering rods. To keep the original Qjet, it will be necessary to tune it for the cam. This will require first, setting everything back to stock; the jets, metering rods, power piston springs, everything. Also, as previously noted, one change must be made at a time, then evaluate the changes. First, check the factory metering, from throttle transition, main metering, everything but idle. This includes the secondary performance. The idle will be addressed separately. The lowered manifold vacuum will adversely affect the main metering, and may require a weaker power piston spring. Most importantly for idle, the throttle must be very close to the factory position for best performance. It may also be necessary to enlarge the idle passages, to allow additional fuel at idle. First and foremost, return to the factory pieces and go from there. The additional cam overlap will require additional timing. I believe the factory setting for this engine was 4 deg before TDC, with the vacuum hose disconnected. Start with 8 deg before, checking the manifold vacuum there, 10 deg, and 12 deg. If the additional timing helps at idle, it will be necessary to adjust accordingly. Since Chevy engines run strongest with about 36 deg of total advance, this must be checked as well. You might try running the vacuum advance to manifold vacuum, allowing additional advance at idle. This sounds like a bunch of info, but sort it out, one piece at a time. Start with the factory settings, first and foremost. Then change one thing at a time, trying to understand the changes made. I have mentioned this several times, and if necessary I will say it again. Make one change at a time, starting from the factory settings. Make the ignition timing changes first; they will directly affect the manifold vacuum, which affects directly the carb's performance. The Qjet is probably the most sensitive to manifold vacuum, due to its operation with a power piston, and the manifold vacuum will need to be stable. Ray
The timing was at 8 degrees to begin with;bumped it to 10 and 12 ;also installed the vacuum advance to manifold vacuum;able to reach 13 in.of vacuum.Installed a weaker power piston spring;appeared to have no change in operation.will get it on the open road this week and check secondary performance.also what am I looking for in the factory meteringand main metering.the throttle transition is above off-idle discharge port;which is at 1200 rpm at idle;which at the moment is the only way to get decent operation out of the car Beginning to look like i might have to enlarge the idle passages.Thanks for the help;sorting it out 1 by one.chris
