Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: Chevynation on October 14, 2023, 07:39:28 AM
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Hello, I have a quadrajet that runs on when I have the idle set at 800 which is where the engine likes to idle. If I lower the idle to close the throttle blades then it will not run on but the idle will be too low at that point. I increased my initial timing but it’s still not enough. No vacuum leaks and I even tried using manifold vacuum on the vacuum advance. Idle mixture screws have full affect and I can’t see any dripping from the boosters. I read about idle bypass air on my carb model but how can I increase it so I can close my throttle blades to prevent run on at shutdown? I have a manual transmission as well.
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Correction, I know I can increase them by opening the size of the holes on the base plate. Depending on the size of the holes how much do I need to increase by in order to see a difference? I shouldn’t need to increase idle fuel correct? The carb was rebuilt and runs great all around. I’m just trying to slightly tweak it to correct this issue run on issue that’s all. I know another option is an anti dieseling solenoid but I prefer not to run it.
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Correction, I know I can increase them by opening the size of the holes on the base plate. Depending on the size of the holes how much do I need to increase by in order to see a difference? I shouldn’t need to increase idle fuel correct? Cliff rebuilt the carb years ago and it runs great all around. I’m just trying to slightly tweak it to correct this run on issue that’s all. I know another option is an anti dieseling solenoid but I prefer not to run it.
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What are the specs of the engine? How much timing are you running at idle with vac. advance?
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383 288HR cam. Duration @ 0.006":288° / 294°Duration @ 0.050":236° / 242°
Max Lift w/ 1.5RR: .520" / .540" Lobe Separation:110°. I have 1.6 rockers. Dual plane edelbrock performer rpm manifold. AFR 195 heads 10.5:1. Had 18 initial and 28 with advance plugged in. I limited the vacuum advance to 10 degrees. I currently have 22 initial and the total is 34 degrees which the engine likes best with the efficient chamber heads.
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No expert but 22 initial is a whole lot of base timing. My inefficient heads don't need that much. Start by backing the timing down around 14-16 initial with no vacuum. And try resetting idle screws.
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Since I don’t have enough idle bypass air when I decrease my initial I have to open the curb idle screw too far and expose too much transfer slot so it starts dieseling. The cam really likes 18 initial but I just bumped it up to 22 for a test. I took the carb off to see how much transfer slot is exposed and it’s too much.
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Does the idle speed change much when you increase the timing from 18 - 22 - 26?
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Absolutely. At 18 I could only get it to not run on at about 600 rpm or so and at 22 I can get it to idle at 750 without running on. 750 isn’t bad but it’s a little low at times especially with heat on. It likes 800 but it will run on. So I just need a small amount of additional air so I don’t have to have the idle screw open as far. I also want to see why my full manifold vacuum port on the side of the carb doesn’t have any vacuum. Neither of my ports have vacuum but the carb has a different main body than it originally had. Hopefully it’s just the gasket that needs a hole drilled but it shouldn’t be too difficult to fix. I’m not worried about the ported vacuum port so that will just remain inoperable. Will .010 increased idle bypass size be sufficient in your opinion? Depending on how the size of hole in the baseplate is I’ll either install a brass set screw so I can drill to the size I want but if the hole is small and just has epoxy then I’ll just drill it a little larger.
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Have you tried more timing just to see what happens?
What size is your IAB now? Guessing that can you probably have 10-11 in. of vac at idle.
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Not sure what the current IAB size is. Carb was done two years ago if I remember correctly. 12” vacuum at idle. If I increase initial then yes it won’t run on but it won’t run on all the time. It doesn’t run on if I decrease the idle either but it’ll just be idling too low. If this was a double pumper I could simply open the secondary curb idle screw to add some air while keeping the correct amount of transfer slot exposure on the primaries.
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What carb-#?
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7045213 it originally had a 7040203 body but it was replaced.
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I had the same problem with my previous 350. Tried every trick in the book and
then some. Even tried different carbs and distributor. Ended up using an anti deiseling
solenoid which worked. Problem with mine was horrible quench distance combined with
factory dished pistons. New motor has .036 quench and flat tops. no more deiseling,
even with exhaust heat under the intake and a 195 degree stat.
IAB mods probably not going to help as you will still be letting the same amount of
air into the engine for the same 800 rpm just from a different path.
The idle solenoid may be the best solution for now especially since it is running fine otherwise.
QJ
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Doesn’t run on with a different carb. My quench is tight. No carbon buildup on pistons. Why would you have to change the distributor? If I increase the idle bypass air by .010 then how would the engine be getting the same amount of air? I should be able to increase the idle bypass air to the point that the engine has a fast idle that can’t be lowered other than reducing the idle bypass air hole size.
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My point with the distributor was that I was willing to to try anything at that point.
The distributor had nothing to do with it.
Sounds like your on the right track.
QJ
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Watching this tread with interest as I am having the same issue. Nervous to increase by-pass air any more to the point where I went too big. At .116" already. How large should one go before looking elsewhere in idle calibration? Such as in the idle air bleeds or idle tube restriction size?
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7040203 and 7045213 has the same specs, .092" upper idle air bleeds, .063" lower idle airbleeds, .039" idle tubes and .051" idle down channels.
I would start with this recipe for your carb on your engine.
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7040203 and 7045213 has the same specs, .092" upper idle air bleeds, .063" lower idle airbleeds, .039" idle tubes and .051" idle down channels.
I would start with this recipe for your carb on your engine.
Thanks Kenth. I’ll be sure to check the idle bypass air hole size when I take the carb off tomorrow. The carb is great everywhere else and Cliff really was spot on with the recipe he put in so I don’t want to mess with it. I’ll just spec it out and do one change at a time. Idle bypass air increase by .010 should hopefully cure it depending on the current size. I’ll keep you posted tomorrow. Thanks.
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.063 Idle fuel tubes and 0.196 idle bypass air. Took the carb off without touching the curb idle screw and the transition slot didn’t look bad. It must have been getting too much air because that slot exposure is with the idle screw backed all the way out. Im not too familiar with quadrajet but should there be transfer slot exposure with the throttle blades closed? I know holleys you can close the blades and have zero exposure. I shaved some of the gasket off to fix the manifold port on the carb because it was just blocking the hole. I was shocked to see how big the bypass holes in the base plate were. Correct me if I’m wrong but I traced the idle bypass holes from the main body down to these two holes on the baseplate and they’re a good size. I went to touch the secondary air flap and the tension spring broke. Good thing it broke now instead of on the car. Pretty weird but no big deal. I’m thinking that I go down .010 instead of up on idle bypass air but I would like an answer regarding the slot exposure with the blades shut. Any opinions?
https://ibb.co/7ndt1HP
https://ibb.co/VCLp3gy
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The bypass holes are already pretty big it appears. If my idle screw was backed out that far and it was still idling at 750 then I would say I have too much bypass air and it’s running on from that and not the transfer slot which I thought before I removed the carb. Doesn’t anyone know the amount of transfer slot exposure for a quadrajet? Throttle blades are 90 degrees at WOT and fully close no light can be seen. The stop that’s on the primary shaft that the idle screw makes contact with looks like it should make contact with the main body for a stop but I’m coming up a little short. Not sure if this is a big deal or not but just something I noticed.
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I think you are measuring the idle tubes in the upper end? The restriction is in the lower end.
As for the off-idle slots they are exposed some in some units adding some idle mixture, use the idle needles to adjust the amount of idle mixture.
And the idle bypass air restriction is measured from underneath on the plate.
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I've always seen more transfer slot on a Qjet vs. Holley, & it seems the later ones have more yet.
What you're showing for transfer slot is pretty good IMO.
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It looks like the base plate idle air holes moved in 73 and my main body is a 75. So really my idle air holes between the body and base plate are misaligned. I looked at the carb and it looks like the gasket has enough of an opening to allow air to still allow it to work. Just thought I’d share that. I’m measuring the idle bypass holes on the bottom now. I looked at another quadrajet and there will be transfer slot exposure with the blades closed so that’s fine.
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0.136 Idle bypass air. I’m not positive but the hose on my choke pull off valve was loose and I put my vacuum gauge in it while on the choke pull off and it wasn’t holding. Tried a different hose and it worked fine. I’ll put it back together as is to see if that was a vacuum leak.
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Use the correct throttle body gasket and your idle bypass air will be fine. Wrong gasket will block the hole on passenger side on your throttle plate.
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Use the correct throttle body gasket and your idle bypass air will be fine. Wrong gasket will block the hole on passenger side on your throttle plate.
I currently have the 7038396. That’s why I had to shave some of the gasket so my manifold vacuum port would work.
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You are using the correct gasket for your plate, no "shaving" should be needed.
Pictures of your shaving?
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You are using the correct gasket for your plate, no "shaving" should be needed.
Pictures of your shaving?
The holes were completely covered before I trimmed. I cleaned the gasket up a little bit so it didn’t look as ragged.
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Found an issue. I’m only measuring 3” vacuum at the manifold vacuum port on the carb but I’m getting 10” at the choke pull off port on the carb. I pulled the tube out of the side of the carb for the manifold vacuum and it’s not blocked or anything. Before I shaved the gasket it wasn’t getting anything because it was blocked but now it’s so low that even my aftermarket vacuum advance can can’t add timing at idle. It’s not a straight shot down through the gasket from the main body bypass holes to the base plate. There is a recess in the base plate or main body I can’t remember which and the bypass holes are offset because the main body is in a different location than the baseplate due to the year difference but since there is a recess air should still get by I would have thought. I could easily blow air through the manifold vacuum port now whereas before I could not. Not sure what I could do to be able to use vacuum advance besides using a tee fitting at the choke pull off.
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It’s almost like it’s a bleed off port or something. Not sure how they routed vacuum lines for the original vehicle. The carb only has three ports. The port on the front for pcv, the port on the passenger side which I thought was full manifold vacuum, and the port on the driver front which should be ported. I didn’t test that one but it didn’t appear to be manifold the way it was routed. I don’t have any ports on the baseplate. My manifold has a hole behind the carb so I installed a fitting for my power brake hose connection. Same way it was from the factory on my car. Do I have to drill the baseplate for a manifold vacuum port?
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Now you are "robbing" air from the idle bypass source, that´s why you see low vacuum in the side manifold vacuum source.
Instead of cutting the gasket you should have drilled the hole a bit further to the next location in float bowl. Then you will get full vacumm signal in the side manifold port, and a fully functional idle bypass air.
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Now you are "robbing" air from the idle bypass source, that´s why you see low vacuum in the side manifold vacuum source.
Instead of cutting the gasket you should have drilled the hole a bit further to the next location in float bowl. Then you will get full vacumm signal in the side manifold port, and a fully functional idle bypass air.
I’ll get a new gasket. You’re a genius. That next port would’ve been perfect so I’ll drill to that hole.
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I might just epoxy the vacuum port hole thats currently exposed from shaving the gasket and drill to the next hole like you said so I don’t need a new gasket at the moment. However, if I need a new gasket should I shave it so that the bypass hole on the base plate is fully showing and not just maybe 1/4 of the hole. The majority of the gasket is just covering the hole. It is fine on the main body and the driver side hole is centered perfectly on the base plate. I would’ve thought it shouldn’t be blocking the hole. Is this not considered a restriction and should be left alone? Thanks again.
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No need for epoxy.
A new gasket will cover the unwanted hole and solve the idle bypass issue as the slotted hole in gasket acts like an air channel leading air to were it´s needed.
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No need for epoxy.
A new gasket will cover the unwanted hole and solve the idle bypass issue as the slotted hole in gasket acts like an air channel leading air to were it´s needed.
For the time being I just drilled to the next hole and put epoxy in the first hole so it’s not robbing the idle bypass air. Everything works as it should now. Can’t wait to take it out and get gear soaked to see how it acts.
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Everything went smooth but when I set the idle and drive it and it gets hot it will go up. I figured it was normal since it heated up. So I lowered it and then I get down the street and it’s higher again. I’ll set it at 750 and it will go up to 1000 by the next time I stop. My throttle return spring is pulling the lever back to the screw so I’m not sure why the idle keeps going up. I can’t find any vacuum leaks. Should I try to reduce the idle bypass holes? My curb idle is pretty much backed all the way out at this point. If I pull on the throttle lever towards the stop then the idle will drop but no spring will be able to pull the lever back that tight to lower the idle. My throttle shaft bushings were replaced when it was rebuilt so I’m not sure why I can lower the idle by forcing the lever to the closed position. Idle bypass holes at .136 currently.
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Check for weak distributor centrifugal weight springs, or binding weights, not returning the advance when letting off to idle speed.
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Tried that. I’m using a billet distributor and I’m using a heavy and medium spring. Repeatedly tried to see if anything was sticking. I can’t see me having to use two heavy springs.
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Went up to a heavier spring. I’ll post back later this evening.
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Two heavy springs worked. I’m still thinking about taking some idle bypass air out. With 18 initial plus 10 manifold vacuum advance it will idle no lower than 750 without touching the curb idle screw.
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Set initial to 10-12° and use the ported source for vacuum advance.
If this don´t work, check the sizes of upper and lower idle circuit airbleeds, idle tubes size (at the lower end), downchannels size and idle mixture needle holes size.
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I’ll have to measure everything because it’s not happy at all at 10-12 degrees initial. It likes the 18 initial. Even with 18 initial and removing the vacuum advance I can’t notice an idle drop but looking at the timing it does retard so my pickup is indeed advancing when connected. I limited it to 10 degrees all in at 1.5 - 2” below idle vacuum. The ported vacuum advance port does not work with the carb since the main body was replaced. I never tried to make it work since I was only worried about manifold vacuum to my vacuum advance. I’m not a fan of ported to be honest. I was under the impression that I could simply remove some idle bypass air since it only affects how much the primary blades have to be open in order to get it to idle and nothing else. Essentially like closing the throttle blades.I would imagine doing so would require me to have to close the idle mixture screws in. The rest of the carb works fine. Responsive everywhere etc. Idle mixture screws are responsive as well. It can just idle with the throttle blades closed which is almost where I want the idle but not quite and I want the throttle blades to be open a small amount at idle i would think. I don’t want to recalibrate the carb and mess with everything that’s working fine. Just reducing the idle bypass air hole sizes in the baseplate by maybe .010 at a time won’t fix the issue?
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Just some info for those reading and following this thread.
ALL of the issues here are fundamental. Too much camshaft and LSA too tight for the CID and compression ratio of the engine.
For sure "rowdy" tight LSA cams sound great but the overlap causes reversion, low vacuum at idle/poor signal to the carb and loss of throttle response and power at low RPM's.
Doesn't mean we can't make things work but for sure the engine is NOT going to do well without running a LOT of initial timing and opening up the idle system to supply it with enough fuel to be happy at idle speed.
It will also like a lot of idle bypass air to keep the throttle angle low so too much transfer slot isn't exposed and they don't "run-on" or "diesel when we shut they down........FWIW.....