Quadrajet Idle Question
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From: Hillard Ohio
Quadrajet Idle Question
I heard about this site from some of the guys on the Team Chevelle page. Having read some of your stuff I know that you know your stuff about Quadrajets and hope that you can answer an "Brand-X" question for me.
I have a 67 Chevelle with a 327 that I rebuilt over the summer. The engine runs and drives great, but I have one small problem: I am having a little difficulty getting the idle adjusted to my liking. I am running a brand new 1969 Corvette 350 horse Quadrajet. I set my idle speed at 650 in drive. The idle surges more than I would like. Adjusting the idle mixture I found that I am able to turn the idle mixture screws in just about all the way and the engine will still run. In fact right now one of the needles is only 1/2 turn out and the other is approx. 1 turn out. I am wondering if the cam might have something to do with it. It is an Crane Energizer 272H, which is fairly mild kind of like the 327/350 h.p. cam that Chevy used in the 1960's. The duration @ .050" is 216/216. 110 degrees lobe seperation, and the lift is .454".454" Is it possible that this cam could cause Quadrajet idle problems? I have about 13-15"h.g. vacuum @ idle. I have had the carb apart to veify that the float is set correctly and all passages are clear.
Thanks!
Rocky
I have a 67 Chevelle with a 327 that I rebuilt over the summer. The engine runs and drives great, but I have one small problem: I am having a little difficulty getting the idle adjusted to my liking. I am running a brand new 1969 Corvette 350 horse Quadrajet. I set my idle speed at 650 in drive. The idle surges more than I would like. Adjusting the idle mixture I found that I am able to turn the idle mixture screws in just about all the way and the engine will still run. In fact right now one of the needles is only 1/2 turn out and the other is approx. 1 turn out. I am wondering if the cam might have something to do with it. It is an Crane Energizer 272H, which is fairly mild kind of like the 327/350 h.p. cam that Chevy used in the 1960's. The duration @ .050" is 216/216. 110 degrees lobe seperation, and the lift is .454".454" Is it possible that this cam could cause Quadrajet idle problems? I have about 13-15"h.g. vacuum @ idle. I have had the carb apart to veify that the float is set correctly and all passages are clear.
Thanks!
Rocky
Drifting
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Re: Quadrajet Idle Question (rocky1)
Sounds like it could be lean for one reason or another.A quick test while idling would be to put your hand over the primaries and see if the idle picks up.Might have to adjust your hand a little closer and farther,well you know what I mean. Then could spray carb cleaner around intake manifold runners where the meet the heads and see if it picks up.Make sure pcv valve is not stuck open and no lines leaking vacumm..Lastly clamp off the power brake booster hose to see if the booster is leaking.Are you certain though that the primary throttle shafts are not worn and leaking? iIknow you said it was new but new could mean to me rebuilt without attention to worn throttle shafts.There is more but try these tests first.
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Re: Quadrajet Idle Question (rocky1)
I have played with a few Q-jets in my days.............Most of that surge comes from worn throttle shafts. The Aluminum base plate just doesn't last with steel shafts going through it. Gee, I wonder why :eek:
As already stated, check for slop there. Most of the better rebuilders/remanf. of carbs put bushings there, but you get what you pay for.
Sounds like a vacumn issue, but not from that cam. I have a .472 SIG II in my 327 68' Camaro w/o issue. Mine was holding a steady 17 at idle.
As said check all your connections including any intake manifold vacum tees. All it takes is a pin hole to have a surge or miss.
Worse case, swap it out with one of your buds carbs, sometimes that what it takes to isolate these little things.
[Modified by ALLTHROTTLE&NOBOTTLE, 7:34 AM 3/15/2002]
As already stated, check for slop there. Most of the better rebuilders/remanf. of carbs put bushings there, but you get what you pay for.
Sounds like a vacumn issue, but not from that cam. I have a .472 SIG II in my 327 68' Camaro w/o issue. Mine was holding a steady 17 at idle.
As said check all your connections including any intake manifold vacum tees. All it takes is a pin hole to have a surge or miss.
Worse case, swap it out with one of your buds carbs, sometimes that what it takes to isolate these little things.
[Modified by ALLTHROTTLE&NOBOTTLE, 7:34 AM 3/15/2002]
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From: Hillard Ohio
Re: Quadrajet Idle Question (ALLTHROTTLE&NOBOTTLE)
Thanks for the ideas so far. Let me give you a little more background information:
I am 99.99% sure that it is not a vacuum leak, nor is it worn throttle shafts. I have had two different Quadrajets on the engine (same part #7029207). One was a used one rebuilt by me, the other was a brand new (not rebuilt) unit. I have also had two different intakes on the engine: cast iron 1968 Corvette intake, and an Edelbrock performer. When installing the intakes and when installing the carbs a new gasket was used. I also was careful to use the specified Felpro Print-O-Seal gasket with the Edelbrock Performer intake. When assembling the engine I used all new vacuum line. Power brake booster and the vacuum modulator hold vacuum. In fact the idle problem remains even with every vacuum port on the carb and intake is plugged. A check using carb cleaner also showed no vacuum leaks.
If I put my hand over the airhorn I can make the engine stall. Finally, the fact that I can turn the idle mixture screws in just about all the way is odd.
Thanks!
rocky
I am 99.99% sure that it is not a vacuum leak, nor is it worn throttle shafts. I have had two different Quadrajets on the engine (same part #7029207). One was a used one rebuilt by me, the other was a brand new (not rebuilt) unit. I have also had two different intakes on the engine: cast iron 1968 Corvette intake, and an Edelbrock performer. When installing the intakes and when installing the carbs a new gasket was used. I also was careful to use the specified Felpro Print-O-Seal gasket with the Edelbrock Performer intake. When assembling the engine I used all new vacuum line. Power brake booster and the vacuum modulator hold vacuum. In fact the idle problem remains even with every vacuum port on the carb and intake is plugged. A check using carb cleaner also showed no vacuum leaks.
If I put my hand over the airhorn I can make the engine stall. Finally, the fact that I can turn the idle mixture screws in just about all the way is odd.
Thanks!
rocky
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Re: Quadrajet Idle Question (rocky1)
check to make sure that your idle transfer slots are not open more that .020/.030,if they are you will need to drill holes in your primary butterflys to get the idle speed up while you close the butterflys. :chevy
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Re: Quadrajet Idle Question (rocky1)
All the above are good tips for checking out the carb, but I have to take the exact opposite approach. I don't think it's running lean if you have to shut off the gas with the idle mixture screws. If air were getting into the engine, other than through the carb, you'd have to OPEN the idle screws to supply fuel to air that wasn't passing through the carb. I would start with the ignition timing. Use the timing marks only for reference. The ring on your damper may have rotated, or a non-compatable timing pointer may have made the timing light inaccurate. If you crank in some advance at idle, the idle will want to speed up. Then turn the idle speed back down with the idle speed adjustment screw. You may have to do this a couple of times to get it right. This should strengthen the vacuum signal to the carb. The idle vacuum should increase and the idle should smooth out. Because the throttle plates are further closed under these conditions, the idle circuit in the carb should be more responsive. Less fuel wlll be supplied through the "off idle" circuit, and more will be needed through the mixture screws. Try it. No guarentees, but I think it will help. My $0.02 worth. Satisfaction, or double your money back.
[Modified by CFI-EFI, 4:32 PM 3/15/2002]
[Modified by CFI-EFI, 4:32 PM 3/15/2002]
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From: Hillard Ohio
Re: Quadrajet Idle Question (CFI-EFI)
Thanks - that is why I was sure to mention that the mixture screws can be turned in more than I have seen in previous experience.
The harmonic balancer/timing pointer suggestion is a good one. The balancer is a brand new GM unit - heck the whole engine is, but that is no guarantee that the balancer is showing true TDC! So, next time I have some garage time I'll verify TDC.
The harmonic balancer/timing pointer suggestion is a good one. The balancer is a brand new GM unit - heck the whole engine is, but that is no guarantee that the balancer is showing true TDC! So, next time I have some garage time I'll verify TDC.
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Melting Slicks
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From: Roanoke Virginia
Re: Quadrajet Idle Question (rocky1)
Rocky, I have the same Q-jet (1969 Corvette 350/350) on my 1972. Lars rebuilt it for me, and works great, but I had similar problems with idle. I found that because my cam is pretty darned radical 1970's technology, I had to crank open the idle plate to get good idle. Then the mixture screws didn't do jack.
I have since raised ignition timing (per Lars) to about 20*, and cut back the total timing curve to stay out of detonation. This allowed me to back off the idle stop screw, and actually get the car to idle about 650-700. It's still a little spastic when warmed up, but a big improvement. The mixture screws still don't do too much though, but I think my problems are just cam related, as I'm only getting about 10 inch Hg at idle.
I have since raised ignition timing (per Lars) to about 20*, and cut back the total timing curve to stay out of detonation. This allowed me to back off the idle stop screw, and actually get the car to idle about 650-700. It's still a little spastic when warmed up, but a big improvement. The mixture screws still don't do too much though, but I think my problems are just cam related, as I'm only getting about 10 inch Hg at idle.
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From: Roanoke Virginia
Re: Quadrajet Idle Question (rocky1)
Somebody in the past owner history built or had the engine built, which I don't have any record of since the car went throuh one of the "Corvette Dealerships" and what little documentation I have doesn't metnion a lot.
It's a 400 bored 0.030, recently I put on original 400 SBC heads to reduce the compression since somebody was nice enough to zero deck it. Comp ratio is now down to 9.7. It was at 11.5 :crazy:
I removed the Holley single-plane manifold and 650 CFM double pumper, and replaced them with an Edlebrock performer and Lars rebuilt Q-Jet.
I don't know what cam is in the car. I'm still debating whether to remove it or just drive it and rough idle be damned. I did try and measure the lift and duration. Not sure how accurate I was (I did it 3 times). My measurements were: lift with 1.5 ratio: 0.44 intake, 0.49 exhaust Duration at 0.05: 250 intake, 270 exhaust. From that info, I've assumed it's a dual-pattern solid lifter cam with long duration and moderate lift. Probably something aftermarket from the 1970's or early 1980's. I set the valve lash at 0.026" both intake and exhaust.
It's a 400 bored 0.030, recently I put on original 400 SBC heads to reduce the compression since somebody was nice enough to zero deck it. Comp ratio is now down to 9.7. It was at 11.5 :crazy:
I removed the Holley single-plane manifold and 650 CFM double pumper, and replaced them with an Edlebrock performer and Lars rebuilt Q-Jet.
I don't know what cam is in the car. I'm still debating whether to remove it or just drive it and rough idle be damned. I did try and measure the lift and duration. Not sure how accurate I was (I did it 3 times). My measurements were: lift with 1.5 ratio: 0.44 intake, 0.49 exhaust Duration at 0.05: 250 intake, 270 exhaust. From that info, I've assumed it's a dual-pattern solid lifter cam with long duration and moderate lift. Probably something aftermarket from the 1970's or early 1980's. I set the valve lash at 0.026" both intake and exhaust.
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From: Hillard Ohio
Re: Quadrajet Idle Question (rocky1)
Looked down the carb last night with the engine idling. I have some nozzle drip going on. I'll let you guys know if I can get it to stop.
rocky
rocky
