At my wits end with this QuadraJet

[pontiacgp]

I run mine in the 2-2.5 range and I think my holes under the screws are .090 and my bypas holes are like .110 and idle restrictions are .036

I make sure all the quadrajets I work on are opened to .090 and set the adjustment screws 2.5 and adjust from there

 

[1bad79]

I make sure all the quadrajets I work on are opened to .090 and set the adjustment screws 2.5 and adjust from there

The biggest thing is having the throttle plate as low as possible so not to much of the transition slot is exposed the more you expose the more drip your going to have on the primary at idle witch will cause a pig rich idle so all the restrictions and bleeds need to be right on for high performance,for guys like us some slight tinkering can get it in the ballpark

 

[1bad79]

The one I did for my car is a 78 800 cfm and I remember it being ridiculously lean on the idle circuit even for the 403 it came off from

 

[pontiacgp]

The biggest thing is having the throttle plate as low as possible so not to much of the transition slot is exposed the more you expose the more drip your going to have on the primary at idle witch will cause a pig rich idle so all the restrictions and bleeds need to be right on for high performance,for guys like us some slight tinkering can get it in the ballpark

you're exactly right, some don't think the idle circuit has anything to do with the A/F mixture once you open the primary circuit. My A/F gauge tells shows me how the idle mixture affects the A/F at cruise. It's useless trying to tweak a quadrajet if you havn't got it right at idle.

 

[olds307 and 403]

I found my Qjet ran best at about 5 turns out on the 8 to 1 cammed 350 and 403. Since recalibration, about half that. I bet no number of turns make much of a difference.

 

[HAFROD]

I did a cam and intake swap on a stock SBC years ago. Went with a nice sounding cam right at the brink of needing a convertor change. When it all went back together, did not idle right and just died. Had an older guy come over and we took it apart. I was early 20s then. He cut the metering rod spring saying it would fix it. It did not. That's when I found out that spring has to be matched to your combo. I ended up putting a Holley on. I have Cliffs book and have read it a couple times. Does not make me an expert but with my personal experience with a car that ran fine before to a cam screwing stuff up. I learned something.

 

[pontiacgp]

I did a cam and intake swap on a stock SBC years ago. Went with a nice sounding cam right at the brink of needing a convertor change. When it all went back together, did not idle right and just died. Had an older guy come over and we took it apart. I was early 20s then. He cut the metering rod spring saying it would fix it. It did not. That's when I found out that spring has to be matched to your combo. I ended up putting a Holley on. I have Cliffs book and have read it a couple times. Does not make me an expert but with my personal experience with a car that ran fine before to a cam screwing stuff up. I learned something.

you are right about spring, it's not hard to test that the piston is held down by the vacuum to make sure the rods are sitting in the jets where they are supposed to be. When the "older guy" cut the spring he would need to stretch it a bit which would make it a heavier spring making it harder for the vacuum to keep the piston down. If he didn't stretch it then the rods never rose high enough to give you gas when you needed it. I have a kit with springs and use the strongest one without interfering idle so when I hit the gas the piston lifts the rods quickly.

 

[1bad79]

you are right about spring, it's not hard to test that the piston is held down by the vacuum to make sure the rods are sitting in the jets where they are supposed to be. When the "older guy" cut the spring he would need to stretch it a bit which would make it a heavier spring making it harder for the vacuum to keep the piston down. If he didn't stretch it then the rods never rose high enough to give you gas when you needed it. I have a kit with springs and use the strongest one without interfering idle so when I hit the gas the piston lifts the rods quickly.

this is exactly right

 

[Rktpwrd]

There’s some awesome info flowing now guys, thanks!!

View attachment 90163 View attachment 90162
The holes are where the air comes from and how it bypasses the throttle plate I just bought this carb it’s a 73 olds 750 cfm auto carb and has factory bypass air not
all have it

Thanks for the info, it’s proving to be invaluable. I just pulled my carb back off and compared the underside of it to your pic, mine definitely does not have the bypass air ports you pointed out:

So now what does that mean? I’m I screwed already before I even get started?

That is a lot of compression and cam for a carb that came off a 8 to 1 350? with a 180's duration cam, Either pick up Cliff Ruggles book, mine was ruined in the 2013 flood or I would send it or contact Techg8.

Again, I’m not certain on the CR, it’s just a half educated guess at this point. My estimate is only a point higher than what that 350 would have come with stock, with the steel shim head gaskets and the smaller combustion chamber volume of the Vortec heads, I don’t think my guess is too far out to lunch.

I was gonna say, 3-3 1/2 turns seems like a lot. I always run mine 1.5-2.5 max.

Thanks for the info. I’m trying to determine the number of turns mine are at right now, but I seem to have lost my adjusting tool!

I run mine in the 2-2.5 range and I think my holes under the screws are .090 and my bypas holes are like .110 and idle restrictions are .036

Great info here, thank you!
So just to be sure before I just start drilling larger holes in my carburetor, what I’m going to be drilling is the existing holes underneath where the idle mixture screws are currently located?

And there doesn’t appear to be any bypass holes to enlarge...

I found my Qjet ran best at about 5 turns out on the 8 to 1 cammed 350 and 403. Since recalibration, about half that. I bet no number of turns make much of a difference.

You’re right Christian, turning them past a certain point seems to have little to no effect.

 

[1bad79]

There’s some awesome info flowing now guys, thanks!!



Thanks for the info, it’s proving to be invaluable. I just pulled my carb back off and compared the underside of it to your pic, mine definitely does not have the bypass air ports you pointed out:

View attachment 90190

So now what does that mean? I’m I screwed already before I even get started?



Again, I’m not certain on the CR, it’s just a half educated guess at this point. My estimate is only a point higher than what that 350 would have come with stock, with the steel shim head gaskets and the smaller combustion chamber volume of the Vortec heads, I don’t think my guess is too far out to lunch.



Thanks for the info. I’m trying to determine the number of turns mine are at right now, but I seem to have lost my adjusting tool!




Great info here, thank you!
So just to be sure before I just start drilling larger holes in my carburetor, what I’m going to be drilling is the existing holes underneath where the idle mixture screws are currently located?

And there doesn’t appear to be any bypass holes to enlarge...



You’re right Christian, turning them past a certain point seems to have little to no effect.

Look at the pic adding bypas it shows the baseplate you have also why I asked about the gaskets because there are a few different base plates there’s specific instructions for doing that mod on that plate