carburetor jet and needle combinations.
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Junior Member
Joined: Apr 2009
Posts: 72
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From: hillsboro aka hillsburito Oregon
Car: 83 berlinetta
Engine: 305 roller motor
Transmission: th350
Axle/Gears: 10 bolt 3.08
carburetor jet and needle combinations.
I have searched these forums for info on the best combinations of jets and needle sizes both primary and secondary for a Quadrajet. what size needle goes with say a #71 jet and then what would the corresponding arm and needle be for the secondaries. I have a Quadrajet from a 78 pontiac firebird and I want to retune it to go on a chevy 350 with 1.94/1.50 heads and a thumper cam my trans is a th350 9inch tail and 3.08 rear gears I just want to know what y'all think I should do for the best possible combination for the other components I have set up.
thanx in advance for your help
thanx in advance for your help
Supreme Member
Joined: Sep 1999
Posts: 7,072
Likes: 13
From: Philly, PA
Re: carburetor jet and needle combinations.
That's difficult to say. You're basically asking "how should I tune my carb" even though that the carb isn't original to the engine and the engine is modified with performance parts. It isn't answerable in a direct way.
First off, you realize that a Pontiac QJet and a Chevy QJet have a different fuel inlet location (straight forward vs. 90* to the side). There may also be differences in the throttle arm- the style and location of how the cables hook up. I'm just talking about the basics of hooking up the critically important stuff.
Putting at that aside for a minute, in general, just about any stock QJet is going to be a little on the lean side on a modified performance-oriented engine. "A little bit richer" is about the only general advice I could give as a starting point for your tuning efforts. Generally this means about 4 jet sizes richer than stock (to a 75 in your case), continuing to use the original primary rods. On the secondary side of the carb a set of either DR or DA rods (have both sets on hand and try both) on a G hanger will cover most performance-oriented QJet applications.
The modifications required to get best performance go beyond just jets, rods and hangers, however. The idle circuits often need to be fattened up a bit. The secondary air valve tension and vacuum canister often need to be worked on to give best performance, etc, etc.
I don't mean to make this sound impossible. Most of the mods are of the "works almost every time" variety. But nobody can give you an exact tune to put into that carb based on a quick description of what you're putting it on top of.
Throwing in the towel and saying "F-it! I'm putting a Holley on top!" won't get you around this problem, either. ANY carb will need tunign to get all the performance you paid for. Although in this case, an aftermarket universal-replacement performace carb will probably run better than a stock emissions-era QJet if all you want to do is bolt the carb on top and run it as-is.
First off, you realize that a Pontiac QJet and a Chevy QJet have a different fuel inlet location (straight forward vs. 90* to the side). There may also be differences in the throttle arm- the style and location of how the cables hook up. I'm just talking about the basics of hooking up the critically important stuff.
Putting at that aside for a minute, in general, just about any stock QJet is going to be a little on the lean side on a modified performance-oriented engine. "A little bit richer" is about the only general advice I could give as a starting point for your tuning efforts. Generally this means about 4 jet sizes richer than stock (to a 75 in your case), continuing to use the original primary rods. On the secondary side of the carb a set of either DR or DA rods (have both sets on hand and try both) on a G hanger will cover most performance-oriented QJet applications.
The modifications required to get best performance go beyond just jets, rods and hangers, however. The idle circuits often need to be fattened up a bit. The secondary air valve tension and vacuum canister often need to be worked on to give best performance, etc, etc.
I don't mean to make this sound impossible. Most of the mods are of the "works almost every time" variety. But nobody can give you an exact tune to put into that carb based on a quick description of what you're putting it on top of.
Throwing in the towel and saying "F-it! I'm putting a Holley on top!" won't get you around this problem, either. ANY carb will need tunign to get all the performance you paid for. Although in this case, an aftermarket universal-replacement performace carb will probably run better than a stock emissions-era QJet if all you want to do is bolt the carb on top and run it as-is.
Thread Starter
Junior Member
Joined: Apr 2009
Posts: 72
Likes: 0
From: hillsboro aka hillsburito Oregon
Car: 83 berlinetta
Engine: 305 roller motor
Transmission: th350
Axle/Gears: 10 bolt 3.08
Re: carburetor jet and needle combinations.
That's difficult to say. You're basically asking "how should I tune my carb" even though that the carb isn't original to the engine and the engine is modified with performance parts. It isn't answerable in a direct way.
First off, you realize that a Pontiac QJet and a Chevy QJet have a different fuel inlet location (straight forward vs. 90* to the side). There may also be differences in the throttle arm- the style and location of how the cables hook up. I'm just talking about the basics of hooking up the critically important stuff.
Putting at that aside for a minute, in general, just about any stock QJet is going to be a little on the lean side on a modified performance-oriented engine. "A little bit richer" is about the only general advice I could give as a starting point for your tuning efforts. Generally this means about 4 jet sizes richer than stock (to a 75 in your case), continuing to use the original primary rods. On the secondary side of the carb a set of either DR or DA rods (have both sets on hand and try both) on a G hanger will cover most performance-oriented QJet applications.
The modifications required to get best performance go beyond just jets, rods and hangers, however. The idle circuits often need to be fattened up a bit. The secondary air valve tension and vacuum canister often need to be worked on to give best performance, etc, etc.
I don't mean to make this sound impossible. Most of the mods are of the "works almost every time" variety. But nobody can give you an exact tune to put into that carb based on a quick description of what you're putting it on top of.
Throwing in the towel and saying "F-it! I'm putting a Holley on top!" won't get you around this problem, either. ANY carb will need tunign to get all the performance you paid for. Although in this case, an aftermarket universal-replacement performace carb will probably run better than a stock emissions-era QJet if all you want to do is bolt the carb on top and run it as-is.
First off, you realize that a Pontiac QJet and a Chevy QJet have a different fuel inlet location (straight forward vs. 90* to the side). There may also be differences in the throttle arm- the style and location of how the cables hook up. I'm just talking about the basics of hooking up the critically important stuff.
Putting at that aside for a minute, in general, just about any stock QJet is going to be a little on the lean side on a modified performance-oriented engine. "A little bit richer" is about the only general advice I could give as a starting point for your tuning efforts. Generally this means about 4 jet sizes richer than stock (to a 75 in your case), continuing to use the original primary rods. On the secondary side of the carb a set of either DR or DA rods (have both sets on hand and try both) on a G hanger will cover most performance-oriented QJet applications.
The modifications required to get best performance go beyond just jets, rods and hangers, however. The idle circuits often need to be fattened up a bit. The secondary air valve tension and vacuum canister often need to be worked on to give best performance, etc, etc.
I don't mean to make this sound impossible. Most of the mods are of the "works almost every time" variety. But nobody can give you an exact tune to put into that carb based on a quick description of what you're putting it on top of.
Throwing in the towel and saying "F-it! I'm putting a Holley on top!" won't get you around this problem, either. ANY carb will need tunign to get all the performance you paid for. Although in this case, an aftermarket universal-replacement performace carb will probably run better than a stock emissions-era QJet if all you want to do is bolt the carb on top and run it as-is.
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