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Quadrajet Tuning question

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Old 05-16-2016, 02:30 PM   #1  
mobird
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Default Quadrajet Tuning question

Hey guys, I just installed Headers and Side Pipes on my 79 corvette. It has a Qjet with the number 7059216 on it. I opened it up to rejet it and found that it currently has size 72 main jets. What size should I change up to compensate for the new headers and side pipes?


Not sure if it's important but my car is a 4-speed (however the carb is from an automatic), the rest of the engine is stock at the moment.


Thanks
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Old 05-16-2016, 04:19 PM   #2  
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Also, I plan to switch intakes to a Edelbrock Performer RPM soon. With that setup, will I need to rejet, and if so, what size?


Thanks
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Old 05-16-2016, 05:14 PM   #3  
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Impossible to tell. It might need SLIGHTLY more fuel, but the only sure method of checking is with a A/F meter while the engine is at idle and WOT. Increasing the jet size will enrich the mixture at all ranges, decreasing the rod size will enrich the "operational" ranges only (assuming you use .026 tip rods). If you make any changes, do them in small steps.
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Old 05-16-2016, 06:04 PM   #4  
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You need to look at Lars' jet/rod charts and find a combo that richens it no more than 10% as recommended by Lars. And that's assuming your running rich....

Last edited by jim2527; 05-16-2016 at 06:05 PM.
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Old 05-16-2016, 06:15 PM   #5  
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Quote:
Originally Posted by mobird View Post
Hey guys, I just installed Headers and Side Pipes on my 79 corvette. It has a Qjet with the number 7059216 on it. I opened it up to rejet it and found that it currently has size 72 main jets. What size should I change up to compensate for the new headers and side pipes?


Not sure if it's important but my car is a 4-speed (however the carb is from an automatic), the rest of the engine is stock at the moment.


Thanks
You may need to go with smaller primary and/or secondary metering rods to richen things since you are removing all that backpressure. Lars would know.

this may be of use: http://www.route66hotrodhigh.com/PowerTuneQjet.html

More... A LOT MORE!!! : http://www.florida4x4.com/tech/quadrajet/

Others here have added side pipes and should be able to recommend metering rods/jets combinations to get you in the ballpark of tuning.

Last edited by TedH; 05-16-2016 at 06:19 PM.
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Old 05-16-2016, 07:38 PM   #6  
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carbs are jetted for sea level so if you are above sea level your carb is already richer than it needs to be so changing jetting may not be necessary for your exhaust change. the formula for changing jetting for altitude is you reduce the jetting area 2% for every 1500 feet you are above sea level.
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Old 05-16-2016, 09:01 PM   #7  
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Quote:
Originally Posted by jim2527 View Post
You need to look at Lars' jet/rod charts and find a combo that richens it no more than 10% as recommended by Lars. And that's assuming your running rich....
Thanks, looking at that it looks like going to a 74 jet should get me about 8% richer. I'm thinking that will be a good starting point.

If anyone else has headers and side pipes, I"m interested to hear what your setup is and particularly if you changed the metering rods.

Last edited by mobird; 05-16-2016 at 09:02 PM.
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Old 05-16-2016, 09:02 PM   #8  
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carbs are jetted for sea level so if you are above sea level your carb is already richer than it needs to be so changing jetting may not be necessary for your exhaust change. the formula for changing jetting for altitude is you reduce the jetting area 2% for every 1500 feet you are above sea level.
Good info, thanks. I'm about 1100 feet above sea level.
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Old 05-16-2016, 09:08 PM   #9  
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Drive it first, it may or may not need re jeting. If it does a 73 would be the first I would try.
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Old 05-16-2016, 09:39 PM   #10  
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Little, if any, primary mixture change is needed for part throttle cruising. With reduced back pressure at full throttle, enrichment may help. The beauty of the Quadrajet is that you can make changes with the secondary rods easily without disassembling the carb. Try this first before you dig into the primary side.

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Old 05-17-2016, 07:42 AM   #11  
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Little, if any, primary mixture change is needed for part throttle cruising. With reduced back pressure at full throttle, enrichment may help. The beauty of the Quadrajet is that you can make changes with the secondary rods easily without disassembling the carb. Try this first before you dig into the primary side.

WOT is where I would check with a sparkplug color check. if it is lean at cruise you will feel a surge.
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Old 05-17-2016, 08:10 AM   #12  
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Thanks for all the info guys.


Do you have a recommendation for a cheap 02 sensor and gauge so I can get my A/F ratio? I'd rather have a number to tune off of then a spark plug.
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Old 05-17-2016, 08:35 AM   #13  
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The cost of a dyno pull will be less($60 to $75 for 3 pulls) than buying a A/F setup. Unless you plan on continued major modifications such as heads or total engine rebuilds, consider getting a dyno test. Plus the tech may make other suggestions as to timing or ignition problems that only show up at max output.
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Old 05-17-2016, 09:24 AM   #14  
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The cost of a dyno pull will be less($60 to $75 for 3 pulls) than buying a A/F setup. Unless you plan on continued major modifications such as heads or total engine rebuilds, consider getting a dyno test. Plus the tech may make other suggestions as to timing or ignition problems that only show up at max output.


I do actually plan to put some aluminum heads and a better cam in in the next few months, so I would like to have a way to read A/F. Seems like there has to be a company making a cheap usable o2 sensor and gauge setup that I can have welded into my collector. I have very little experience with this so I would appreciate reccomendations.
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Old 05-17-2016, 09:40 AM   #15  
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Can I just buy any 02 sensor and bung and have it welded in and then attach a cheap A/F gauge to it? Or is it more complicated than that?
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Old 05-17-2016, 10:26 AM   #16  
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Just weld it in place at least 18" from the cylinder head. The meter instructions will recommend exact placement. But I think you will find that the A/F meters aren't so cheap! If you do plan on engine mods, keep the carb untouched until you get the engine running, and then adjust the carb.
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Old 05-17-2016, 12:12 PM   #17  
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Just weld it in place at least 18" from the cylinder head. The meter instructions will recommend exact placement. But I think you will find that the A/F meters aren't so cheap! If you do plan on engine mods, keep the carb untouched until you get the engine running, and then adjust the carb.

Ok thanks. Did a little research, looks like I can get the AEM wideband kit for a little under $200.


Do you know if I can leave the wideband 02 sensor in at all times? I read a forum post or two that said the sensor doesn't seem to last long so some people were only keeping it in when they were tuning. If that's the case, do I just plug the Bung with a bolt that is the same thread size as the sensor? Or do they make something to plug the 02 sensor bung with?
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Old 05-17-2016, 12:24 PM   #18  
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O2 sensors last for 100K miles in new cars. They can be removed and plugged if you wish, but then the wiring connection would be exposed to the elements. Install the bung, do a clean professional job on the wiring and "forget about it"- as my New York born and raised wife says.
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