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#1 |
Registered User
Join Date: Jan 2011
Location: Glendale, AZ
Posts: 3,178
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Carb jet size
What size carb jet are you running on your stock 4 barrel Quadra jet? I keep failing smog on my stock 350. I'm at .07 and it won't pass.
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#2 |
Registered User
Join Date: Dec 2017
Location: Elkhart, Texas
Posts: 1,699
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Re: Carb jet size
Changing jets isn't all there is to cleaning up the exhaust emissions. Timing, timing curve, PCV valve condition, ignition components (spark plugs, wires, coil, cap, rotor) condition, and last but not least -- overall engine condition... All have a direct bearing on emissions.
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#3 |
Post Whore
![]() Join Date: Feb 2016
Location: Portland Oregon
Posts: 11,114
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Re: Carb jet size
What Rusty said. Can you post the printed results? What kind of test does your state use.
Unless you've swapped carburetors and the emissions testing is done on a dyno changing main jets isn't likely to get the truck through. You may have to make some temporary steps to get through For example to get my 91 4.3 S10 to pass I have to set the timing at 8 degrees, disconnect the vacuum advance, and plug the PVC hose between the valve and the intake.
__________________
Thanks to Bob and Jeanie and everyone else at Superior Performance for all their great help. ![]() RIP Bob Parks. 1967 Burban (the WMB),1988 S10 Blazer (the Stink10 II),1969 GTO (the Goat), 1970 Javelin, 1952 F2 Ford OHC six 4X4, 29 Model A, 72 Firebird (the DBP Bird). 85 Alfa Romeo If it breaks I didn't want it in the first place The WMB repair thread http://67-72chevytrucks.com/vboard/s...d.php?t=698377 |
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#4 |
Senior Member
![]() Join Date: Jan 2001
Location: Orem, Utah
Posts: 7,905
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Re: Carb jet size
It's important to know what part of the emissions test your truck is failing. Posted results would be very helpful.
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I have sworn upon the altar of God, eternal hostility against every form of tyranny over the mind of man. - Thomas Jefferson |
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#5 |
Registered User
Join Date: Mar 2010
Location: Oregon
Posts: 189
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Re: Carb jet size
The last time I had to mess with my Q-jet carb from a different vehicle to pass smog, it was the primary air bleeds that helped it to pass. I actually made screw in air bleeds similar to mods I have seen on the internet. It was a pain, but as I worked in a shop at the time, at least I was able to have access to the analyzer to see my results.
Bob |
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#6 | |
Senior Member
![]() Join Date: Feb 2002
Location: Tucson, AZ USA
Posts: 7,260
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Re: Carb jet size
Quote:
What year is your truck? 1967-68 had White plates with Black letters and numbers. 1969-72 had Lemon Yellow plates and Black. The collector insurance is a lot cheaper than regular car coverage.
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Every 25 years I like to rebuild that 292, whether it needs it or not. |
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#7 |
Registered User
Join Date: Jan 2011
Location: Glendale, AZ
Posts: 3,178
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Re: Carb jet size
I have adjusted everything to the best of my ability. I think the jet size might get me there. What size are you guys running in your Q jet
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#8 |
Registered User
Join Date: Jan 2011
Location: Glendale, AZ
Posts: 3,178
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Re: Carb jet size
Stock 1970 with a 350. The smog cut off in AZ is 67 I believe. I'd like to get it to pass so I can just insure it normally. I'm not a fan of the classic insurance rules they have as far as where the truck is stored and how many miles you can put on it.
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#9 | |
Registered User
Join Date: Jan 2011
Location: Glendale, AZ
Posts: 3,178
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Re: Carb jet size
Quote:
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#10 | |
Senior Member
![]() Join Date: Feb 2002
Location: Tucson, AZ USA
Posts: 7,260
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Re: Carb jet size
Quote:
And the policy I have with Hagerty has no restrictions. It's my daily driver. 1971 GMC Jimmy, K/1500, 350 V8. I had a hard enough time passing emissions in Pima County. Once I tried going thru in Maricopa -- they were way more stringent.
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Every 25 years I like to rebuild that 292, whether it needs it or not. |
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#11 |
Registered User
Join Date: Jan 2011
Location: Glendale, AZ
Posts: 3,178
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Re: Carb jet size
Can anyone answer my original question. Lots of views but no answer. What size carb jet are you guys running in your Q jet carb. I'm going to downsize mine to .067 or .068 and I wanted to see what others are running
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#12 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,602
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Re: Carb jet size
Post your carb number and we’ll be able to tell you stock jetting.
A pic of your carb helps too. Last edited by geezer#99; 04-07-2023 at 08:40 AM. |
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#13 |
Registered User
Join Date: Mar 2010
Location: Oregon
Posts: 189
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Re: Carb jet size
There are many aspects to carb air/fuel metering. You may have a correct or "smog worthy" jet size, however the power valve piston spring may be incorrect for the proper A/F mixture. This can cause an improper A/F mixture due to the vacuum required to lift the power valve needle when needed. Power valve rods also have different calibrations as well.
Carb calibration can be tricky with out a dyno as many factors are influences of the fuel metering system. In response to the jet sizes I use, or am familiar with of hand, are 70 -72's. That's not to say that they are correct for my applications, they just seem to common ones I have used in the past. Bob PS - I rarely use stock model or year correct carbs as most Chevy parts interchange. I also try and use later model carbs to avoid worn carbs from the late 60's era. This will affect jet sizes as I may use, or have seen common in my carbs. There's a Rochester Carburetor book out that may help with tuning and understanding how the Quadrajet meters fuel delivery under the range of uses, and carb fuel delivery circuits involved. Last edited by HotWheelsFan; 04-07-2023 at 05:40 PM. Reason: Added PS information |
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#14 |
Post Whore
![]() Join Date: Feb 2016
Location: Portland Oregon
Posts: 11,114
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Re: Carb jet size
In my Burban's Qjet I'm running 64 jets and B32 rods. It is a pretty generic 350 with 3.73 gears and a T350 transmission. The carburetor is off of a 1978 BBC and as a result we've done a fair amount of work on the idle circuits to get it to work properly.
__________________
Thanks to Bob and Jeanie and everyone else at Superior Performance for all their great help. ![]() RIP Bob Parks. 1967 Burban (the WMB),1988 S10 Blazer (the Stink10 II),1969 GTO (the Goat), 1970 Javelin, 1952 F2 Ford OHC six 4X4, 29 Model A, 72 Firebird (the DBP Bird). 85 Alfa Romeo If it breaks I didn't want it in the first place The WMB repair thread http://67-72chevytrucks.com/vboard/s...d.php?t=698377 |
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#15 |
Registered User
Join Date: Jan 2011
Location: Glendale, AZ
Posts: 3,178
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Re: Carb jet size
Thanks for all the input. Downsized my jets to .067 and I passed
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