Carb jet size

[Tuff Gong]

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.

[RustyPile]

Originally Posted by Tuff Gong

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.

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.

[HO455]

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.

[truckster]

It's important to know what part of the emissions test your truck is failing. Posted results would be very helpful.

[HotWheelsFan]

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

['68OrangeSunshine]

Originally Posted by Tuff Gong

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.

Get Year Of Manufacture license plates. Get collector car insurance. And you'll be exempt from ADEQ emissions, even in Pima and Maricopa Counties.

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.

[Tuff Gong]

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

[Tuff Gong]

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.

[Tuff Gong]

Originally Posted by '68OrangeSunshine

Get Year Of Manufacture license plates. Get collector car insurance. And you'll be exempt from ADEQ emissions, even in Pima and Maricopa Counties.

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.

Will the year of manufacture plates make me exempt? I thought it was by year only?

['68OrangeSunshine]

Originally Posted by Tuff Gong

Will the year of manufacture plates make me exempt? I thought it was by year only?

Yes they will.
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.

[Tuff Gong]

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

[geezer#99]

Post your carb number and we’ll be able to tell you stock jetting.

A pic of your carb helps too.

[HotWheelsFan]

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.

[HO455]

Originally Posted by Tuff Gong

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

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.

[Tuff Gong]

Thanks for all the input. Downsized my jets to .067 and I passed