How Do I adjust the...

[CenValyCali]

Mixture screws? What is everyone using to do this? Sockets won't fit, neither will any of my tools. It's a rectangle and I can't get it out with needle noses. Please help. I'm doing a rebuild. O yeah it's a Rochester carb. Thanks

[ray_mcavoy]

I have a newer 80-something Q-Jet carb on my 77 plow truck that has those oddball mixture screws too.

I ended up making a tool out of a scrap piece of brake line tubing. 3/16" I think. Just squashed the end a little bit in a vice to give it sort of a double-D shape that fits the heads of those screws.

[datguysaL]

get a piece of rubber hose, small enought to fit tight. and adjust as needed, some auto parts store also carry the a set with a variety of small sockets with a long flexible scredriver type thingy.

[tucsonjwt]

You can get a double D carb adjustment tool from Autozone, Pep Boys, etc. for cheap. Still, I find the adjustment is still iffy getting it to stay in place while doing the adjustment, but if the carb is off the truck - should be no problem.

[85Bowtie]

I thought the Quadra Junks were pre-set from the factory and had no provisions for adjusting the idle mixture without drilling something out or something funky like that.

Are you attempting a rebuild on the carb or the whole motor? If you're unsure of how to adjust the idle mixture on a Q-Jet you might as well forget about rebuilding it and just get you an Edelbrock.

[CaliDude76]

This is what I use to adjust the air mixture screws on my 84's Q-Jet... The tool has different sizes at both ends and flexes.. works like a charm... My dad got it from Napa Auto Parts in Visalia, Ca.. a while ago

[CenValyCali]

Thanks everyone for the ideas. Cali dude does one of those ends look like it would fit a rectangle?

[CaliDude76]

Originally Posted by CenValyCali

Thanks everyone for the ideas. Cali dude does one of those ends look like it would fit a rectangle?

Yes... sorry bro.. that one close-up didnt work to good....., but it does have rectangular ends on both sides.. apparently some carbs have different size deals to adjust. good luck

[CenValyCali]

Ok Got my Carb put on and the truck is running, I just have one problem It seems my back barrels aren't open. I've pulled the throttle and the barrels open maybe a 8th of a inch. What am I doing wrong here. I'm guessing all my lines are run properly. My cjeck engine light turned off and I'm happy about that, I'm just not having any power when I punch it. Also it back fires at the carb when I punch it. Hope someone can help.... Thanks

[ray_mcavoy]

Q-Jet carbs have a mechanical linkage connecting the primary (front) throttle blades to the rear (secondary) throttle blades. But there is a lockout mechanism that keeps the secondaries from opening if the choke isn't fully open. So make sure your choke mechanism is hooked up correctly and fully opening.

Q-Jet carbs also have secondary air doors above the secondary throttle blades. The air doors are spring loaded and only open based on engine airflow demand. So it's perfectly normal for the secondary air doors (at the top/rear of the carb) to stay closed or only open a tiny bit if you simply rev the engine with the trans in park/neutral.

Is the backfiring something new since the carb rebuild? Or was it doing that before too?

[CenValyCali]

Originally Posted by ray_mcavoy

Q-Jet carbs have a mechanical linkage connecting the primary (front) throttle blades to the rear (secondary) throttle blades. But there is a lockout mechanism that keeps the secondaries from opening if the choke isn't fully open. So make sure your choke mechanism is hooked up correctly and fully opening.

Q-Jet carbs also have secondary air doors above the secondary throttle blades. The air doors are spring loaded and only open based on engine airflow demand. So it's perfectly normal for the secondary air doors (at the top/rear of the carb) to stay closed or only open a tiny bit if you simply rev the engine with the trans in park/neutral.

Is the backfiring something new since the carb rebuild? Or was it doing that before too?

It's something new. I've never ran this carb on my truck before either. I found it at a junk yard and rebuilt it. I've tried to adjust mostly everything but have no ****ing clue what a good starting point is.

I've got my jets set a 2 and a half turns
I'll be setting my idle adjuster on top of the carb to four turns tomorrow.

I've got to fix that choke. It's suppose to open all the way up right(up and down)? Right not it opens up at it's fullest at about 100 degree angle. I feel like it should be more of a 90 degree angle. Please help. I need to get this think Smogged

[ray_mcavoy]

Originally Posted by CenValyCali

It's something new. I've never ran this carb on my truck before either. I found it at a junk yard and rebuilt it. I've tried to adjust mostly everything but have no ****ing clue what a good starting point is.

The rebuild kit should have included a paper with all the various adjustment specifications for all the carb #'s the kit fits. Look up your carb # (located on the driver side of the carb ... starts with a 170) on that paper and it'll give you all the settings for float height, choke adjustment, and such.

I've got my jets set a 2 and a half turns

I assume you're meaning the idle mixture screws are 2 1/2 turns out since the jets are inside the carb and have a fixed size orifice. If that's the case, 2 1/2 turns should be enough to get the engine to start & run but might need some fine-tuning for a good idle. Adjusting these won't change the way the engine runs at higher speeds though.

I'll be setting my idle adjuster on top of the carb to four turns tomorrow.

Not sure what you're referring to here ... Idle speed screw over on the driver side of the carb?

I've got to fix that choke. It's suppose to open all the way up right(up and down)? Right not it opens up at it's fullest at about 100 degree angle. I feel like it should be more of a 90 degree angle. Please help. I need to get this think Smogged

Yes, the choke should be pretty much straight up & down when it's fully opened.

There's a good carburetor troubleshooting chart at: http://www.recarbco.com/technical/newtrouble.html that you might find helpful.

[CenValyCali]

Originally Posted by ray_mcavoy

The rebuild kit should have included a paper with all the various adjustment specifications for all the carb #'s the kit fits. Look up your carb # (located on the driver side of the carb ... starts with a 170) on that paper and it'll give you all the settings for float height, choke adjustment, and such.

The paper was included with the kit but it was hard to understand. I still have it and will give it another shot. If I come up short I'll post my problems.


I assume you're meaning the idle mixture screws are 2 1/2 turns out since the jets are inside the carb and have a fixed size orifice. If that's the case, 2 1/2 turns should be enough to get the engine to start & run but might need some fine-tuning for a good idle. Adjusting these won't change the way the engine runs at higher speeds though.

That's exactly what I'm talking about. I'm guessing one is for air and the other is for fuel? Passenger would be fuel and driver side would be air?


Not sure what you're referring to here ... Idle speed screw over on the driver side of the carb?

I was told not to mess with that screw on the drivers side but if I wanted to mess with the idle I could turn the screw on the top of the carb, dead center in front of the buttlerflies.


Yes, the choke should be pretty much straight up & down when it's fully opened.

I'm thinking I didn't set the choke right and will work on that first.

There's a good carburetor troubleshooting chart at: http://www.recarbco.com/technical/newtrouble.html that you might find helpful.

Thanks, I answered your questions and have more up top in the quote.

[68 TT]

You really need a vacuum gauge to adjust the mixture screws properly. Set one screw for max vacuum then back it off until it starts to drop, then adjust the other the same way. Once you have done them both go back and do it again. The setting of the second screw will throw off the first a little.

After that is done set your timing to factory specs (unless you have a bigger cam) then adjust your idle screw to get the desired idle speed. Don't forget to disconnect and plug the vacuum advance hose when you set the base timing.

If the mixture screws don't alter the vacuum then the passages behind them are gummed up and it needs cleaned out better.

[68 TT]

Originally Posted by 85Bowtie

I thought the Quadra Junks were pre-set from the factory and had no provisions for adjusting the idle mixture without drilling something out or something funky like that.

Are you attempting a rebuild on the carb or the whole motor? If you're unsure of how to adjust the idle mixture on a Q-Jet you might as well forget about rebuilding it and just get you an Edelbrock.

I know some Q-Jets had small caps put over the mixture adjustment screws to prevent people from tinkering with them. You can knock them out and get to the screws though. I had an emissions inspector try to fail a car once because the plugs were missing. I had to go to his supervisor and reason with him to get past it.

[ray_mcavoy]

Originally Posted by CenValyCali

That's exactly what I'm talking about. I'm guessing one is for air and the other is for fuel? Passenger would be fuel and driver side would be air?

No, both screws do the same thing. They adjust the air/fuel mixture at idle. There's one for each primary carb barrel. Turning the screws in (clockwise) leans out the mixture and turning them out (counterclockwise) richens the mixture. As 68 TT said, you'll want to hook up a vacuum gauge to get these adjusted correctly.

I was told not to mess with that screw on the drivers side but if I wanted to mess with the idle I could turn the screw on the top of the carb, dead center in front of the buttlerflies.

I think you've got that backwards.

You mentioned a Check Engine light in one of your earlier posts so I assume you're dealing with an electronic feedback carb. Those do have a couple of adjustment screws accessible from the top for adjusting the lean and rich limit stops of the mixture solenoid. Those are the ones I wouldn't recommend messing with unless you have a specific reason and know exactly what you're doing. I think they're normally capped off from the factory anyway.

The screw over on the driver side of the carb is the one you want to turn if you need to raise or lower the idle speed.

[85Bowtie]

Originally Posted by 68 TT

I know some Q-Jets had small caps put over the mixture adjustment screws to prevent people from tinkering with them. You can knock them out and get to the screws though. I had an emissions inspector try to fail a car once because the plugs were missing. I had to go to his supervisor and reason with him to get past it.

I knew it was something like that. I haven't looked at one in 10 years, I wised up and went to an aftermarket carb. It seems alot of people nowadays are wanting to run these things and I have no idea why.