I just got a new quadrajet for my 350 Chevy. It bolted on and ran fine. It is a little rich though. There is a hint of black smoke coming out my exhaust, and I can't find the conventional adjustment screws. How do I change the mixture of my carb? Also, which vacuum port does the vacuum advance go to? The PCV valve? Engine breather?

There are no conventional adjustments you can make to a Q-jet other then filing down the metering rods which I would leave up to a pro.

There are a few tricks to get them to preform better though. I took mine and modified it so the secondary choke butterlies just drop open at full throttle insted of opening via the vac. pull off.
Well that was the one I USED to have. Gonna have to do that all over again when I get one for the SB in the Toy.:D

You should be able to hook the vac. advance to any port on the carb itself.

The adjustment screws are on the bottom plate at the front of the carb. Theres several types of screw head out there all can be adjusted. Some are covered with plugs, once you remove those plugs you can adjust the carb.

The srcews at the front of the carb adjust the idle mixture only. If it runs rich at idle, turn these clockwise, that will increase the air/fuel ratio. If it runs rich at speed, or while acclerating, then you need to change jets/rods. I don't know if this is possible with a Q-jet, as it is with a Edelbrock. You may need to take it to a speed shop, as ForestCam suggested.

Yeah I saw those screws, but they just seem so damn small that I didn't even fathom that those were my adjustment screws. They did seem to be in the right spot though. I'll try them and then if that doesn't work, bring it to a speed shop.

(Iv'e never had to have someone else make adjustments to anything before, it feels weird not doing it myself:confused:)

Originally posted by ForestCam
There are no conventional adjustments you can make to a Q-jet other then filing down the metering rods which I would leave up to a pro.

There are a few tricks to get them to preform better though. I took mine and modified it so the secondary choke butterlies just drop open at full throttle insted of opening via the vac. pull off.
Well that was the one I USED to have. Gonna have to do that all over again when I get one for the SB in the Toy.:D

You should be able to hook the vac. advance to any port on the carb itself.

Bullshit. The Q-jet is fully adjustable throughout. And only an idiot would file down metering rods. Edelbrock sells a "strip kit" that has an assortment of primary jets, primary and secondary metering rods, and power piston springs. Also see http://www.quadrajet.com for a full assortment of parts. Edelbrock bought castings and most other parts from Rochester at the end of production; Edelbrock kits will fit Rochester carbs (because they are the same). All primary rods are not interchangeable; one series fits 70xxxx carbs and the other is for 170xxxx carbs. This is why Edebrock sells two different strip kits.

The vacuum advance must be hooked to a port that is above the throttle plates, not below. There is exactly ONE such port on a Q-jet, it's on the front, drivers side, about 1/3 of the way up. This port has vacuum when you open the throttle, not when closed. Otherwise you get full advance at idle and opening the throttle causes the timing to be RETARDED (just like some people...) I won't even start on the secondaries.

Mike's right about the location of the idle mixture screws. Also check for vacuum leaks; if the throttle plates are open more to compensate, it uncovers the transition circuit slot, and will cause a rich idle condition. I'd bet that this is your problem.

HPPublishing has a great book on Q-jets, get it. I think the author is Doug Rowe. The adjustment screws are for fine tuning only, you need to get the right combo of Primary jets and rods. I like to read the plugs for this step. Once the primary side is diailed in, you can work on the secondarys. You can use different hangers, rods, and vacum pots to to dial them in. It takes a bit of trial and error. Keep in mind your a Wheeler not a drag racer, so you may not want the secondarys hitting to fast and hard off road.

Don't file down or alter the parts, figure out what you curently have, and get an assortment of parts to work with. You already said your rich, so get some stuff to lean it out a bit at a time. Once you get the hang of popping the top off a q-jet, it's easy to do on the engine, just be carefull and don't drop any parts down the intake ...... :rasta:

Doug Roe's book is outstanding.

Originally posted by Lloyd
The vacuum advance must be hooked to a port that is above the throttle plates, not below. There is exactly ONE such port on a Q-jet, it's on the front, drivers side, about 1/3 of the way up. This port has vacuum when you open the throttle, not when closed. Otherwise you get full advance at idle and opening the throttle causes the timing to be RETARDED (just like some people).

I was setting up my q-jet Sunday, and found that that port has no vacuum ever... even went so far as to hook the gauge to it and drive.

I understand not hooking the dizzy to the other ports, makes perfect sense...

The carb is NOT dirty, and when I ran a screwdriver into that port (thinking maybe something was blocking it), it went pretty far in (seemed like it would poke into the venturi with another 1/16" of depth), and was CLEAN when i took it out. when the screwdriver bottomed out, i was hitting metal. does this port not run straight, and instead take a series of curves?

I'm at a loss.. and would REALLY like to have the advance working!

Originally posted by Eskimo


I was setting up my q-jet Sunday, and found that that port has no vacuum ever... even went so far as to hook the gauge to it and drive.


Something isn't right there. While the vacuum signal isn't quite as strong as full manifold vacuum at idle, it should definitely show up on the gauge and be easy to detect with your finger.


The carb is NOT dirty, and when I ran a screwdriver into that port (thinking maybe something was blocking it), it went pretty far in (seemed like it would poke into the venturi with another 1/16" of depth), and was CLEAN when i took it out. when the screwdriver bottomed out, i was hitting metal. does this port not run straight, and instead take a series of curves?



I think that passage is pretty straight, but necks down to an orifice inside the venturi throat. It'll be about 0.040" at the end, so you'll need a damn fine screwdriver if that's your tool of choice. ;) Music wire (guitar string, etc.) also makes a good probe for this, but it would be better to start from the inside and see if you can push something out. Compressed air (bent needle) and/or carb cleaner with the flexible straw is something else to try. It definitely sounds like something exactly the right size to plug the orifice got in there (which is actually fairly common). Poking at it from the outside won't help, and it's a bitch to get to the inside of that passage, but you could try applying vacuum to the outside also. Hope this helps.

I appreciate it...

Is the 0.040" hole (damn that's small) visible from looking down the venturi throat, or should I just start poking thing wire around where the hole *should* be?

The only source of vacuum I have is my big 'ol 6.5hp shop vac.. that sucker CAN make some good vacuum, so hopefully that and some wire will get it for me.. Too cheap to buy a vacuum pump.

Yeah, it's not too hard to see. It'll be a round hole; the little slot close by is the feed port for the transition circuit. Shop vac and a stiff bent wire should do the trick. :)

Thanks, I actually DID break down and buy a vacuum pump, figuring it'll make bleeding the brakes much easier...

Hope this works, dont' see why it wouldn't.. :)

OK, this was WEIRD!!! I tried and tried cleaning it, using the vac and the vacuum pump, no go. so I take the carb off, and still, something is blocking it... So, I take the carb apart... (my first time doing THAT).. What I found was that the hole went nowhere! There was no provision in the venturi for air to go to that port! Going backwards, air goes in the vacuum port, takes a 90 degree turn upwards, through a gasket, and into a dead-end hold in the carb. Nowhere did it ever make a path into the venturi.

SO, I decided to do a little surgery. I cut a small slit in the gasket between the throttle plate (lower 1/3'd) and the middle (venturi area) 1/3'd of the carb body, and re-assembled the carb, after crapping myself when a buncha little pieces fell out, but they all went back in OK.

Now, I can pull air out of the port. The slit I cut is above the throttle plate, so I'm hoping that it will provide the correct signal. If not, it's only the gasket that I modified, instead of the carb housing. A quick drive up the block should provide proof whether it works right or not (leaving the vacuum gauge connected)

So, what should I be looking for? I'm guessing:

Little to no vacuum at idle
a large amount off-idle
tapering down to the manifold pressure the more I open the throttle

Sound right? I currently have my gas tank out, as the previous owner used regular rubber plugs on the spare tank vents, and they rotted out and were leaking badly... I think I lost over a gallon of gas at Uwharrie!

I've never seen one without a functional vacuum advance port. What is the number on that carb? (Stamped into the casting on the LR corner, should be 70xxxxx or 170xxxxx.) Sounds like you understand what the vacuum signal should be doing. Only application I can think of that might have had a carb like that would have been an old high-performance car with a dual-point distributor from the factory (and thus no vac. advance can). This is really weird - learn something new every day.

UPDATE: Success!!! The 305 runs much better now in part-throttle application... The vacuum advance gives nothing at idle, some, proportional to the amount of throttle as I dip into it, and tapers back once the secondaries open..

Between that and the black magic cooling fan I installed yesterday, she no longer runs hot OR crappy.. almost too nice for a trail rig! :rasta:

It appears that somewhere down the line, someone changed out the venturi section of the carb for one that doesn't have the vacuum advance port milled in, and didn't realize it. Several signs make me think this, most notably, some scratches /discoloration on the carb throttle plate section that don't continue to the venturi section...

But, my little redneck vacuum advance port works, which is all I care about! Looks are WAY down the line.

It still bogs a little when the secondaries open if I just romp on the go pedal, but if I roll it on, it's smooth... Aaaaah, nothing like a small block chevy at 4500rpm without mufflers (well, they have 5" holes in them), singing with the intake of a q-jet secondaries!

Thanks again to all for your help...