Where is the Mixture Screw

devilock76 · #1 (**permalink**) 06-21-07, 10:00 AM

I know my car is now running rich that I have traced and fixed all the vacuum issues. Idiot mechanics the prior owner had. My mechanic buddy said it is near the bottom on the carb on my 307. Can't see it and well using the Exploded View from my service manual isn't helping. Can someone here point a better internet finger for me?

Ken

The Ape Man · #2 (**permalink**) 06-21-07, 06:48 PM

They might be capped off. They are located on the front side of the baseplate like all clodrojets. If you only see 2 pot metal or steel plugs then they are still sealed and not likely the cause of your rich condition.

devilock76 · #3 (**permalink**) 06-22-07, 09:39 AM

Quote:

Originally Posted by The Ape Man

They might be capped off. They are located on the front side of the baseplate like all clodrojets. If you only see 2 pot metal or steel plugs then they are still sealed and not likely the cause of your rich condition.

Do you know which one is idle mixture? Left or Right?

Ken

N0DIH · #4 (**permalink**) 06-22-07, 01:59 PM

They are hidden under the caps, you must remove carbie, and remove them (sort of brutal method). You have to punch out the aluminum base to get access to the pressed in plug. It won't be "pretty" anymore....

I honestly haven't seen hardly any that NEED to be adjusted, ususally there is other issues that are the real problem. But if they have been messed with, then that could be, they should be left alone.... It is not for bandaiding things....

Get Brad Urban's book on Rochester Carburators, you can't go wrong, best $$ on info on the Q-Jet you will ever get....

There is a left and right, the carb has a left venturi and a right, they are individual in operation. And with a dual plane intake (most have) they each feed 4 cyls.

Again, most the of the time the problem is more like a worn throttle shaft (Q-jets are notorious for this) causing an air leak at the criticial sensitive low air through the carb time. So richening the idle mix can help, but the real problem is the throttle shafts need to be bushed.

devilock76 · #5 (**permalink**) 06-25-07, 10:28 AM

Quote:

Originally Posted by N0DIH

They are hidden under the caps, you must remove carbie, and remove them (sort of brutal method). You have to punch out the aluminum base to get access to the pressed in plug. It won't be "pretty" anymore....

I honestly haven't seen hardly any that NEED to be adjusted, ususally there is other issues that are the real problem. But if they have been messed with, then that could be, they should be left alone.... It is not for bandaiding things....

Get Brad Urban's book on Rochester Carburators, you can't go wrong, best $$ on info on the Q-Jet you will ever get....

There is a left and right, the carb has a left venturi and a right, they are individual in operation. And with a dual plane intake (most have) they each feed 4 cyls.

Again, most the of the time the problem is more like a worn throttle shaft (Q-jets are notorious for this) causing an air leak at the criticial sensitive low air through the carb time. So richening the idle mix can help, but the real problem is the throttle shafts need to be bushed.

Well to clarify the situation I know the carb was rebuilt by someone who probably was not right to do the job and then tuned up by even worse people. Since owning it I found a ton of vaccuum leaks and since fixing that I think that the mixture is set too high from compensating for them before.

Ken

88landyacht · #6 (**permalink**) 06-25-07, 05:11 PM

I hate it when people mess with stuff they don't know about, Thats why the only person to detail my Caddy, and the wifes Lincoln, is ME!!

devilock76 · #7 (**permalink**) 06-26-07, 10:37 AM

Quote:

Originally Posted by 88landyacht

I hate it when people mess with stuff they don't know about, Thats why the only person to detail my Caddy, and the wifes Lincoln, is ME!!

Well this is all from the people who conned the previous owner. Just the slow process of righting things.

Ken

devilock76 · #8 (**permalink**) 07-16-07, 10:32 AM

OK after a little looking myself and talking to a mechanic friend (who is a bit out of the way for me) aparently I need a special tool for the mixture screws, this oval head bendy thing or something. Don't verbally slap me if I am wrong but if I am right where can I find said tool. I am in the Raleigh NC area.

Ken

N0DIH · #9 (**permalink**) 07-16-07, 10:37 AM

NAPA used to carry them, but being the Q-Jet is so old now (17 years since the last one in a car left the production line), it may be getting hard to get.

Call around to each part store and see what they have. I actually got a Q-Jet that was a training carb at GM school that was factory notched out for the screws and have screwdriver/nutdriver adjustable screws for it. Not sure the application, but it was closer to my Turbo 301 carb, but a Chevy fuel inlet.

Mud · #10 (**permalink**) 07-16-07, 01:48 PM

Is this an electronic carb? If so, then are other components that can cause it to run rich, especially if the rebuilt was not done correctly. I just rebuilt the one on my 86 and I can tell you that care needs to be taken when going through the carb.

I just ordered this tool set to make my life easier to adjust all carb components:
http://www.tooldiscounter.com/ItemDi...?lookup=THE362

The Doug Roe book on Q-jets is worthwhile reading.

Those mixture screws only set the base idle mixture. Other items that can cause your problem:
Choke flap not opening fully due to bad choke pull-off.
Secondaries not closing entirely.
Float set too high or inlet needle not seating properly.
Mixture control solenoid bad.
TPS bad and or misadjusted.
Incorrectly installed/adjusted air bleed assembly.
Primary metering rods messed up, stuck, misadjusted APR setting.

Most of the items above can be checked at rebuild time - based on what you said about past owner/rebuild I would be very leery of what the internal condition of the carb is. My suspicion is that adjusting the idle mix screws is only treating the symptom and not affecting the real cause.

The Ape Man · #11 (**permalink**) 07-16-07, 05:59 PM

Mud is right on the mark. Chances are very good that the mixture screw's adjustment isn't the problem.

Brother_B · #12 (**permalink**) 07-16-07, 10:14 PM

Well, I would take Mud's advice and try to do some troubleshooting, but if you do end up wanting a tool for adjusting (_if_ your mixture screws are not capped) I think this one is sweet. (Your friend may have been talking about a special tool for using when the screws are capped or somehow made to be difficult to adjust. This one is just for going around corners and stuff.) I already have a bendy-type one that I got at Advance Auto Parts, but I don't really like it. This one looks like it would work way better:

http://buy1.snapon.com/catalog/item....re&dir=catalog

Mud · #13 (**permalink**) 07-17-07, 08:18 AM

I have also made a tool for adjusting the mix screws - use a piece of 1/4" brake line tubing and cut a notch in the end of it.

N0DIH · #14 (**permalink**) 07-18-07, 12:50 AM

Sounds like a winner. See also if you can find an old GM dwell tool that is flexible and adapt a new end to get into the mixture screws. I have one somewhere that had removeable tips, but I don't know where it is anymore. It was handy being able to change tips and a flex shaft.