Gain extra benefits by becoming a Supporting Member Click here find out how!
Silver
Gold ***Platinum***
andylee32Boosted54 DOC396 (9)blue56 (12)pistol (13)
Gremlin (3)bobyone4dad (2)PLS (3)55sedanman50Ragtop (6)
MoboMac (3)jrogers885 (3)
longboard (6) jel1957 (6)
Rick Dorion (2)
Bowtie Crazy (5)
nauta (9)
cyclops (3)fonc (2)
Classic Performance Products Classic Parts
Ciadella Interiors American Auto Wire Art Morrison.com
Exile® Battery Keeper™ 6/12 volt charger w/ LED battery monitor
Low priced Genuine GM Auto PartsEcklers AutoMotive
Hellwig Products IncPerformance Rod & CustomClassic Industries
Impala Bob's Bob's Chevy Trucks Bob's Chevelle Parts Bob's Classic Chevy
 

Recent Hot Topics
Current Quote
"Another reason of why this site and the people that participate in it are great. I have been in need of a good running engine for the fleetline and thanks to Patgizz I now have one. He was kind enough to give a nice 250 I6 motor. Thanks again Pat!!!"
~ Member
Recent Topics
LMC Truck Join the Community today Join Us Now �



�
Username Post: adjusting the idle mixture screws        (Topic#210039)
tamccain 
Contributor
Posts: 137

Reg: 02-02-09
02-19-09 11:56 AM - Post#1633727    

I need to adjust the idle mixture screws on my quadrajet. The problem is, on the air cleaner it says, "Idle mixture screws are set from the factory and sealed" Because of this, I cannot seem to find these idle mixture screws. It seems like there is some "cap" over them. With that being said, how do I go about adjusting the idle mixture on this carburetor.
1993 K3500
454/4l80e
The Beast


 
LMC Truck
CaptainStompAndSteer 
Junior Contributor
Posts: 451
CaptainStompAndSteer
Age: 33
Loc: Redding Ca
Reg: 01-29-08
02-19-09 12:13 PM - Post#1633742    
    In response to tamccain

You have to dig the epoxy out first. Second of all if you dont know what you are doing you should leave that to someone that is good and I dont mean just someone that can adjust a holley or what ever. I would find someone that knows Q jets to try that. Besides that you should have your ignition timing spot on before you adjust your carb. Just my 2 cents. I know a guy that took his truck to some guy in the sticks for the carb adjustment and his truck has ran really good and it gets really good mileage also.
"May the sun shine in your face and the sparks fly far behind you"


 
tony1963 
Frequent Contributor
Posts: 1206

Loc: Birmingham AL
Reg: 10-11-08
02-19-09 02:09 PM - Post#1633820    
    In response to CaptainStompAndSteer

Whenever I hear of someone wanting to adjust their idle mixture, I always ask for the symptom.

Keep in mind that before touching this, you need to, as mentioned above, make sure that the other engine adjustments are at par.

The idle mixture will only change the air/fuel ratio at idle, not above. Once you open the throttle blades, even a bit, you're running on the size of the openings in the carburetor.

If you find that you must adjust the idle mixtures, the best solution is to start with a rebuild kit. Open it up, clean it out, assemble with new parts. On the base plate of the carb you will find where the mixture screws are leaded over. You can use a saw and cut out around those mixture screws.

To begin, turn them in until they seat, but don't turn hard or you will ruin the screws. Then, back out exactly 2 turns, start 'er up. Turn the screws in until you lose idle speed, then turn them back out 1/2 turn. You're dialed in.
1964 Chevrolet Impala SS 327 Powerglide (SOLD!)
1981 Ford Granada 3.1 L (200 inline 6) (SOLD!)
1985 Chevrolet Suburban C20 454 Turbo 400
1999 Mercedes E300 (SOLD!)
2008 Mercedes E350
2012 Mercedes S550



 
tamccain 
Contributor
Posts: 137

Reg: 02-02-09
02-19-09 02:49 PM - Post#1633851    
    In response to tony1963

I have been doing a lot of thinking through all the work I need to do on this truck, so my question was more for my knowledge than anything. BUT, I do have an issue of the truck blowing out black smoke at startup. I am going to replace spark plugs and wires(they look pretty old), and if that doesn't fix it then I may look at a rebuild for the carb.

I am somewhat of a knowledge junkie, so just knowing makes me feel better.
1993 K3500
454/4l80e
The Beast


 
The_Hunter 
Contributor
Posts: 984
The_Hunter
Loc: Manitoba, Canada
Reg: 02-14-05
02-19-09 04:50 PM - Post#1633951    
    In response to tamccain

I had that black smoke startup on my truck several years ago. I brought it 2 a local shop since I knew nothing about carbs. All it was was the choke pull off needed to be replaced. I would start by checking your chokes operation.
86 C10 Short Box

Computers are like air conditioners - They can't do their job properly if you open windows.


 
McLeod 
Member
Posts: 238
McLeod
Loc: North Carolina
Reg: 01-10-05
02-19-09 06:07 PM - Post#1634012    
    In response to tony1963

OK...when turning them back in, do you turn both a little bit, or do you turn one until the idle goes down and then adjust the other? Thanks! Mc
McLeod -- 1999 Tahoe LT, '03 2500HD Ext. Cab.


 
CaptainStompAndSteer 
Junior Contributor
Posts: 451
CaptainStompAndSteer
Age: 33
Loc: Redding Ca
Reg: 01-29-08
02-19-09 06:12 PM - Post#1634018    
    In response to McLeod

I allways do one at a time turning it in until the motor starts to just run rough and then open it up just a tiny bit past where it starts to smooth out. Then I adjust the other screw the same exact way and never look back. I learned tuning a 327 with a tunnel ram and duall 4160s.
"May the sun shine in your face and the sparks fly far behind you"


 
maximus86 
Forum Newbie
Posts: 18

Reg: 02-20-09
02-21-09 08:28 AM - Post#1635198    
    In response to The_Hunter

  • The_Hunter Said:
I had that black smoke startup on my truck several years ago. I brought it 2 a local shop since I knew nothing about carbs. All it was was the choke pull off needed to be replaced. I would start by checking your chokes operation.



When you say "choke pull off" Do you mean the actual choke, or is there something else? I am getting the same symptom.
Chris
Ixonia,WI
1986 GMC High Sierra
2005 Dodge Magnum R/T
A couple motorcycles
A snowmobile
and a boat.


 
gchemist 
Senior Chevytalk Moderator
Posts: 22095
gchemist
Loc: Austin, TX 78748
Reg: 05-09-00
02-21-09 08:34 AM - Post#1635202    
    In response to maximus86

I tune by carb with a vacuum guage.
http://www.centuryperformance.com/tuning-with-a-va...

Does it work? HMMMM, I can get 19.5 MPG out of my ZZ4 355+ HP!!
'83 Silverardo XST - ZZ4 powered
'96 GMC Jimmy LS Ret. @236651 miles


 
Vaughn 
"13th Year" Gold Supporting Member
Posts: 15834

Loc: Colorado Springs, CO
Reg: 08-08-04
02-21-09 08:59 PM - Post#1635677    
    In response to gchemist

A choke pull off diaphragm is used to pull open the choke when the throttle blades are shifted to wide open throttle while the engine is still cold. It prevents the choke from stalling the motor out.

The choke pull off diaphragm is usually screwed to the outer passenger side of the carb at the top of the air horn, and interacts with the choke linkage. It usually has a large slot in the side of its linkage that the choke rod goes through or connects to.

This diaphragm works the opposite of what most people think of a vacuum diaphragm doing - it starts to operate when there is no intake manifold vacuum (WOT) but shuts off when there is high intake vacuum, like when idling. It is also hooked to a thermostatic vacuum valve, ie. it only works when the motor is cold (and some specific emissions related conditions).

 
raceman6135 
Senior Member
Posts: 891
raceman6135
Loc: Calgary, Alberta, Canada
Reg: 11-04-02
02-22-09 07:28 AM - Post#1635791    
    In response to Vaughn

  • Vaughn Said:
A choke pull off diaphragm is used to pull open the choke when the throttle blades are shifted to wide open throttle while the engine is still cold. It prevents the choke from stalling the motor out.

The choke pull off diaphragm is usually screwed to the outer passenger side of the carb at the top of the air horn, and interacts with the choke linkage. It usually has a large slot in the side of its linkage that the choke rod goes through or connects to.

This diaphragm works the opposite of what most people think of a vacuum diaphragm doing - it starts to operate when there is no intake manifold vacuum (WOT) but shuts off when there is high intake vacuum, like when idling. It is also hooked to a thermostatic vacuum valve, ie. it only works when the motor is cold (and some specific emissions related conditions).





 
maximus86 
Forum Newbie
Posts: 18

Reg: 02-20-09
02-22-09 08:40 AM - Post#1635841    
    In response to raceman6135

Thanks for all the pics and advice guys! I'm going to fix that and then check all the vacuum lines.
Chris
Ixonia,WI
1986 GMC High Sierra
2005 Dodge Magnum R/T
A couple motorcycles
A snowmobile
and a boat.


 
RunAndGun 
Contributor
Posts: 410
RunAndGun
Age: 47
Loc: Mckinney Texas
Reg: 05-16-09
06-12-09 05:34 AM - Post#1715656    
    In response to gchemist

Thank you all for pointing the way for me to adjust my Quad!

Every time I think I have achieved all I can out of my little motor, knowledge I gain from this board proves me wrong.

It runs better than ever now. I am getting ever so slight of what feels like an intermittent miss at low idle, so I think I might need to adjust a little more, but not enough for it to be a priority at present.

I have read many times on this board that adjusting the idle mixture screws only affects the performance at idle. If this is the case, why does my 305 now perform better all the way around?
1984 4WD Short Step Side
305 4Bl Quadrajet, 700R4


 
LMC Truck
Icon Legend Permissions Topic Options
Report Post

Quote Post

Quick Reply

Print Topic

Email Topic

7559 Views
FusionBB
FusionBB™ Version 2.1
©2003-2006 InteractivePHP, Inc.
Execution time: 0.136 seconds.   Total Queries: 13   Zlib Compression is on.
All times are (GMT -0800) Pacific. Current time is 09:15 AM
Top