Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

Setting Up and Adjusting Idle

(1/8) > >>

defiant1:
Need to know the proper procedure to set up the idle and subsequent adjustments on the 17080204 Q-jet.

Background: 
Sent carb to have throttle shafts redone via Cliff
Purchased rebuild kit from Cliff
Oh, and bought Cliff's book
APT plug was removed and set screw installed

Carb specs:
#17080204
Idle Tube: .036" (I believe this is correct unfortunately I forgot to measure before install and Cliff's packages don't come labeled)
Idle Down Channel: .055"
Upper Idle Air Bleed: .070"
Lower Idle Air Bleed: .068"
Idle Bypass Air: .081"
Mixture Air Screw Holes: .096"
Float Height: 5/16"
APT screw: 4 turns from seated
Main Jets: #73
Secondary Rods: CH
Secondary Metering Rods: #44
Idle Mixture Screws: 4 turns out from seated

Motor Specs:
SBO 350
Rebuilt Cylinder Heads
9.0:1 CR

Cam Specs:
LSA: 108 +2 (Installed straight up)
Duration: 217/224
Rocker Arm Ratio: 1.6:1

Ignition:
Pertronix III with Flamethrower Coil
I installed heavy mechanical advance springs in the distributor.
Spark Plug Gap: .042


Ok-I don't have any documentation on how to set the idle (either curb or fast idle).
The electric choke works well--I was concerned as I have never used one.
The motor starts on the first turn of key.  All I have to do to push accelerator pedal down once. (I know, I know, like its suppose to-- but when you have never had that in a carb before it is a welcomed change)

The motor runs smooth at higher rpm but at idle it shakes like I got bad motor mounts (mounts are new).
It runs a little rich based on what the plugs look like.
I did the "tip in" procedure and there was no change in rpm.

So before I start turning screws randomly, I am looking for the procedures to set the idle scews and make adjustments from there to remedy the rough idle.

Thanks

omaha:
   Vacuun gauge and an RPM gauge. Id set the initial at 12 to start with. Youll be looking for the highest vacuum reading at the appropriate idle RPM . You can get the gauges at sears or auto zone. you may nees to go back and forth with the adjustment cause adjusting your mixture may effect the rpm  to go higher. so you gotta adjust the rpm speed back down, then go back with the mixture adjustment and etc.
over and over til its just right. Ill bet it will smooth right out.

defiant1:
Where am I pulling vacuum from (carb, manifold)?  Also is 12 the vacuum reading or the initial timing you mentioned I start with??

Is there a chart that correlates RPM with vacuum?

Thanks

omaha:
...sorry for confusion. 12 would be the timing advance setting while the engine is idling. (with distributer vac advance disconnected; plug the vac line). this is just a base to start with and really just a guesstimate but for your engine combo. Use a manifold vacuum source with your vacuum gauge to set your idle mixture. As far as a correlation of rpm to vacuum, that would be impossible to do because every engine is different. The engine will make whatever vacuum it wants to. this goes for any engine. Even engines that are identicle from the factory will vary! they'll be close but not identicle. Your job while setting the idle mixture is to obtain the highest vacuum that you can (observed on the gauge) while maintaining a predetermined rpm (some engines with big cams do not like to idle at a slow speed). My old pontiac 350 liketo idle at about 650 rpm. this was aparantly the factory specification for the idle speed for this engine equipped with an auto trans. At this speed it made 18 inches of vacuum. (extremely good reading for a 40 year old engine and     that had never been rebuilt) Idle speeds are set after teh engine has warmed up to operating temp. If you look in a chiltons book or similar, it will give yu the specification for the idle speed for STOCK engines from the factory.  So, you want to keep theidle as slow as practible for reduced wear, emissions , fuel consumption, drivetrain wear etc. BUT, you do not want it to be too slow or it will stall out, be impossible to set mixtures, and the alternator needs some rpms to make voltage and other various things. So you need to figure out at what speed you want it to idle because of the modifications to the engine such as the cam. You will git an idea of what you think you can live with when you start to do the tuning. The idea is to get the highest vacuun reading you can at the lowest rpm that are PRACTICLE for you engine. Guys with very good skills at setting up the idle circuit can get their cars to idle at a lower speed and still pull decent vacuum. what you are actually doing is finding the optimum burn in the cylinder and the 2 things it boils down to is ignition timing and mixture quality.

defiant1:
Ok-still working on getting a vacuum gauge. 

In the meantime, I turned in the idle mixture screws in all the way and the motor rpm did not change.

The carb sounds like it is pulling vacuum hard (very noisy).  I don't want to spray carb cleaner on the intake manifold due to new paint.  I have a 1" phenolic spacer.  Could the gasket in between bottom of the carb and top of the spacer be the problem?

I will still check vacuum after I get the gauge.

d1

Navigation

[0] Message Index

[#] Next page