Setting Up and Adjusting Idle

[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

[omaha]

   I think I would do this; check for a vac leak that might be a vacuum hose, it kinda sounds like you problem..Also, if the idle mixture needles are not seating all the way, the spring on them may be at coil bind. You may need to trim a coil or two from them. You shoukd be able to kill the engine with the turning in of the idle screws. Of course, you wont get a super smooth idle with  the cam you have but it should be close to a factory performance cam, pretty smooth in the case of an olds. As far as the gasket goes, yes this may have a vacuum leak but unless you can check for a leak....in the old days, I used to use an unlit propane torch to check for leaks but everyone seem to think it is dangerous. I guess maybe but the only other way I know of is some type of spray. Another guy I know says he sprays water to check for leaks. I have never tried it but who knows. Dont forget to check the timing and you should have 12 at idle, (vac. adv. disconecttd.) Try 14 degrees and see if it helps, if not, put it back. check plug wires too, firing order and connections. If the spacer is good quality, it should work. But, you might try it without the spacer just to see if it makes a difference.

[defiant1]

Ok - update:

- Can't install carb without spacer, due to throttle linkage hitting intake. So could not attempt that setup.
- Installed different gaskets (thicker base gasket).
- 10* at 1100 rpm for timing based on what the manual is telling me to set it at.  
- Vacuum gauge read 10.5" at 700-750 rpm.  18 inches @ 2000 rpm.  No idea what that means in the large scheme of things.
- Idle mixtures screws DO now affect rpm when screwed in, whereas before they did not.  So I am gaining there, I think.

I am going to do a compression test on the cylinders and see what I come up with.

Forgot to add:  I used a stronger power piston spring.  It was the same length as the factory spring, but there were more coils (if that makes any sense), basically a stronger spring.  Should I put the factory or replacement equivalent in and see if that helps with the idle.  Of course this is assuming the vacuum numbers listed above are ok.

d1

[jamesF]

Make sure the flaps are closing all the way. Work them a little and push them closed as you run it to see if they are not fitting tight or closing 100%.

[defiant1]

Not sure what you mean by moving the flaps or attempting to move them.  Can you elaborate?  Side to side movement of throttles shaft??

Well I did the following this afternoon:

BTW: I have the Petronix III unit with flamethrower coil.  I ran this setup with my former 2BBL/intake (same motor w/o head work) at 10* BTDC at 1100 rpm (dist. vacuum capped off).

I just finished my compression test, ran motor to operating temp and took readings:

#1 - 159
#3 - 155
#5 - 155
#7 - 162
#2 - 157
#4 - 159
#6 - 163
#8 - 160

Not too bad for a 40 yr motor with no work done to the bottom end.
I also checked the plugs and the #1 plug was the worst looking-had a black coat on the ceramic.  The remaining plugs had a light to dark brown ceramic coating.  Nothing was covered in soot.

When motor was running I got brave and squirted starting fluid around base of carb.  No noticeable difference in rpm occurred.

The local mechanic thought the vacuum was pulling from w/in the carb itself.  If that is the case I have no idea what circuit inside the carb controls that.  BTW I had every vacuum port plugged with the exception of the PVC and the power brake hose which I used to hook up my gauge.

I noticed this time around my idle Hg reading at 750 was a steady 11".

I thought I had fixed my idle mixture screw problem, but I screwed them in all the way and the idle did not change (well maybe 50 rpm--but it should have  made a noticeable difference).

New items:
4BBL Q-jet
Cam: custom grind 217/224 @.050
Edelbrock Performer intake

I am open to suggestions.

d1

[defiant1]

Ok- trying to eliminate things that could cause problem.  I took a skinny screwdriver and placed it in the vent tube to ensure the power piston goes up and down (motor off). 

With motor on, I did the same thing and the power piston is fully seated.  So the light blue spring I am using is not affecting the idle circuit from what I can tell.

Adjusting the idle mixture screws in or out does little to nothing to the RPM.

I am out of ideas.

- 10.5" Hg at 750 rpm - not sure if that is good or bad
- Idle does not improve when I cup my hand over primaries
- Sprayed starting fluid around carb and rpm did not increase

What else can keep the idle mixture screws from doing their job??? 

d1

[omaha]

  If you have chenged the cam to one that is larger, you will need more initial advance. I would try 14 or 15 degrees initial advance. Also, the idles RPM speed ay end up a little bit higher. Since this cam is something that is new, it changes the parameters of the engine characteristics. The only other thing that I can think of would be the idle tubes not being large enough but it seems like this would be not the case.
What is the exact advertised duration of this cam? (the specs yu gave are duration at .050 lift I assume).
The 108 centerline is just a bit tighter than most of the cams out today that are made with 110. This small amount more overlap may be causing some of  the issues with the idling.  The vac reading sure seems a bit low.  I would expect ~12-14 inches with a 217/224 @ .050 cam but what is the advertised duration? Bump the initial timing to 14 or 15 at Idle.  try to stay below 1000 rpm when setting the initial timing. (although you can tell when it starts to advance with the timing light.)
      Just trying to get an idea of what cam you have to figure out what you can expect with the idle quality. Some cam will make the engine shake a bit.

[defiant1]

The camshaft is a custom grind.  Erson Cam- 268/284  LSA: 108+2  Valve lift: intake-.494  exhaust-.504  Overlap: 60

Intake center line: 106 @ .050

I degreed the cam and verified all the specs.

When I get home tonight I will try the 14-15* at lets say 900 rpm and go from there.

Is there a correlation between how far the throttle plates are open and the Q-jet's idle circuit when the motor is at idle?

[jamesF]

I meant the Secondary Flaps. On mine I had trouble initially getting the idle mixture to respond to the screws. THe flaps were not closing by the smallest bit. They needed some "working" open / close - open /close  etc.

[defiant1]

Ok- tonight I retorqued my intake manifold.

I plugged off both valve cover grommets and left the hose for the PCV attached to carb and left the other end open.

I started motor and it was pulling a steady 11" vacuum with hose open.  When I put my finger over the hose and plugged it the vacuum jumped to a steady 15".  Not sure what this exactly means but just an observation.

Ok hooked up everything else back up and ran motor.  Set the timing at 14* at 850 rpm.  The vacuum at this setting is a steady 12.5".  I played with the timing up and down for awhile and this seemed to be the happy medium.  The motor has an occasional slight even shudder at times but it was not abrupt.  Although a shudder to me still screams vacuum leak...so still not sure.   

It would run rougher when I turned in the idle mixture screw all the way in, but still would not kill the motor or make a substantial decrease in rpm.

It dieseled a little when I shut it down.  But I guess this could be attributed to making multiple adjustments to the timing.

I might be gaining, but still not completely confident I got the idle figured out.

On another note:  Does it matter what port on the carb (Chevy Q-jet) I hook the PCV hose to?  I am using the one in the front on the baseplate, it that right?  I plugged the rear port and am using the intake manifold for the power booster.

d1

[defiant1]

I was referred to another carb builder, I explained the issues I was having and he suggested the  throttle plates were open slightly interrupting the idle circuit.

He said this would contribute to the following issues:
Low vacuum
Idle mixture screws having no affect

He suggested I:
Replace the power piston spring with the lightest spring to help keep to ensure the needle stays seated
Ensure that the throttle plates are covering from the middle to lower third of the transition slots (vertical slots in bore).
Idle Bypass Air hole should be drilled a little larger. Mine is .081" He recommended .093" I will drill the hole incrementally until it runs the best.
And verify that fuel is not dripping from nozzle

I will make these mods and see what happens.

d1