Idle Screw reaction

[PaulProe]

enjoying the tuning process of my webers. I am finding my idle mix screws have little to no effect on the idle speed or smoothness.

I've checked the throttle blades to make sure the progression holes are covered and everything looks good. At idle, the idle speed screw is "in" not quite 1/2 turn. If the car is running off sucking air past the progression ports, I don't think I can get the blades to fit any tighter.

On a Holley, I'd be getting suggestions to crack open the secondaries or drill the throttle plates to move a little more air into the engine. But this isn't a Holley. I hesitate to drill the plates just yet.

Car will idle relatively smooth at 850-900. Mixture is a little rich but I get stumble around 2500 rpm as the car transitions. I can baby it and I don't get any stumble. If I romp on it, the squirters do their thing and everything is fine. If I try to accelerate with medium peddle, it's like riding a bucking horse

Ideas?

Paul

[jhirasak]

Paul,

First, do you have a Air Fuel Ratio meter that you can use to record how your AFR varies as you drive? If not, I would highly recommend that you purchase one and use it to assist in your Weber tuning process. Without knowing if your AFR is too rich or too lean in your problem areas, it is difficult to provide you with accurate recommendations to correct the problem.

With respect to your idle mixture and idle speed questions, it appears that the idle fuel jet that you are currently using is causing a slightly rich condition based on your mixture screw setting of 1/2 turn out. Normally with your throttle plates fully closed, the transition ports should not be providing any air to control your idle speed. The air at idle should be provided by the idle jet holder and the air normally leaking past the throttle plate.

With respect to the transition problem near 2500 RPMs, it is likely due to your choice of emulsion tube or fuel level in the bowls if you can ease through that speed range. Without data on your AFR readings (too rich or too lean), and information on your current tube number, I cannot recommend which tube could improve your situation. Please post information on all of the jets that your are currently running.

Cheers,

John

[PaulProe]

John
I am using a LM2 AFR to check my progress. I am running about 11.5-13.0 most all times, except when it transitions and then I am seeing spikes to 18/20

I am currently using these variables:
Idle Jet: 60
Idle Holder: 90
Main Jet: 155
Air Corrector: 120
Pump Jet: 50
Pump Bleed: 35
eTube: F11
Mix Screw Turns: 7/8

I have repeatedly confirmed fuel level. I could go to a smaller idle jet but it would really fly in the ones reported by everyone else with my engine size.
432" FE / mild roller cam (285Adv. - 225 @ .050) Almost everyone is showing 65-70 on idle jets and I am already below that.

The manifold has a vacuum plenum built into it to power the PCV system and the motor generates about 12" Hg at idle. Vacuum signal isn't a problem.

I tried a set of F16 tubes and it got worse. Tried blocking the top holes of the F11's and it didn't make much difference.

It acts like the mains need to come in a little sooner, so I tried a smaller set of air correctors (100). But that flies against the rule of thumb of Main +60 for the correctors.

How much change should I see in the engine when turning the idle screws? Right now, I can screw them all the way in and the engine idle speed/smoothness stays the same.

I've attached a shot of the transition holes, taken from above, to give you an idea on how they align with the throttle plate.

Paul

[jhirasak]

Paul,

I sent you a PM. I do have some E-tubes and idle jet holders that you can try to see if they make the situation better or worse.

John

[zrayr]

Originally Posted by PaulProe

enjoying the tuning process of my webers. I am finding my idle mix screws have little to no effect on the idle speed or smoothness".......

Your throttle plates are not open equally. You will have to twist the shaft slightly until the non responding hole opens up some more. Even if they all show equal with a balance tool, they are obviously not getting enough air to combust the fuel you are metering in with the idle mixture screws.

To back up a minute, you have to be certain that there are no clogs in the idle mixture passageways and that fuel can flow thru the metering hole at the base of the carb, and that you don't have any issues with ignition, etc.

If you are getting no response on all of the cylinders then you need to adjust the linkage until they are open more.

Why are you talking about going to a smaller idle jet when you are too lean on the transition ? Forget about the A/F numbers at idle. Idle A/f ratio is the least important consideration for you.

I had similar transition symptoms and ditching the 60 idle jet Improved things dramatically. A 65/100 combo would be the first thing I'd try in your situation. A 70/120 or a 70/110 (a drilled out 100) would be next up.

Z

[zrayr]

Have you tried adjusting the carb idle mixture screws with the PCV system plugged so all the air the engine is getting is coming thru the carbs ?

Z

[*13*]

If you aren't getting any response from the idle screw, it's likely the passage is blocked. Pull the idle screw & clean the hole.

[jhirasak]

Paul,

I think that Z and Hyde have a very good point in that you should resolve your idle mixture screw response problem before attempting to iron out the transition roughness issue. Since the idle circuit performance overlaps the main circuit transition region, the idle and off idle mixture control must be corrected first. Otherwise, you will be chasing a number of rabbits that are difficult to catch.

John

[saltshaker]

I Went thru this...I couldn't shut down a barrel by turning the mixture screw all the way in, so i took the tops off all the carbs removed the squirter and float. I pulled the idle jet stack out and got a can of carb cleaner and with a long straw that fit in the spray head i up it in the passage that feeds the idle circuit... There is one on each side of the fuel bowl opposite side of the exhaust jet. Squirt the cleaner in there till it starts to come up where the idle jet screws in.... Blow air back down it to finish clearing the passage. I would do all 4 carbs. To get the idle circuit working correctly I took out the main jet stack so the idle circuit would work on it's own..... The start to work on the main circuit.
My 428 was 435 ci w/ a cam 236@.50
My combo worked good for me
65/120 idle
160/175 main
F11 emulsion tubes
7/8 turns out.....
Good Luck, Jon

[PaulProe]

Figured it out - the carbs originally came with 120 main air correctors (all new 48IDA's bought in the box do) and everyone reported don't worry about the correctors, they only affect the high end of the main mix. That is incorrect!

If the air corrector is too small and you have a strong signal (vacuum), it will cause the mains to feed slightly, even at low/idle speeds. And it causes your mixture to run extremely rich. Once I opened up the air correctors to around 200, the mains & idle jets began to operate as you would expect.

As I understand it, the air correctors not only control the high end mix of the mains, they also determine when the mains begin to come on line. Too low and they are "on-line" all the time.

I'm not done yet, but I am once again moving forward. Hope my experiences will help another newbie.

Paul

[saltshaker]

Paul, Emulison tube determine when the fuel starts to enter the main circuit. The 120 is a idle jet holder for the idle curcuit and the air corrector is for the main circuit, they are a aid to either lean or richen the jet. They have nothing to do with when the fuel transfers from idle to main... That is done by the emulsion tube. If your motor is a FE then I as many others found F11 tubes to work
You can call me if you want to talk
Jon
4844109570

[mike cain]

this is just a suggustion. I was working with 45 dcoes on a four cylinder engine. Could not get the idle right, final found that someone had screwed the idle adjustment screws so tight that all four of the tips had broken and remained stuck in the idle jet holes and would not allow gasoline to past.
also 45 dcoes series 152 use a different thread on the idle adjustment screw so you need to turn them out appox. 3 turns rather then 1 1/2 turns like the manual. Don't know if the later model IDA have the same idle adjustment screw.

[zimmy]

Don't get too hung up on the old "rule of thumb" thoughts for # of turns out etc. Today's fuels are very different and can have very big effects on the behavior of the IDAs. Today's fuel has a lot of oxygenators in the fuel meaning that you have to run a bit richer to compensate. My 454 with 48 IDAs runs and idle best with the idle mixture screws at about 1 1/4 turns out - not the "gospel" 3/4 of a turn. It really is necessary today to use these old rules as baselines, but go with what your engine wants to run with. Don't lock yourself into only what historically was the answer. Times have changed!!