Weber Idle Circuit Mystery Options

[Conrad192]

I installed a new set of Weber IDF 40's on a bone stock '76 2.0

One of the carbs appears to have a dead idle circuit. (IMG:style_emoticons/default/WTF.gif) Car runs like crap below 2000 rpm's. It's happy spot is 3000. idle it's rough. Pull the wire on cylinder 1 or 2 and it makes zero difference in the idle. Pull 1 or 4 and it immediately changes the idle.

Here is what I know:

Floats height is set at 10MM

Carbs are spotlessly clean. Every orifice has been blown out with carb cleaner and check with guitar wire. Then blown out again with 100 psi of air.

New wires, spark plugs, rotor, distributor cap, and new 009 dizzy.

Removing plug and grounding to engine, the spark plugs are firing.
Compression is 130 lbs.

Valves are adjusted properly.



Air by-pass screws are FULLY closed.

Butterly is FULLY closed

The fuel adjustment screws were originally set to 2.5 turns out, but they make absolutely zero difference in the idle speed.

pump circuit works correctly.

I have absolutely no idea what is wrong with this carb. Given that they have never ran correctly it makes me wonder if there is a factory defect and the intersecting bores are not drilled properly.

[Superhawk996]

Not enough info.

What is jetting is in the carbs? What venturis?

Have carbs been synchronized properly? Linkage adjusted properly? Plugs on cylinders that are not firing - wet, dry, carbon fouled?

Pictures or video is helpful to help you diagnose.

[rhodyguy]

“Changes” is vague at best. What actually happens to the idle? When you get to the 3k mark you are off the idle circuitry, past the progression ports and on the mains. How closely are the carbs balanced, all 4 Venturis, at 3k? Then at 1k. Use one idle speed adj screw to turn the idle up. The other one will be off the stop and out of play. The linkage is in a working state. Now measure the flow again. If something is out of wack it will be evident.

[Conrad192]

Not enough info.

What is jetting is in the carbs? What venturis?

Have carbs been synchronized properly? Linkage adjusted properly? Plugs on cylinders that are not firing - wet, dry, carbon fouled?

Pictures or video is helpful to help you diagnose.

Venturi - 28
Main Jet - 200
Idle Jet 55
Air Jet 115
Emul Tube - F11

All spark plugs are firing. Dry, but all are black. Obviously rich.

Carbs are not sync'd whereas cylinders 1 and 2 are not firing at idle due to lack of fuel.

Linkage has been very carefully adjusted.

I believe the idle jets are too fat and need to be reduced to 50's. But why would one side run fine and the other side not, given that everything is identical?

[Conrad192]

“Changes” is vague at best. What actually happens to the idle? When you get to the 3k mark you are off the idle circuitry, past the progression ports and on the mains. How closely are the carbs balanced, all 4 Venturis, at 3k? Then at 1k. Use one idle speed adj screw to turn the idle up. The other one will be off the stop and out of play. The linkage is in a working state. Now measure the flow again.

(IMG:style_emoticons/default/agree.gif) the engine runs fine at 3000 on the main jets as you said. It will idle at 500-600 but it is obvious that it is not running on all cylinders. One carb, although not jetted properly and not synchronized functions adequately. The other carb (1&2) appears to have zero fuel output in the idle circuit. With the car idleing at 5-600 rpm a spark plug wire on cylinders 1 or 2 can be pulled and it makes zero difference in the rpm. Pull 3 or 4 and it will immediately drop the rpm and begin to chug until the engine dies. every symptom seems to point to the idle circuit on the one carb.

[rhodyguy]

Why do want it to idle at 500-600RPM? Or expect it too? Stock with FI is 950. The driver’s side carb seems to be the issue. Turn the idle speed adjusting screw in. Force the idle speed to increase. Leave the idle air mixture screw alone for now. Measure the flow with your synchronizing tool.

Fuel adjustment screws aren’t really a thing. There are idle air mixture screws and there are idle speed adjustment screws. Which ones are you turning in and out?

Are these new NOS Italian/Spanish carbs or Empie’s version?

[rfinegan]

Spray a little carb cleaner in the bore of the carb that is not "working"

Just a blip on the can and the RPM should come up. This will confirm no fuel on the bore Idle circuit, try it again on the adjacent bore.
Even with CLEAN clean carbs the idle jets pug and clog
VERY EZ.

[Superhawk996]

Not enough info.

What is jetting is in the carbs? What venturis?

Have carbs been synchronized properly? Linkage adjusted properly? Plugs on cylinders that are not firing - wet, dry, carbon fouled?

Pictures or video is helpful to help you diagnose.

Venturi - 28
Main Jet - 200
Idle Jet 55
Air Jet 115


Emul Tube - F11

All spark plugs are firing. Dry, but all are black. Obviously rich.

Carbs are not sync'd whereas cylinders 1 and 2 are not firing at idle due to lack of fuel.

Linkage has been very carefully adjusted.

I believe the idle jets are too fat and need to be reduced to 50's. But why would one side run fine and the other side not, given that everything is identical?

Jetting looks reasonable but main jet is too large. Rule of thumb; main = 4 or 5x the Venturi. Clearly not your idle problem though.

What do you mean carbs are not synched? Carbs must be synched before linkages are even connected. Improperly synched carbs will do exactly what you’re describing.

2.5 turns out on idle mix screws is too much - something not right. Yes probably need to go down a jet size but should still be running and making a difference when you pull plug wires.

Highly unlikely the issue is within the carb as a defect.

Do you have equal flow on snail at idle across all 4 barrels with no linkage connected?

[technicalninja]

He's got the numbers swapped between mains and airs most likely.
If not, then the mains are HUGE, and the air jets are tiny.

Those don't matter at idle-2k rpm.

I'd use a pen light and look down the bores as I was adding throttle.

I believe you should be able to see the activity at the progression holes below the venturi if you look closely. I'd see if I had significantly different activity between carbs and then I'd take the non-functional carb off and really go over the idle and progression circuits.

You might be missing one of the tiny lead plugs that make that system vacuum tight.

You might have some obstruction in the circuits.

Sounds like an internal issue with one of the carbs to me...

[porschetub]

Not enough info.

What is jetting is in the carbs? What venturis?

Have carbs been synchronized properly? Linkage adjusted properly? Plugs on cylinders that are not firing - wet, dry, carbon fouled?

Pictures or video is helpful to help you diagnose.

Venturi - 28
Main Jet - 200
Idle Jet 55
Air Jet 115
Emul Tube - F11

All spark plugs are firing. Dry, but all are black. Obviously rich.

Carbs are not sync'd whereas cylinders 1 and 2 are not firing at idle due to lack of fuel.

Linkage has been very carefully adjusted.

I believe the idle jets are too fat and need to be reduced to 50's. But why would one side run fine and the other side not, given that everything is identical?

Hi,think you have mixed up air-jet with main jet ,should be air jet 200 and main jet 115 ,cheers.
Edit,sorry late reply,wonder if the OP has checked for inlet vacuum leaks, had this recently with one of my Zeniths due to wrong thickness gasket on one manifold , it was ok over 2000rpm but idle curcuit was crap ,picked up the leak when trying to balance them with my snail.
Had little or no response to adjustment on this carb as the OP has ,worth checking for sure.

[rhodyguy]

Air Correction Jet.