Weber Carburator HELP!!

[914obsession]

I have a 78' 3.0 with stock CIS pistons and Cams, the engine was entirely rebuilt by me. I installed a set of rebuilt webers on the engine and cannot get the mixture screws to do ANYTHING. Carbs were ultrasonically cleaned, blown out and rebuilt with new rebuild kit. The carbs are running 34mm venturis, 150 mains, 180 air corrections and 60 idle jets. This is what I have done thus far:

- Float bowl levels have been set and re-checked
- engine timing is correct
- all gaskets between heads and manifolds and manifolds and carbs are new and not leaking
- set up the geometry for the throttle linkage
- checked for spark in all 6 cylinders
pulled the carbs off (twice now) and blew everything out and looked for any signs of debris or other issues- none found.
- Fuel pressure to the carbs is 3.5 psi.

The engine will not idle unless the idle sets screws are turned in about one full turn OR the air correction screws are turned out a bit. The car runs GREAT (but rich) with the air correction screws each turned out about 1 full turn. I have a O2 sensor that I can hook up to check air/fuel mixture. The car runs at about 13.8 over 3200 RPM (out of idle circuit), a little rich but not bad.

By turning the air correction screws in, the car gets leaner, by turning them out it gets richer (as measured by the O2 sensor). I can turn all 6 mixture adjustment screws in all the way with NO effect on the engine, I can also turn them nearly all the way out with no effect. Any suggestions would be greatly appreciated, I am getting really frustrated...

Dan

[Steve@Rennsport]

Precisely,...how many turns outward are the idle screws (from closed) when it idles OK???.

[914obsession]

Steve, thanks for the reply. There are two answers to your question. With the air correction screws each turned out a bout an 1/8 of a turn (seems like the recommended starting point) the idle set screws need to be turned out one and three quarter turns in order to get the car to idle around 1k.

If I adjust the air correction screws out I can set the idle with the idle set screws not turned out at all. What's odd is, the air correction screws are supposed to cause the car to run leaner as you open them up, on my engine it's the opposite (according to my air/fuel tester).

Nothing that I have read states how many turns is permissible with the air correction screws. Also, nothing I have read helps out with completely non-responsive mixture screws - ( I have tried a set of 55 idle jets and a set of 65's, same result).

Thanks,

Dan

[Steve@Rennsport]

Dan,

Thank you for the very helpful followup information.

Offhand, it sounds like an idle passage is still partially obstructed and this is not uncommon after a cleanout & rebuild. The lead plugs MUST be removed to get these clean as ultrasonic cleaning isn't very effective unless they are gone.

[914obsession]

Steve, thanks for the quick reply! I've blown air through all the passages several times. One of the plugs for the idle circuit was removed, everything cleaned out and then tapped and plugged. Also, the carbs were soaked in a tub of carb cleaner for a week or so before the ultrasonic cleaning. Are there any other possible issues, any way for me to blow air through some passage to determine whether the idle circuit is blocked without removing the carbs from the car? Is there a max number of turns you are allowed to turn the air correction screws before it throws things off?

Dan

[Steve@Rennsport]

Originally Posted by 914obsession

Steve, thanks for the quick reply! I've blown air through all the passages several times. One of the plugs for the idle circuit was removed, everything cleaned out and then tapped and plugged. Also, the carbs were soaked in a tub of carb cleaner for a week or so before the ultrasonic cleaning. Are there any other possible issues, any way for me to blow air through some passage to determine whether the idle circuit is blocked without removing the carbs from the car? Is there a max number of turns you are allowed to turn the air correction screws before it throws things off?

Dan

You could remove the idle jets, progression hole covers, and idle mixture screws to blow air back and forth through those holes.

Air correction screw position isn't real critical; idle mixture screw position is!

Air correction screws are simply there to balance airflow when the throttle blades are closed so you need to make sure each butterfly is closed and all of them are the same at rest. This involves taking each carb off and peering down each throat using a flashlight at the opposite end. The throttles MUST be closed for the air correction and idle mixture screws to respond.

If the throttle blades, shafts, and bushings are worn, you will play hell getting them to properly respond to the idle mixture screws. Same goes for any small vacuum leak between the carbs and the heads.

[914obsession]

Steve, thanks again for the quick reply!! I have blown everything out several times, but will try your method as well.

just to be clear, I can get the car to idle great and run awesome (albeit very rich) simply by backing off the air mixture screws. You are saying that I can have some of them turned out two full turns and some only a quarter or so as long as all the cylinders are pulling the same amount of air? Does my jetting seem way off? I forgot to mention that I am running F3 emulsion tubes as well.

Thanks again,

Dan

[snbush67]

This is a good reference site that I have found to be very helpful.

Tuning the Weber Carbs

[Steve@Rennsport]

Dan,

Yes, they can be out two full turns. Just make sure you don't have any vacuum leaks,....

Your jetting looks very close to me although I'd be using 155 mains and 55 idles.

[tab tanner]

Looking at the picture you have the air bleed screws, and the idle mixture screws in the wrong places. The air bleed screws go in the top hole on the carb body and the mixture screws go on the bottom. Turn them around and I'll bet you will fix your problem

[2.70Racer]

Nice catch Tab. Did a double take of the picture, completely slipped past.

[tab tanner]

I have seen this happen more than once??

[914obsession]

THANKS!!! That's probably it!! I'll mess around with them when I get home tonight. This board is Awesome!!

Dan