weber idle air correction screws

[356robo]

I've followed the process to adjust webers outlined in numerous threads. Here's my background and question:

floats ok, accel pumps ok, linkage disconnected, idles at about 900.

Using a Redline synchrometer and adjusting the idle air correction screws, all barrels were adjusted to flow showing "4" on the synchrometer EXCEPT #2. It shows about 4.75 with the screw turned all the way in.

Is there something wrong with #2 or should I adjust all the other barrels to match #2? The others are turned out several "1/8" turns.

Thanks,
Rob

[1QuickS]

It seems like the throttle butterflies are not sync'd but difficult to be realized as 1 & 2 as well as 4 & 5 are on a common shaft, typically #3 is out of whack with 1 & 2 and # 6 is similarly out of whack with 4 & 5.

Pull the carb off, adjust throttle stop screw to be 1 turn from closed (idle position) and look for equivalent light rings around the two butterfly valves in question. They should be pretty close to the same, try it again with butterfly valves fully closed.

Air screws make up for throttle plate timing variations but not larger issues. Also, if the butterflies are not perfectly adjusted in their bores and to each other (throttle shaft twist) your air screw adjustment will only be correct for one idle speed as any change in rotation of the throttle shaft will produce unequal air flows.

I think I'd get them all close as you can and as long as there isn't any spitting and the car runs well then call it close enough.

Paul Abbott
Performance Oriented

[356robo]

Thanks Paul,

It's a 911. It seems to run great. My only complaints are inconsistant idle speed and slight minimal backfiring (more like a burble) through exhaust on decel.

Are readings of "4" and "4.75" on the Redline synchrometer really significant?

Checking and correcting the butterflies would be more than I'm willing to do at this time.

Rob

[Shoepop]

Originally Posted by 356robo

Thanks Paul,


Checking and correcting the butterflies would be more than I'm willing to do at this time.

Rob

Then just take them all to 4.75. Are you pulling linkage off and setting each bank independently? If not do so.

[T77911S]

here is a clip from the site below.

According to the Weber operation manual, proper throttle valve position at idle should block the first progression hole with the bottom edge of the throttle plate aligned to be level with the bottom of the first progression hole. On a new 40mm Weber this would correspond to an idle stop screw adjustment of about 1/2 turn from throttle plates completely closed upon the ID of the 40mm bores in the throttle bodies. This is a physical setting that is not to be adjusted.

Tuning the Weber Carbs

this opened my eyes to carbs. i wish i had read this when i had webers.

[azasadny]

Great info here! Thanks!

[1QuickS]

The info T77911S offered from my site is a topic dear to me and as I mentioned assumes new condition carbs. However the throttle plates do wear along their edges due to axial movement of the throttle shaft and this wear provides air to the engine regardless of how closed the butterflies are. So, you may get plenty of air to idle the engine with both the butterflies and air correctors closed, especially if you have a 2.0 engine. Larger engines demand more air so this clearance works in your favor; however this edge wear goes hand-in-hand with throttle shaft/bushing wear which tends to be an erratic air supply causing sniffing at idle and tuning variations.

Larger engines demand more idle air so it is typical to use the throttle stop screws to adjust idle speed. This in turn may expose the first progression hole which is a problem to be aware of. Once the first progression hole is exposed you will supply fuel from the idle mixture and the first progression hole. To control the richness at idle you would be tempted to install a smaller idle jet so your mixture screws are the typical 1 1/2 to 2 1/2 turns out. Now you have a small idle jet so you may experience a lean progression and transition to the mains.

The idle air screw hole is too small to allow enough air during idle to prevent this situation (assuming new Webers as mentioned above) so a fix is to drill a small hole into the throttle plates to allow more idle air without adjusting the butterflies open. Recommended hole size to start is 1mm and may be increased; location is diametrically opposed to the side where the progression holes are located and about 2 to 3 mm from the edge. You may solder the hole shut so don't worry about permanently damaging your carbs.

Best check for all of this is to get your engine to the idle speed you like with the air screws as closed as possible and then take the carbs off and have a look to see if the first progression hole is exposed. You may then plan a course of action. Better to have the throttle plates below the first progression hole than to have it exposed at idle.

Paul Abbott
Performance Oriented

[T77911S]

paul, that is your site? awesome. i love reading about the webers. got more info on how they work? i would love to read anything you have on the webers.

i wish i had this info back when i had webers! as i read your article, i see conditions that i created, like setting the idle with the idle stop screws, then adjusting the mixture and nothing happening, or opening the air corrections screws to the point of the noise they make. that was my first dealings with webers and even though i learned a lot with trial and error, mostly error, reading your post really brought things together.
reading about the rest of the carb, i realize how much carbs can be as sensetive to air leaks as much as CIS, and how much more complicated and definately how much harder to tune they are than CIS.

[1QuickS]

Hi T77911S,

Send an email and I'll provide my list of Weber reference info for late night reading.

Cheers,

Paul Abbott
info@PerformanceOriented.com

[2.70Racer]

Paul Abbot,
I just read the the info presented at your site regarding the operation, machining and tuning of Weber carburetors.
I found the information presented to be the most thorough and well written of any I have read regarding Weber carburetors.
I was particularly surprised to find you are boring 40mm carbs to suit various engine sizes and configurations.
I spent some time configuring my 40IDA-3C Webers on a 2.7 with 9.5:1 JE's and Solex cams.
Learning about Weber's was a real journey for me, resulting a faster race car at the high altitudes of Albuquerque.
I read everything I could get my hands on, gleaning a little bit from each writer.
Your information presented at your site, has all the key information, presented in a well thought out manner and exceedingly well written.
What you have presented demonstrates your intimate knowledge of the subject.
I'm pleased you are in the business and willing to share your knowledge.
Thanks,

[1QuickS]

Hi Doug,

I sent a PM to express my thanks for your comments.

Cheers,

Paul Abbott