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Discussion Starter #1
Hi, I'm new to dual carburettor synchronizing.
Following many instructions on the forums, the web, and in weber carb books, I'm a little confused.
The reason for the confusion, is that I have 2 sets of carbs(4 total), on 2 different cars, and all of my carbs do not have the 'Air Bleed adjustment and lock nut'.

To tune 'my carbs', to me, the only external adjustment are mechanical linkages, and the 'Idle Air Mixture' screw. No 'Air Bleed adjustment'.

So, is this the way the came from factory, without the lock nut, and just a cap screw on the air bleeds?
Or have then been rebuilt, and wrong parts put in?

Or do I just tune them, only adjusting the idle air screw?
 

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Which version of Weber carbs do you have? There will be numbers stamped on the top cover of the carbs e.g. 40 DCOE 32. The earlier carbs do not have the air bypass screws. I believe the air bypass screws were introduced on later versions to help with CO emissions at idle.
 

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I think that most 40DCOE's from 40DCOE72 on and maybe some earlier had the idle air bypass screws. They were intended to fine tune the carbs for emissions compliance. I suggest that you set them fully closed.
 

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I think that most 40DCOE's from 40DCOE72 on and maybe some earlier had the idle air bypass screws. They were intended to fine tune the carbs for emissions compliance. I suggest that you set them fully closed.
My 44 / 45 have them. Possibly the lowest number car to have them?

I'm struggling with the generally accepted logic of closing them, rather than opening them (less as possible having sync'd the 2 carbs against each other and set the idle speed) to obtain the best even vacuum across all 4 barrels.

Clearly closing them means that the carbs work like a classic Weber.
 

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But Mad North-Northwest
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I'm struggling with the generally accepted logic of closing them, rather than opening them (less as possible having sync'd the 2 carbs against each other and set the idle speed) to obtain the best even vacuum across all 4 barrels.

Clearly closing them means that the carbs work like a classic Weber.
Correct, if you close them the carb basically works like a classic Weber. If you have them you should use them, though: they are a good thing and help you get a more even idle and lower idle emissions. Makes the process slightly more complicated but it still ain't rocket science, and there's no real disadvantage. I suspect folks at the time saw them as just an "emissions" thing and rejected using them out-of-hand rather than learning how to use them properly as a way to better tune the carbs.

Dellortos have them and the easiest way to balance them is with a manometer. You match the lowest vacuum throat on one carb to the lowest throat on the other carb using the balance screw. Then you bring down the high throat on each carb using the bypass screw to match the low throat on the same carb. When you're done only one bypass should be open on each carb and all throats should show the same vacuum. You can fairly easily get them all to within 10 mmHg vacuum.

I use a Carbtune Pro 4 on the Dellortos and it's an awesome tool. If the emissions Webers have the vacuum takeoffs that's how I'd recommend doing it, otherwise you can likely do it using a standard carb synchrometer. In that case (and assuming I'm thinking about it correctly) you'd want to first match the high flow throat on each carb using the balance screw. Then you'd open the idle bypass on the low flow throat on each carb to bring it up to match the high flow throat.

 

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Correct, if you close them the carb basically works like a classic Weber. If you have them you should use them, though: they are a good thing and help you get a more even idle and lower idle emissions. Makes the process slightly more complicated but it still ain't rocket science, and there's no real disadvantage. I suspect folks at the time saw them as just an "emissions" thing and rejected using them out-of-hand rather than learning how to use them properly as a way to better tune the carbs.

Dellortos have them and the easiest way to balance them is with a manometer. You match the lowest vacuum throat on one carb to the lowest throat on the other carb using the balance screw. Then you bring down the high throat on each carb using the bypass screw to match the low throat on the same carb. When you're done only one bypass should be open on each carb and all throats should show the same vacuum. You can fairly easily get them all to within 10 mmHg vacuum.

I use a Carbtune Pro 4 on the Dellortos and it's an awesome tool. If the emissions Webers have the vacuum takeoffs that's how I'd recommend doing it, otherwise you can likely do it using a standard carb synchrometer. In that case (and assuming I'm thinking about it correctly) you'd want to first match the high flow throat on each carb using the balance screw. Then you'd open the idle bypass on the low flow throat on each carb to bring it up to match the high flow throat.

Thanks, the process that you described is exactly what I have done on mine on the basis of the more even I can get the vacuum (airflow) the better the idle will be. I can see no downsides if done correctly and providing the screw don't move after adjustment and locking. I have notice a "chuff chuff" noise if the bypasses are opened a lot.
 

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But Mad North-Northwest
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Actually the same seller has it cheaper on eBay if you want to buy the manometers:

 

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Discussion Starter #10
Which version of Weber carbs do you have? There will be numbers stamped on the top cover of the carbs e.g. 40 DCOE 32. The earlier carbs do not have the air bypass screws. I believe the air bypass screws were introduced on later versions to help with CO emissions at idle.
The numbers are 40DCOE32, as in your example. All these posts, and I'm still confused.
OK, so, early carbs, no air bypass screw.

Am I correct in thinking that all I need to do now, to tune the carbs is adjust just the 'Idle mixture screws'?
 

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Correct. 40DCOE32 will not have idle bypass screws.
The main adjustment on your carbs are the idle mixture screws (1 screw per barrel).
You will also need to ensure the 2 carbs are synchronised to each other i.e. the throttle plates on both carbs open in the same way. You also have a idle speed screw (throttle stop). This guide may help http://www.webercarburetors.com/ppw/html/tech/dcoe_adjustment_layout_typical_i.htm
 

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WWhite (Walter?)

I know some consider it heresy, and me a heretic, but I've been tuning Webers since the early 70s, and stopped using flow meters not long after I started. I actually have a nice four-tube standing manometer that is a very precise instrument, but have developed my own methods that, as far as I've observed, are faster and deliver more consistent results than synchrometers.

It's too late here on Xmas Eve to launch into it, but if you want, send me a PM with your email and I'll send off guidance and pictures, and a video or two.
 

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Discussion Starter #13
Correct. 40DCOE32 will not have idle bypass screws.
The main adjustment on your carbs are the idle mixture screws (1 screw per barrel).
You will also need to ensure the 2 carbs are synchronised to each other i.e. the throttle plates on both carbs open in the same way. You also have a idle speed screw (throttle stop). This guide may help http://www.webercarburetors.com/ppw/html/tech/dcoe_adjustment_layout_typical_i.htm
Thanks, so since there are no idle bypass screws, I just skip that step whenever it is mentioned in the guides.
Appreciated, thank you.
 
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