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additional needle screw on weber 45 dcoe 152?

ol doc gully

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Doing a slow restore on a 72 240 i picked up. Weber 45s were having some trouble sneezing and leaking so took them off to clean em up and replace gaskets, set floats etc. Noticed these adjustment screws which i find no mention of in Haynes manual, or any online reference ive found. These are the later 152s with nitrophyl/spansil floats so im thinking it was a modification on later models?



has a white plastic hat/cover. one on each throat.



with plastic cover removed



needle screw with fastener nut



needle pokes through main venturu/choke into carb throat (upper right)


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I believe thats the air-bypass screw...comes on 151's and 152's...should normally be closed and should be final adjustment for synchronization.  Its on both barrels, correct?  


You can see it in the exploded view on piercemanifolds website.  http://www.piercemanifolds.com/category_s/305.htm  should be #25-27, as well as here: http://www.carburetion.com/Weber/Tech/dcoe_adjustment_layout_typical_i.htm

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Ah, thank you very much gents - yes, on both barrels.


So this screw is an additional air inlet that bypasses the throttle plate? Doing some more reading I've seen two different explanations of intended use: One, to use it as a last step for balancing, or two that it could/should be used so that throttle plate stops can be set to where the plates are just covering the first progression holes; then the idle mixture and air bypass screws can be used to tweak mixture and rpm?


I still have a way to go before these are put back on the car or tuned but I'm trying to use the cleaning process as a time to familiarize myself and learn as much as possible about the carbs.

Edited by ol doc gully
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The air bypass screw and throttle position screw (not the technical term...can't remember) should be two different adjustments.


Lining up the throttle plate with the first progression hole is a carb off the engine setting, done on your workbench. Match them up all the same.


Set idle mixture screw at 3 or 3.5 turns from fully seated (I believe the 152's have a finer idle mixture screw vs older dcoe's)...exact number of turns for "normal" settings should be available on the internets...I have the older model so I can't say for sure. Air bypass should be closed.


Try to fire her up...you may find that your RPM's are really high or low. Use best lean idle method to try and get mixture right...or possibly back the throttle plate off a little if the RPMs are REALLY high. Use your sychrometer to match the highest flowing barrel to the lowest. Last adjustment should be air bypass. You may find yourself fiddling w everything over and over again initially.


This is where a $150 investment in a wideband O2 sensor really pays off. Tuning DCOE's aren't bad once you get the hang of them. Disclaimer: I'm sure someone else out there has a more thorough process...I'm just going off basic memory.

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