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Re: Breather tube

To: "Ray Wygonik" <>, "mgs" <>
Subject: Re: Breather tube
From: "Paul Hunt" <>
Date: Mon, 14 Aug 2006 16:53:25 +0100
A breather port will exhibit a constant low level of suction regardless of
throttle position, on an SU at least, don't know about Weber.  A carb vacuum
port exhibits negligible vacuum at idle, rapidly increasing as the throttle
is opened to about 1/4 or 1/3rd, then gradually reducing as the throttle is
opened further.

If you don't connect a breather port to the crankcase then you can get
condensation and hence corrosion in the crankcase.  You *don't* get a
pressure buildup by not connecting the carb breather, as the crankcase
breathing has two ports - one from carb or PCV suction and the other to let
fresh air in.  Just by not connecting the carb breather you now have two
fresh-air ports, but no through flow to scavenge fumes and condensation.
Even if you sealed the carb port on the engine you still have the fresh air
port, unless you seal that as well.  A PCV valve on the inlet manifold
whilst possibly being the only alternative to no breathing of the carb
doesn't have a port, is a much less satisfactory solution than using a carb

By not connecting some vacuum, either carb or inlet manifold, to the
distributor vacuum capsule, you sacrifice a significant amount of
part-throttle acceleration performance and cruising economy.


----- Original Message ----- 

> I have a Weber 32/36 DGV downdraft carb.  (too lazy to mess with the SUs)
> Not aware of any vent fittings on it for the bowls.  There is one fitting
> the base of the carb that (I hope) is vacum. I plan to attach it to my
> advance on the dizzy.  I was drawing off the intake for the vacum but
> that port to the anti-run on valve.
> Is there a vent on the Weber that should be connected to the carbon
> canister?
> Is the port at the base of the weber on the engine side a vacum port?
> What benefits will I get from venting the crankcase?

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