[TR] Carb Vent Tubes (was: Non TR3 Question)

[Alan Myers]

 Hi Alex,

I believe you are looking at the fuel/float bowl vent tubes on those SU carbs.

This vent is only there to allow air pressure to equalize in the bowls, to
prevent a vacuum from forming (that would stop proper fuel flow).

Plugging the vent tube with your finger causes that vacuum and prevents fuel
from flowing out of the bowl into the venturi of the carb, causing the car to
stall.

SU are passive carbs... They don't have any sort of internal accelerator pump
and simply use the engine's own vacuum to draw fuel through the carb (metered
by the carb), through the intake manifold, and on into the combustion chamber.
As you might imagine, this isn't a particularly strong flow, so any sort of
counter vacuum in the fuel bowl can stop the flow of fuel pretty quickly.

The reason a vacuum would form in the fuel bowl is that as the fuel is drawn
out and the level drops, air needs to be let in to replace it.

So there's really little or no need to filter the air flow into the fuel bowl
through that vent. Most older carb designs have some sort of vent for their
fuel/float bowls, that's simply open to the atmosphere (modern carbs might be
better sealed or filtered, but that's more to prevent hydrocarbon emissions
from fuel evaporation than to avoid particulates entering the bowl). The air
in the fuel bowl is not directly any significant part of the fuel/air mix
that's eventually finding it's way into the engine.

Note: If there is a problem with the needle valve in the float/fuel bowl not
doing it's job - due to wear or incorrect fit or damage to the needle or it's
seat - or if the fuel pressure from the fuel pump exceeds the sealing
abilities of the needle valve, then yes, that float/fuel bowl vent tube might
become an overflow tube, with excess fuel forced out of it. Not a good thing,
so that's why it's important to not over-pressurize the fuel system and to
insure the needle valves are working properly.



Alan Myers
San Jose, California
amfoto1 at aol.com
'62 TR4 CT17602L

  http://www.triumphowners.com/640


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

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Message: 14
Date: Mon, 24 May 2010 10:04:56 -0400
From: "Alex" <ambritts at bellsouth.net>
Subject: [TR] Non TR3 question
To: "*Triumphs List" <triumphs at autox.team.net>
Message-ID: <3B10C026DF4D4BAE89DB1290500B026A at AlexPC>
Content-Type: text/plain; charset="us-ascii"

Hello all,
Not a TR3 question but a TR6 question.

Moss Motors sells an upgrade SU carb setup. My friend just bought a 1970 TR6
with this SU setup. The PO had to install a pressure regulator off the new
mechanical pump from Moss,because it was pumping close to 7 lbs. This caused
gas to pour out of two brass tubes coming out of the top of fuel bowls. Once
installed the problem no longer existed.

These tubes are the kind that would have some kind of tubing attached to it.
It reminds me of the vent tube on a TR3 that comes from the fuel bowl to the
airfilter.

My concern was the direction these tubes point. Right to the exhaust
manifold.
Then I put my fingers over the tube openings and the car stalled within 10
seconds. So they are definitely sucking air. I was told that nothing needed
to
go over these tubes and Moss did not supply directions. They (Moss) also
indicated that they never heard of the gas problem coming from these tubes.

At the very least I would want to filter these tubes as they are pulling air
in. The original carbon canister was taken off. I am considering reinstalling
the canister (if we can find it) and venting these two tubes to the canister.

Anyone have any experience or thoughts.

Alex Manzo
72 TR6
59 TR3A


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