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Re: Anti run-on valve circuit

To: Bud Krueger <>,
Subject: Re: Anti run-on valve circuit
From: Barney Gaylord <>
Date: Sun, 01 Apr 2001 19:44:33 -0500
At 01:09 PM 4/1/2001 -0400, Bud Krueger wrote:
>.... regarding the source of manifold vacuum.  There are only three hoses
feeding into the charcoal cannister which, in turn,  feeds the anti run-on
valve, viz., one from the fuel tank vapor separator, one from the
carburetor's float bowl vent and one from the valve cover.  How can
manifold vacuum come into the system function? ....

Ah yes, well I may have mispoke with somewhate confusing utterance here.
The vacuum gets where it's going by a more circuitous route.  There is a
pipe/hose routed from the front tappet cover to the intake manifold just
inboard of the carbutor.  This applies a vacuum to the crankcase, which is
sealed.  There is another hose from the top of the rocker cover to the
runninng-on control valve that supplies the vacuum to that valve from the

When the engine is running the control valve is de-energized, and I believe
all ports are common and open.  The tube running from the control valve to
the float chamber is small bore and has virtuly no air flow in either
direction, except to allow for minor variations in fuel level in the float
chamber.  The other tubes on the valve are larger, allowing (a small amount
of) air to flow through freely starting from the open end of the vent pipe.

Air flows in through the vent pipe, through the valve, through the
running-on control hose, through the carbon canister, through the purge
line to the valve cover, through the crankcase, then through the breather
pipe on the tappet cover and into the intake manifold.  The flow rate is
fairly small and limited by a restrictor orifice.  At least some of these
systems had the restrictor in the end of the vent pipe on the valve cover.
This small amount of air flow serves to purge previously collected fuel and
vapors from the carbon canister, while at the same time carrying away
vapors from the fuel tank.

When the ignition is turned to the OFF position the valve is energized,
which closes the open inlet vent pipe while leaving the other two pipes
ported together.  With the inlet vent closed the crankcase vacuum is
immediately applied to the small hose going to the float chamber(s),
effectively killing the engine.  When the engine stops the oil pressure
switch breaks the ground connection and de-energized the valve, at which
time all hoses are again vented to atmosphere.

With the engine off any vapors from the fuel tank are free to pass through
the carbon canister on the way to escaping via the valve and the open vent
pipe.  In the process much of the fuel contained in those vapors is caught
in the carbon canister.  Then the next time the engine is started the
venting of the cannister through the crankcase into the intake manifold
recovers the fuel from the canister to be burned in the combustion process.

There are a couple of strange quirks to this system.  If the float valve in
the carburetor should fail in the open position while the fuel pump is
running (whether the engine is running or not), then liquid fuel can be
vented from the float chamber into the carbon canister.  While your engine
may be running too rich, it may not be enough to cause you to stop driving,
in which case the canister could fill with fuel to the point of overflowing
liquid fuel into the crankcase, which will severely dilute the engine oil,
which can result in failure of the crankshaft bearings, camshaft and
tappets, etc.  In other words, your engine is programmed to self destruct
in the event of a float valve failure, rather than spilling fuel into the
open environment.

Just a thought,

Barney Gaylord
1958 MGA with an attitude

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