Robert Allen writes:
> Matt Liggett wrote:
> > This regards the 1275 in my '70 Midget. I'm concerned about the big
> > "breather" on the front of the timing cover. If I understand
> > correctly, this should be a PCV valve, correct? That means a one-way
> > valve that flows only from the crankcase OUT. I thought mine might be
> > broken, so I pulled the line off that goes to the carbs and sucked on
> > it and blew through it. It was clear both ways. Does this mean my
> > crankcase might be sucking in fuel-air mixture through this valve?
[ Nasty emoticon snipped. ]
> Crankcases don't suck. They blow. Ever heard of blow-by? The crankcase
> is where it goes as it blows-by the rings. Anything that gets passed the
> rings builds in the crankcase and has to go somewhere.
Right so far.
> And that crankcase breather off the timing cover is the normal 'vent'
> for an 'A' series engine. In the emission era I think it went to the
> 'gulp valve' which was a funny looking, pancake-topped thingy that
> screws into the middle of the log manifold (British PCV valve).
[ Note: The following technical information is based upon my knowledge
of MGB engines. I have every reason to assume that the corresponding
model year Midget engines are very similar. ]
Wrong. The pancake-topped thingy is called, correctly, the PCV Valve.
It's vacuum-operated, letting in varying amounts of crankcase gasses
based upon manifold vacuum. Fitted from '68 until '71 or so. You have
to replace the diaphragm every so often in order for it to work
properly.
The gulp valve is the canister-looking thingy (to use the technical
nomenclature) that lets fresh air into the manifold from the air pump
on high vacuum (i.e., decel) conditions. Fitted from '72 on (I
believe). It doesn't serve a PCV function at all, rather, it simply
dilutes the (overly-rich) mixture during high-vacuum conditions. It
also causes nasty backfiring on overrun.
> At least
> I would assume it does. On my midgets it was long gone. The crankcase
> breather on my last midget has a 'Y' connection where little pipes fed
> it in just past the dampner pistons on the SUs. It doesn't hurt anything
> there.
This is the correct description of the PCV system fitted after '71. It
doesn't use an actual valve, just some simple hoses. It also forms part
of the Evaporative Emission Control system, by drawing gasoline vapors
from the charcoal canister, through the crankcase, into the manifold to
be burnt again.
Full, explicit, and graphic details can be found at the SOL Web Site:
ftp://ftp.team.net/sol/PCV.txt
(Roger, why isn't this file included in the Tech pages? Perhaps because
it's rather unstructured and rambling?)
> > How do I replace it, and should there be some kind of foam "breather
> > filter" element as in modern cars? I considered just buying a generic
> > import PCV valve and putting it inline in the hose that runs from the
> > timing cover to the carbs. Would this work?
The breather incorporates a flame trap, but it's not a valve, simply a
tangle of steel mesh that somehow guarantees that flames won't travel
down into the crankcase and blow your motor up. Just trust it (and
clean it next time you remove it).
> The PCV valve needs manifold pressure on one side of it. So you would
> have to plumb the other side back into the log manifold.
And I don't think it's quite that simple, either. You'd have to make
sure that you got one with the right sized orifice, the right "rate" of
opening, and the right vacuum "polarity" (i.e., high vacuum opens it).
Then you'd have to cobble up connections, and fine someplace to mount it
securely.
Again, lots of this is hashed out (by several of our esteemed SOL
forebears, may they rest in cyber-peace) in the URL above.
> Bob Allen, Kansas City, '69CGT, '75TR6
>
> PS: Some of the technical wizards will respond that if you had the
> super-deluxe, fancy-smanshy, gee-golly piston rings, then you could, in
> fact, see vacuum in the crancase when decelerating.
I doubt it. TotalSeal rings (that was what you were referring to,
wasn't it?) greatly reduce blow-by, but they don't eliminate it. And
they most certainly CANNOT reverse it. Can you think of any possible
scenario whereby combustion chamber activity could draw a vacuum on the
crankcase?
> To them I can only respond with ;~P*****************
I would expect nothing less of you, Bob. ;)
(BTW, it's "Know", "past", "damper", "shmansy", and "crankcase".)
--
Todd Mullins
Todd.Mullins@nrlssc.navy.mil On the lovely Mississippi (USA) Coast
'74 MGB Tourer with a nice and simple PCV system
That's me at the corner.
That's me at the stoplight,
Losing my transmission...
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