I wrote this up for Marcus, whose PCV setup appears to have gone missing.
This is what I did recently to replace the NLA "flame trap" that hooks
the 1/2" i.d. tube coming off the cam cover on the B to the 1/4" brass
tubes on the SU carbs. Total cost: a couple of bucks, it was lost in
the price of a case of Castrol 5W-50 Syntec (egad!).
Theory of Operation:
The Positive Crankcase Ventilation (PCV) system is meant to remove
overpressures from blow-by past the rings and other causes (such as
piston downstrokes, heat expansion, etc.) from the engine's crankcase.
In the Olden Dayes, this pressure was vented to the air on the theory
that gases being heavier than air would condense, fall to the ground,
and become one with the pavement. That, of course, turns out not to
be the case; the unburned hydrocarbons that result from air-venting of
crankcase fumes are a particularly stinging component of photochemical
smog.
In addition, especially if you're running a high-compression engine,
you want to make sure that you apply some kind of suction to the
crankcase so that the internal pressure is just below atmospheric
rather than just above. If you don't run this suction, the pressure
can force first gases, then later oil itself, past seals and gaskets.
The dilution of the intake charge is negligible, and as a side benefit
you also burn up the vapors in the moto. Also, if you don't have some
kind of suction system that either burns or at least filters the vapors,
your car will smoke noticeably under the hood, and oil (or at least
aporized oil) will be forced out of various orifices to condense on
the underhood environment, where it not only forms an unsightly film
but can even be a fire hazard. In short: install a PCV valve, it's a
big win.
You could run tubing from the crankcase breather directly to the intake
manifold, but there's a risk with that. If you get a backfire through
the carbs, the fire can ignite the oil vapors in the crankcase. The
effect of this is left as an exercise to the reader's imagination. To
this end, many PCV valve applications are known as "flame traps" (well
known to veteran netters :-). The purpose of the PCV valve as flame trap
is to serve as a one-way check valve, that permits gases to flow in
only one direction. Suction from the carburetors will draw the vapors
through the PCV valve into the engine, but if the pressure reverses
itself, as in a backfire, the valve shuts before the flame front reaches
it because the pressure wave precedes the flame front in the tubing.
Here's a laundry list of parts needed to install a working PCV setup on
most dual-carb M.G.s that have SU carbs with the standard brass tubes for
the breather:
About 4" - 6" of 1/2" internal diameter (i.d.) rubber hose
Almost *any* PCV valve with a 1/2" inlet and a 3/8" outlet (I used one
from a Toyota truck, figuring it was close to my 1800cc engine in
size; all this has to do is act as a check valve to keep flames
from a backfire out of the crankcase)
1 foot of 3/8" i.d. hose (of which I've got about 6" left)
1 tee fitting with two 3/8" barbs in line and a 1/4" at right angles
2 feet of 1/4" i.d. hose (of which I've got about a food left)
2 reducing fittings, 3/8" to 1/4" (of which I've got one left, but they
came in packages of 2)
A bunch of hose clamps in two sizes, one for the 1/4" i.d. tubing and
one size for the 1/2" and 3/8" i.d. stuff
The rest should be predictable: the 1/2" i.d. hose goes over the breather,
sticks up about 4", and has the 1/2" end (inlet) of the PCV valve (which,
by the way, stands for Positive Crankcase Ventilation valve, not Pollution
Control Valve valve as some people seem to think :-) stuffed into the upper
end of that hose, held on by a clamp. Then there's about a 4" section of
3/8" i.d. hose clamped onto the outlet end of the PCVv, with the tee in
the downstream end of *that*. The 1/4" part of the tee goes to about a
4" section of 1/4" i.d. hose that is shoved down and clamped onto the
front carb. The 3/8" part of the tee goes into about 8"-10" (less for the
1275, as the distance between the carbs is less than on the 1798) of the
3/8" i.d. hose. At the aft end of that hose is the 3/8-to-1/4 reducer, at
which point another 4" section of 1/4" i.d. hose goes to the aft carb's
breather tube.
My setup actually looks quite respectable now, with new black tubing and
shiny silver hose clamps. In addition, the car no longer smokes while
operating, and performance has not been noticeably affected. If I had it
to do over again, I might have tried to locate *black* 1/2" i.d. tubing
instead of the red stuff I have, but it's not too objectionable, and for
only 4" the stuff can't cost that much.
The long-term problem of not suctioning the crankcase vapors is described
above, but the short-term problem is that people stopped in traffic next
to you tend to shout, "Hey, mister, your car is on fire!" Also you will
get a sore throat from breathing oil fumes, and that will turn into a
chest cold, and if you are enough of a damn fool to drive around with the
top down in -7F weather, you're likely to get pneumonia. (Me, I just
have a bottle-a-day Contac Liquid habit... :-)
--Scott Fisher
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