I don't mean to Flame anyone but....
in response to the recent posting on PCV valves
Scott Fisher writes...
>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...
Well kid of Scott the major function of the PCV valve on an engine
is to act as a variable orifice that decreases its size as the
vacuum level increases on the manifold side.
Fortunately for me when I put a Toyota PCV valve on my '68 MGB
my father had taught an automotive design course in pollution
control devices (my dad was a big fan of PCV systems, claimed
they help engines and cleaned up the air). Anyway the old engines
(MGBs to early '64) had a "road draft tube" system where by air
passing under the car drew the gasses out the tube causing a
suction on the engine and drawing clean air in through the valve
cover from a tube to the air filter.
The PCV system draws the up in to the intake system and
burns them in the combustion process. This causes one problem at
an idle you don't want allot of oily gasses drawn in your intake
causing a rough idle. The solution follows two courses of
action.
The first solution(MGB's '64-'69) is to put a variable
orifice between the crankcase and the intake manifold so that
when the car is idling (HIGH MANIFOLD VACUUM) the valve is drawn
shut limiting the amount of pollutants in the combustion chamber.
At high speeds (LOW MANIFOLD VACUUM) the valve opens allowing
more gasses to enter the manifold where a larger amount of gasses
need to be removed. The size of the orifice and spring etc. need
to be sized to the engine so that you don't end up with a big
vacuum leak that causes your engine to idle rough.
The second method (MGB's '70 on) is to put a port into a
venturi in the carborator (not under the throtle pates!) and
attach this to the crank case then at idle there is low air flow
and (from Bernulli's priciple) the presure drop on the port will
be less that at full throttle where the drop is geater drawing in
more air from the crankcase.
>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.
OOP's now you've conected the PCV from system 1 to a carb in
system 2 (see above). So now when your a idle the PCV is open and
the carb is drowing a small amount of crankcase fumes, but when
the throttle is opened the carb increases its "suction" but the
PCV thought the throttle was set back to idle and closes its
orifice allowing less fumes through.
If you want to use a PCV with the later MGB you must connect
the "suction" side of the PCV to the intake manifold and block
off the crankcase ports of the carbs. As far a s I can tell
Marcus' PCV system was correct.
John "just call me Mr. PCV" Ross
P.S. Newer Volvos use a "fixed" orifice to the intake maifold and
a line to the air intake to the turbo with a "fire trap" (small screen)
in the crankcase line.
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