>> 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.
> Umm, Scott, it sounds to me like you now have the crankcase vent hooked to
> the carb vents. The crankcase vent connections on the carbs are 3/8". The
> carb vents (floatbowl) are 1/4".
Heh. No, I can tell the difference between a float bowl and a throttle
body. These are new HS4s; I don't know whether they're genuine SUs or
Hitachi licensed replicas. They have, for instance, the early-style
fixed needles, plus recessed throttle stop screws that I've never seen
before on a set of old carbs. They seem to work really well, but as I
bought them from a friend who got them in a parts car (you know how that
goes), I don't know much about their provenance.
I have the vapor recovery line to the charcoal canister hooked up to the
float bowl vents, as original.
> As I told Marcus just before my internet gate took a four day vacation, there>
> are two PCV-ish systems used by MG's from 66 - 80. In the pre-charcoal can
> days, the engine breather was hooked to the intake via a rather rude-n-crude
> "pcv" valve. These things had replacable diaphrams, and all kinds of weird
> things like that.
Oh, right. I've seen those, both in person and in the manual, but I've
never worked with one.
> When they went to charcoal cans (68, I think), the system
> changed drasticly. There was no longer a pcv valve in the system. Rather they
> used a metered vacuum leak approach.
There's also the gulp valve on these later ones, but I can't now remember
where or how it fits into the picture. I think it's solely meant to dilute
the intake charge when you let off the gas, isn't it? It's connected to
the air pump, as is the air injection rail that goes to the ports via the
plug side of the head.
> The charcoal canister had two intake ports. One of these went to the
> fuel tank vents, the other went to the two carb vents (only one for ZS cars).
So far, so good. That's all in place.
> The ouput port of the can went to the valve cover, with the metering orfice
> in the V/C connector.
Got that, though I had no idea it was an output port; I'd always assumed
that it was drawing excess pressure off the head via that elbow at the
back of the valve cover.
> Fumes then passed through the engine to the breather
> on the side plate (B) or timing chain cover (Midget). This breather contained
> a wire mesh oil seperator/flame arrestor.
Right; I remember cleaning that wire mesh in the parts washer when I had
the engine apart.
> From here, the fumes went to the
> carbs. The connector is 3/8", and enters the carb bore between the piston and
> butterfly.
Negative on the 3/8, at least on the carbs I've got. I'll have to measure
the others to make sure, but I measured the brass tubes at 1/4", I bought
1/4" vapor line, and it's on there tight as you could want it.
> It's really ingenious. They had to run a purge/scavange line from the carbs
> to the charcoal canister, so they passed the flow through the engine, picking
> up the crankcase fumes in the process. Note that the change required a
> change to the valve cover (restictor in the tube connector). This has caused
> some grief for people with "multi-year" cars, and/or aftermarket valve
> covers.
Interesting... Note also that there are three oil filler caps: a very
early one all steel, a vented plastic one with wire mesh inside and a
pair of breather holes just below the cap itself, and a sealed one.
Obviously the sealed one is required for this setup. Interesting indeed.
I'll have to try swapping the valve covers. I'm using the '72 cover on
the '71's canister/etc. setup; I wonder if the size of the orifice would
make a difference there.
> The ascii-art Marcus originally posted showed this later system. Do you
> know what type of carbs you car has?
As I said, they're modern HS4s (1-1/2" bores, line up with MGB intake
manifold bolt pattern and air cleaner bolts). But I don't know who made
them -- or, for that matter, what needles they have. That's what the
Vericom is for, helping determine whether the car has any flat spots
in the power curve. (Doesn't feel like it, but it'll be nice to measure
it all more carefully later.)
Thanks for the explanation. I'll have to try replacing the PCV valve
with a straight 1/2-3/8 reducer, if you say there's a flame trap in
the cam cover; that was the main reason I went with the PCV, to keep
the car from exploding if I ever did backfire. :-)
--Scott
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