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MG MGB Technical - Need help on a hose routing

Hi
BREATHER PIPE
I am in the final stage of a total engine rebuild on a lady friend's 1975 MGB which she bought new.

The engine had been over heated by a friend of hers and was in bad shape; soft rings, a ground up cam lobes and lifters, bad valves, and rocker shaft, etc. About all that was okay was the crankshaft.

I tore the engine down last August, and am just now finishing the job. I am at the stage of hooking up some hoses, and firing it up.

Here is the problem. Some years ago she had the Emission Control System removed, air pump and all relevant hoses.

When I got the car the Breather zpipe on the Front Tappet Cover at the front of the engine was opened to atmosphere; that is, it had no hose attached to it.

I know that this is part of the Positive Crankcase Ventilation system, and should connect to the intake manifold somewhere but I am not sure where it connects.

I only have a Haynes manual and the internet for information on the vehicle, and the Haynes manual is lacking in many areas.

In addition to this Breather Pipe being opened to atmosphere, their is an intake manifold port(3/8 dia stub) just in front of the EGR valve (towards the front of the engine) that had been blocked off using a 3 inch piece of hose with a bolt stuffed in it.

And approx 2 inches in front of the manifold port (again, towards the front of the engine), their is another intake manifold stub, with a restricted orfice and approx 3/16 diameter. I do not know what connects to either of these ports, and can't get a clear answer either off the internet or the Haynes manual.

My question is, "should the opened breather pipe connect there, and if not, where?"

IDLE AIR REGULATOR
In addition to the Breather Pipe problem I have noticed that the "Idle Air Regulator" on the right side of the Zenith Carburetor(towards the firewall) has been removed and a plate and gasket installed to block off the two ports.

My friend kepted the emission control parts that the mechanic removed, but we could not find the Regulator. I asked her if the car idled okay and she first said, "yes", but then said, "it has been so long...."

The car sat for 9 years in garage before she finally (bugged me for 4 years) talked me into rebuilding the engine. Can anyone speak to this missing Regulator. The mechanic must have thought that it was part of the polution control system which in a sense it is, but it didn't seem necessary to remove it, it fact it seems detrimental to a proper idling engine.

BETTER PICTURES AND DIAGRAMS
Is there some web sites with better pictures of the engine area, especially the hose routing, and is there hose routing diagrams as many manuals provide?

FUEL TANK/PUMP CONCERNS
Would anyone venture to guess if the fuel pump will still work and/or what I will need to do to ensure a smooth start up. I see that there is a drain on the tank, and I think I should drain any fuel that may be still in it, and perhaps purge the fuel line upto the carburator.

After the car sat for 10 years, I am not optimistic that I will get a smooth start-up, and will not fall off my chair if I need to do some carburetor work, etc. I know that the engine is together properly and ready to run, and run well, provided I do not have some fuel problems. The ignition has been rebuild. It's the carburetion that concerns me.

Thanks---Layne Wilson
Layne Wilson

Layne,
Most of those missing items that you mention can simply be left off. The breather on the valve cover can be vented to atmosphere or into the intake manifold or its own separate air filter. This website has lots of pictures in a different section and i know of plenty of MGs up in your neck of the woods. Another thing to consider is taking the original carb and manifold and replacing the single ZS carb with dual S.U.s if its not that way already.

Hope that gives you a good start..

-BMC.
BMC Brian McCullough

Do not leave any vents open to atmosphere. If everything else is connected properly this will be sucking unfiltered air into the crankcase or engine.

With SU or Zenith carbs there should be a crankase ventilation port connected to the front tappet cover. If you have a charcoal canister fitted there should be a connection to that from a port on the rear of the rocker cover, and the oil filler cao should be of the non-vented variety. This provides for positive crankcase ventilation.

If the charcoal canister is not fitted the port on the rear of the rocker cover should be sealed and a vented oil filler cap fitted. Likewise the tank vent should be sealed and a vented fuel filler cap fitted, and the carb float chamber vents be piped down past the engine. The canister also had a connection to the inlet manifold, and without this the manifold port must be sealed off.


Other carbs like the Weber don't seem to have a crankcase ventilation port, so you really ought to fit the earlier PCV valve on the inlet manifold and connect that to the front tappet cover. Without that there will be little or no ventilation of the crankcase and you can get a build-up of condensation as a creamy substance in the oil filler neck. Connecting a pipe from the rocker cover or tappet chest to the air cleaner housing is no substitute, the earliest MGBs were like that and it hgave little or no ventilation, just relieved pressure.
Paul Hunt 2

Hi Paul,

1) Thank you for your response. My thinking is the same as yours, which is to establish a Positive Crankcase Ventilation system where one does now exist.

2) How ever I was not sure how best to establish this. I am well aware of how a PVC system works, and being 65 years old have worked on a lot of older V8's with earlier less complicated ventilation/emission control sysems.

3) What I was hoping for is some more direct information specific to this vehicle. As you suggested, I was not aware of a PCV valve for this vehicle, and was considering experimenting with several purchased at random. Where would I buy a PCV valve specific to this engine?

4) Your statement that the fuel line vent should not be blocked off---the canister is still in the vehicle---and a vent gas cap installed also seems correct. Where would I purchase a vented gas cap? I am not sure if what I need to do is within the law in Canada, however purchasing a new air pump, probably canister, valving, etc and ressurrectiong the old system is not attractive.

5) Where would be the best place to pull the air from? The hose from the canister to the back of the rocker arm cover is still attached. I seems to me that the routing should go something like this: (A)atmosphere (probably air cleaner) (B) into the front tappet cover breather pipe (C) down into the bowels of the engine (D) out the vent at the back of the rocker arm cover (E) into a PCV valve, and (F) into the base of the carburetor directly below the throttle plate.

6) Any comments are appreciated, especially where to purchase any needed components.

Layne Wilson
Layne Wilson

Hi Layne,
If you have acess to the "Bentley" manual. Page 139 shows connections to carb. Pages 282/283 shows emmission/evap connections.
If not available, feel free to e-mail me.
Cheers Ric
R.A Willis

Hi Layne,

You should be able to engineer an early MGB (64 to 68) PCV valve to fit the later inlet manifold I would have thought, especially with the extra emissions stuff pipes having been removed from it.

Vented gas caps should be available for pre-1970 cars, but will probably also need the mating filler neck. I know some have just drilled a hole in the cap, or filler neck, but I wouldn't be happy doing that.

Personally I'd go for the non-emissions route which is a vented and filtered oil filler cap (again standard pre 1970), block off the port on the rear of the rocker cover, and connect the front tappet chest cover to the PCV valve mounted on the inlet manifold. The only drawback with that route is that when the valve fails it applies a large suction to the tappet chest which can suck a lot of oil from it. However the resulting smoke-screen does tell you there is a problem, the reverse route would give you just the significantly weak mixture.

If you have the breather port on the carb(s) then you don't need the PCV valve, which is a much better option. This port is between the throttle plate and the piston on SU carbs, which gives a relatively constant level of low-level suction - exactly what is needed - under all conditions. The port directly under the throttle plate is for vacuum advance and is usually on one carb only, really you need a port on both carbs to retain the balance, unlike the breather port there is no flow through the vacuum port. HIFs also have an internal port just under the throttle plate which is supposed to prevent fuel pooling in the throat during long idles. Best not alter that especially jut on one carb.
Paul Hunt 2

Hi Paul, and others:

1) The car has a one barrel Stromberg/Zenith carburetor on it. It came with an air pump and all the trappings which were removed shortly after it was purchased in 1975.

2) I told our lady friend that the mechanic who told her to take the emission control system off did a bad thing, especially since he (A) removed and blocked off the idle emission adjustment from the side of the carburetor---duh (B) didn't deal with the gas tank vent at the canister, and (C) didn't vent the crankcase properly, and left the breather pipe open to atmosphere and the only place for the crankcase vent from.

3) I told her that the ground up parts inside of the engine did not come from the final overheat as she claimed, and that that surely happen over a long time due to (A) the 160 degree F thermostat, and the poor engine breathing.

4) In the 1950's it was customary to have a breather with a mesh in it mounted on high in the crankcase, on one side of the lower block. The air flowed in this crankcase port and out---hot air wants to rise---breather caps on the front of each rocker arm cover (on V8's).

5) Now then, since the most common location for the PCV valve on American cars is in one of the rocker arm cover(mounted in a rubber grommet), I was think that I could create a mount at the rear of the rocker arm cover where the existing emission tube is---remove it, it is now very non-functional---, and plumb that to the base of the carburetor on the intake manifold side.

6) The crankcase air would then have to enter via the breather pipe, and I probably would need some kind of weather cap in it or perhaps plump it over to the air cleaner, again a routing used for decades on American cars.

7) My question is, am I off my rocker, and would the MG PCV valve, which you say is available, be a 90 degree grommet mount? I have access to a machine shop and could easily alter the rocker arm cover with a machined ring (stiffner), and either weld or braze it in place.

8) Taking the air out of the rocker arm cover (the highest point of the engine and a large domed gathering area, seems to be fundamentally correct. Further, having the air enter lower down also seems correct.

9) You speak of a port between the throttle plate and the piston which is metered. The carburetor has an approx 3/8-7/16 dia port on the right side towards the firwall, which has been stubbed off with a hose and bolt.

10) Could this be the metered port you are talking about, and in essence a ready-to-go PCV valve? If so, then why couldn't I simply plump the rear rocker arm port over to this port, and the breather pipe (now an inlet to the crankcase) over to the air cleaner?

11) My problem is that I have no detail breakdown on the carburetor, and I am not excited about opening it up, if it is not necessary to do so.

Thank you! Layne Wilson

Layne Wilson

This thread was discussed between 16/09/2006 and 27/09/2006

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