MGB & GT Forum

I run a Weber 45MCHH that the PO had installed on my '78 B. I noticed it does however look like when it was installed they opted to vent the crankcase as a "road draft" as opposed to connecting it back into the air filter as I've seen some members on the forum do with this Carb. What I've noticed however though is that with the current setup I'm ending up with over time an oily residue on the side and underside of the engine from where its being vented. So I'm looking at a couple of options that I've seen members try.

1) Longer tube since the current barely clears the distributer and gets nowhere close to outside of the engine bay. Does anyone know the correct tubing ID since I don't have the MG with me and would like to pick it up while I'm close to the parts store.

2) Adding a breather filter to the end of the tube or replacing the tube all together with the filter

3) Connecting the crankcase breather tub back into the air filter on the Weber 45 MCHH. If so any idea where I might can find what I need to be able to do so locally?

Connect it back up to the air cleaner.

Do you have the valve cover connected to the charcoal canister? It should be. Breathe in through the valve cover out through the tappet cover tube.

The oil filler cap should also be non-vented if you do this.

If you decide to just use a filter at the end of the tappet cover tube and there is no connection between your valve cover and the charcoal canister, then go with a vented cap.

yes the valve cover is connected to the charcoal canister.


how did you attached the tappet cover tube to the air cleaner

There was a barb that came with the original kit whereby you drilled the right size hole in the back of the air cleaner and pushed the barb in. That kit barb is long gone, you can use brass elbow or almost any elbow. I now use an aluminum anodized one I got at a speed shop. Be creative.

Now that Ive vented the crankcase back to the air filter on the Weber do I need to look about retuning the carb?

Also will properly venting the crankcase have any overall effect on long term fuel efficiency or anything like that?

I'm going to say no to all Robert. Lots of Luck! Let me know if I can be of any further help.

Guess I should have asked, how does it run now?



Edited 1 time(s). Last edit at 2018-03-27 06:06 PM by lewk.

That connection point on air filter housings is often misunderstood. Its purpose is to be a source of filtered air to be drawn into the engine as part of a positive crankcase ventilation system. It is not for sucking crankcase gases out of the engine. The vacuum on the air filter side of a carburetor is insufficient. It needs to be via a PCV valve into the intake manifold, or into calibrated orifice on the engine side of the butterfly.

Tommy's comment is another opinion, not necessarily fact. Venting the crankcase through the air cleaner was the factory choice on later model engines from, I believe 18GH on. No PCV and no manifold port.

Please take some time off the MG forum and GOOGLE PVC systems. Regardless of how some " off road use only" kits were incorrectly designed, the correct airflow is from the air cleaner into the crankcase, out a PCV valve, into the manifold. Correct selection of the PCV valve is the hard part. The function is a small leak at high vac (Idle) that you balance with the idle mixture, and a larger leak under low vac that is swamped by the larger flow. By 78, you also have positive evaporative control added to the PCV system. I would not copy MG's design as it was very clumsy. Go look at a mid 70's to 80's GM car.

Second, if you are getting that much oil mess, sounds like the vapor separator inside the tappet cover is gunked up or you have a lot of blowby. One may want to put in an external separator ( can cull of a curlly-cate). Modern cars run via PVC into the manifold and rely on the fuel cloud to keep the manifold and valves clean. This became a problem on newer direct injection cars (like my Mini) as there was no fuel to clean the valves, they coked up. I would not want to dump this oily crud into the top of a carb!

Either do a correct draft tube (tapered end, MG even screwed that one up) or do a correct PCV/Evap system.

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Cogito ergo sum periculoso

Scott, Its PCV. Positive Crankcase Ventilation.
Not PVC. PolyVinyl Chloride (white plastic pipe)

As stated earlier, venting to the back of the air cleaner is the best method, and Weber usually provides a "knock-out" to do so. Or you could find an MGA front tappet cover with a road vent tube (less environmentally friendly) but effective. One requires a trip to the hardware store, the other either an Ebay purchase or a trip to the local British repair shop.

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jeff@advanceddistributors.com

Lewk,

So far its run well. Which is why I was really hesitant to mess with it and leave well enough alone at this point and focus on some other things with the car since its getting prime weather to enjoy it round my neck of the woods. .

Venting doesn't really affect drivability - unless you have a major problem. It affects how much oil your car leaks onto the ground versus keeping it in the block.

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jeff@advanceddistributors.com

Glad to hear all's well Robert. Weathers getting better around here as well, starting to get itchy feet. I'm glad you're enjoying that particular Weber of ours. I find it to be a very good and reliable carb, as well as unique. Nice sound and not too complicated. Set and forget. Got lots of oomph for me.

After they moved away from PCV and went to the Evaporative Loss Control system around 1970, crankcase and evaporative gases were fed into the emissions port on the carburetors body, which vents directly into the carburetors venturi behind the throttle plate.

The negative pressure inside the air filter housing isn't enough to be of any real benefit in scavenging crankcase gases, it's just used because it's the easiest option when it comes to aftermarket carburetors.

No argument Tommy, it's just that I've had and seen experiences with manifold draw from the crankcase, without the proper PCV you're drawing way too much vacuum. I find, as have others, that the passive vacuum pressure caused by the air cleaner set up is usually sufficient to draw gases without drawing oil.