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MG MGB Technical - Crankcase Ventilation with DCOE

Is there a proper way to vent the pressure from the crankcase when running a WEBER DCOE carb? I have always run just a open hose from the tappet cover. I have seen on a race car set-up where there was a BIG hole in the valve cover running to a catch tank. How have others done this?


K&N makes vent breathers - I had one on a modified 240Z and I highly recommend it. They look nice too :)

Mike MaGee


I've been thinking about that too.
The plan now is to connect both breather pipes (rocker cover and from the side of the engine) with a Y-shaped connector to a 3rd bit of hose that leads to whatever's under the car.


Doesn't that vent breather get clogged up rather quickly?

Alexander M

If the DCOE (or any other carb) has a PCV suction port then use it the same as the original (post 67) carb ports, with a standard vented oil filler cap if you don't have a charcoal canister, or a standard non-vented cap if you do. If the carb doesn't have a PCV suction port then you will always be worse off. Just leaving the rest of the engine as it is will vent any excess pressure out of the remaining port (vented oil filler cap or charcoal canister), but you will lose crankcase scavenging which removes condensation. Loss of this is a bad thing as it can cause internal corrosion, fitting a pre 67 PCV valve to the inlet manifold would be better than nothing. As long as you have suction the 'other' port (oil filler cap, charcoal canister, etc) won't get blocked.
Paul Hunt 2

Here's what I did on my '78 w/brake booster...

The front runner on the intake manifold has the fitting for the brake booster hose. I pulled the hose from the manifold and installed a "T" fitting. Connected the booster hose to the rear side of the T. Then I bought a 90 degree PCV valve at AutoZone that was about the same diameter as the breather pipe, and connected it to the pipe with a short piece of hose. Then placed a short length of hose from the PCV valve to the front side of the T. I also fashioned a very small restrictor that is attached to the front side of the T to limit the amount of suction (it doesn't take much!).

I've had it that way for a few months now. No change to the way the engine runs or the brake booster functions - and it produces just the right amount of suction. Since the restrictor is so small, I haven't seen any indication that the front cylinders are running lean - which would have been my primary concern. Actually, I drove the car around the block without the restrictor just to see what the result would be. It sucked so much air through the oil filler cap hole it sounded like a tea kettle, the brake boost was greatly reduced, and the engine ran like crap - obviously way too lean on those cylinders. Restricting the flow to a very small level fixed all that.
Joe Reed

Hi Alex,

If your engine is in good shape it doesn't get plugged. Being that nothing is being sucked through it little oil will reach the filter.

I'd buy 2 of the filters - they do look nice and the chrome cap cover enhances and gives sparkle to the engine :)

DCOEs have no PCV ports or any other port except perhaps those that were custom made for a manufacturer like Ferrari or Alfa. A vacuum port adapter is available through a few race shops.
Mike MaGee

Alexander....I wouldn't connect the hoses from the tappet cover and the one from the rocker cover. Air is supposed to go into the rocker cover and out the tappet cover - creating a flow-through ventilation system. Connecting those hoses would give you a closed system, which isn't what you want.

I just looked at your engine photo....nice crossflow setup - especially the monogramed rocker cover!! Does that inlet in your valve cover have a restricted opening - it doesn't appear that way on the photo? Of course, if you're just venting to atmosphere (and not running a PCV valve) it probably doesn't matter. You could just put one of those filters Mike suggests on your rocker cover (for looks....and to keep out any debris), and drop a hose from the tappet cover toward the ground (similar to the setup on the early 3-main engines).
Joe Reed

I was planning on switching to a PCV setup with my '69 MGB, but no less an authority than Joe Curto told me not to bother- he suggested just venting to the air filter.
Adam Birnbaum

Two problems with venting to the filter: one is that I have a foam "sock" type filter and it wont work. Second, every carb I have seen with crank vented is filtly looking after a whiole. That can not be good for the carb? Then again, if Joe endorses the idea....



Thanks for the remarks on my car!

The rocker cover has a vent, the inlet itself hasn't got a vent.I think I might go for your idea: 'filter' connected to the rocker cover and just a hose to the tappet cover vent.

Btw, the Dellorto carbs have a vac takeoff, one per carb so basically if I connected those to the tappet vent, cylinders 2 and 4 would be getting some oil damps in?

Alexander M

My mistake, only one of the carbs has a vac take off...
Alexander M

Where did you get your rocker arm cover? Who makes it?
Steve S.

Unless you've got a serious blow-by problem, oil getting into the cylinders isn't a problem. Using the vacuum port, you'll have to restrict the orifice to a very tiny opening anyway - otherwise you'll such way too much air into the intake. On the vacuum advance and brake booster, you're pulling a vacuum on a closed system so, once it's pulled the vacuum it doesn't continue to suck air. The crankcase vent is an open system, so it'll constantly suck air in large quantities if not restricted.

Pete....yeah, there is some oil vapor that will get into the intake. I've never found it to be a problem - and it's certainly worth it to have a well ventliated crankcase. If you're getting too much oil in there, it's probably time for a ring job...
Joe Reed

Motivation for this project is to try and get rid of the hot oil smell. It did not bother me when the car was mostly a track day car, but now I am trying to drive it more often and I do get tired of the smell on long journeys!



It's an original 60s Alexander Engineering rocker cover, they used to make/sell tuning parts for A- and B-series engined cars, amongst others. I found it on Ebay (not for cheap) and I've never seen another except in a period brochure. There's also a genuine HRG Derrington rocker cover waiting for some refurbishing.


Thanks for the info!

Alexander M

My MGB with a side draft Weber has a port on the intake
with a T. One line runs to the vacuum advance and the other to the side of the engine at the front.It was like this when I bought it and I have always wondered about it. On the valve cover at the back there is a hose that just runs out under the engine. Will this set up cause a problem?

John Dale

With the vacuum advance line, where does the other part finish? The tappet cover? If so I'd imagine you get very little vacuum signal off it? Are there any valves?

As for the DCOE breather, the first picture in the GT album shows the arrangement on my car:

Pipes from the rocker cover and tappet chest vent to an aluminium catch can which then has a large-bore pipe running to air down by the gearbox. Without this, the fumes in the car are pretty hazardous to health.
Steve Postins

The vacuum advance line has to come off the inlet manifold, or preferably the pukka port on the carb if it has one. You need up to 15 in Hg depression for the vacuum advance to work, depending on its reference number, and you won't get anywhere near that at the rocker cover under normal circumstances.

The arrangement of having an open tube near the bottom of the engine (a 'road draught' tube) and a pipe in the air cleaner was how only the very earliest non-positive crankcase breathing system MGBs were plumbed. They very soon changed to the much better positive systems using a PCV valve, and the even better positive systems using the carb breathers.
Paul Hunt 2

So if I have got this right, my rocker cover vent to the ground is an intake and I will be sucking in dirt. The vent from the tappet cover should run out and not to the other end of the T. Should I plug the other end of the T to get the vacuum for the vacuum advance?

John C. Dale

My DCOE engine crankcase breather system utilizes
most of the stock (1971) evaporative equipment
including the charcoal canister, stock valve cover
w/ restricted hose fitting, front tappet cover, etc.

I use an intake manifold that has a home-brewed
balance tube between the runners. On this balance
tube is a fitting with a threaded restrictor made
from a 1/4" NPT allen plug and drilled. A PVC valve
from an 1800cc "pancake" Subaru engine is then
screwed onto this fitting. A hose is run from
this PVC valve to the front tappet cover
breather (with built-in oil separator).

I used an old backyard mechanic "paper test"
to determine the size of the restrictor.

Paper test: With the engine idling - remove
the oil filler cap on the valve cover and place
a flat 1/2 sheet of common notebook paper
over the oil filler tube opening.

If the restrictor opening is correct - the piece
of paper will be held in place, very lightly, by
crankcase vacuum. With gentle tugging, it'll
have the same resistance or "feel" as
a weak magnet.

If the piece of paper can easily fall off, floats
or flutters & flaps, then the restrictor opening
is too small.

If the piece of paper is pulled down smartly,
becomes bowed, or is pulled into the engine,
then the restrictor is too large.

For me, 1/16" to 5/64" works fine. Monitor
the sparkplug color over time and adjust the
idle mixture screws if needed.

Because the fuel bowl breather port on a DCOE
is vented to inside the aircleaner, any fuel vapors
from the fuel bowl will accumulate inside the aircleaner
and be pulled into the engine and burned at startup.
Because of this, the small hose which goes from
the charcoal canister to the stock SU fuel bowls
is not needed and is sealed off and left unused.

This setup is more "active" at scavenging vapors
from the crankcase (and fuel tank) than the
"passive", open hose method.

Retaining the charcoal canister insures that fresh
air entering the crankcase is filtered; and blow-by
gasses as well as raw vapors from the fuel tank
and float bowl are burned in the combustion process.

Sorry for the long thread.
Daniel Wong


No need for apology! Your post is very very useful!
I've got some more inspiration now :-)
Alexander M

This thread was discussed between 15/09/2005 and 22/09/2005

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