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MG MGB Technical - Please explain crankcase ventilation to me...

My car is a 1972 18V UK model (no american air cleaning stuff)

I'm looking for a very simple explanation on crankcase ventiliation for this model and the role of the re-circ pipes and other ways of providing good ventilation. I am experiencing oil leaks which i think may be related to poor ventilation and i want to look into ways of improving this.

Also what effect would unadjusted tappets have on this - i assume that poor valve clearances would make a situation worse as vacuum would be less? Is this correct? (I am sure my valves need re-checking after a rolling road tune and i can't decide if the situation is worse now in comparison to when the tappets were last adjusted).

Thanks in advance ~PHIL
Phil

Phil, I think your engine should have the ventilated oilfillercap that needs replacing during service intervals, this is often forgotten and it may be clogged up, thus making it harder to "ventilate"?
Willem van der Veer

Willem is correct, carb suction from the angled pipes on the carbs, through a Y-pipe to the front tappet cover, and a ventilated, restricted and filtered oil filler cap is how it is.

Valve clearances would have little effect unless they were so bad the engine wouldn't run, I venture to suggest.

Valve clearances are the first thing to set, then plug gaps, points gap/dwell and timing, then carbs air-flow and mixture balance, before any trip to a rolling road.

Where are the oil leaks from? If there is no suction from the carbs because the pipes are blocked then even slight blow-by can push oil out of all sorts of places. OTOH there have been cases of problems with the oil separator/flame trap in the front cover which results in significant amounts of oil being pumped up the breather and into the carbs.

You can check the pipes by removing the oil filler cap with the engine idling. This should result in a slight reduction in idle speed if the mixture is correct, and if you place the palm of your hand or a sheet of paper over the filler you should detect suction. Postive pressure, or no suction, could indicate blocked pipes or so much blow-by that it is overwhelmong the suction. A blocked oil filler cap in itself should not cause leaks, but it may cause excesive oil consumption from too high a vacuum and condensation in the engine, like a creamy scum on the bottom of the cap. You are likely to get the same scum if there is no suction from the carbs.
Paul Hunt

Phil-
It is important to retain the crankcase ventilation system. Properly maintained, crankcase gases are drawn into the combustion chambers of the engine by the vacuum created by the fuel induction system, either through the intake manifold as in the 18GB through 18GF engines, or through the carburetors as in the later engines. This permits the crankcase to function in a partial vacuum which not only reduces power loss due to the pistons, connecting rods, and crankshaft forcing the atmosphere inside the crankcase to move about (technically termed "windage loss"), it also causes oil mist inside the crankcase to condense while being drawn upwards towards the camshaft and tappets. Without the partial vacuum provided by this system, the pressurized gases inside the crankcase of the B Series engine would cause oil on the cylinder walls to be blown past the pistons into the combustion chambers leading to carbon buildup and consequent preignition problems. The carbon can also collect in the groove provided for the compression ring, causing it to seize (Bet'cha can't guess how I know this!). In addition, an excess of these pressurized gases and oil mist would also be vented partially through its rocker arm cover, pressurizing the adsorption canister and interfering with its function, rather than traveling down through the pushrod passages as they should to aid in the lubrication of the lower ball ends of the pushrods and the upper sections of the tappets. For the excess pressurized gases in the crankcase to arrive at the rocker arm cover they would also have to travel up the past the pushrods and the oil drainback holes in the floor of the tappet chest. This means that the excess pressure of the gases would be forced upward around the tappets, depriving their upper sections of the additional lubrication supplied by the oil mist and the oil running down the pushrods from the rocker arm assembly. The pistons would also have to work against the pressure trapped inside the crankcase, retarding their downward movement and thus causing more combustion heat to the transfered to both the cylinder walls and the roof of the combustion chamber, making the engine run hotter. Thus it must be understood that all of this is prevented by drawing all of the pressurized gases inside the engine out through the tappet chest cover and into the induction system under vacuum, and as such the system contributes to long term reliability and a prolonged engine lifespan.
Steve S.

Steve:

Any advice on how to vent crankcase with a Weber DCOE set-up? All I run is a road draft out of the front tappet cover.

Pete
Pete

I need some hints in this department as well.

I thought I only needed to use the tappet chest vent (which I connected to a 'slant' cut tube directed to lead towards the floor) and used some tape to close the vent on the valve cover.
Wrong I guess! On the way back from Silverstone, a rather silent 'bang' and the tape had blown away.
So for now, that one's left open, but I am really looking for a neat way to use both vents.

(just in case: using 2 dell'orto sidedraught carbs, and I don't feel like making a disruption of the flow in one of the manifold runners!)

Thanks in advance!

Alex
Alexander M

This is a thread that I started after my experience with MAJOR EXHAUST SMOKE.
I still haven't gotten a definitive answer yet.

Posted 20 August 2004 at 14:37:42 UK time

Dwight McCullough, Minnesota, dcm@mnautox.com


SMOKING!
Yes major smoking. A fairly new engine. But auto crossed. [slalomed]
One big change, I had a slight oil leakage from the valve cover breather tube. Which I have installed a crankcase filter. It doesn't go anywhere anymore, this is a competition engine.
So I installed a 'stock' valve cover with the small hole in same breather tube.
On my way to the Minnesota Austin Healey club gymkhana, glancing in the rear view mirror, clouds, I say clouds of oil smoke.
Panic!
But continue on, if something has 'blown' then deal with it later. The event went well, only a small amount of smoke during competition, but returned in volumes on the return trip home.
It used over 2 quarts of oil, almost 2 liters to the rest of the world.

Oh dread, oh no, after all that work, I've ruined my engine.
Or so my thought's went.
So I re-installed the valve cover with the large cut-off breather vent, and went for a drive with my 10 year old grandson.
Voila!
No more smoke!
Hallelujah!
Lesson; engine need lotsa crankcase breathing. Add more.
Safety Fast
Dwight McCullough
Dwight McCullough

I am also interested in this problem. I have a warm 18gb B series with a weber DCOE(no PCV valve). Currently I have K&N breather filter on the tappet chest cover pipe but think the engine cannot breathe sufficiently. I got rid of the stock PCV valve after having probs similar to Dwights. BTW engine is a 2100cc so breathing is more important. I was thinking of adding an oil catch tank (part of race regs) BUT after reading Steve S's post am re thinking. With a catch tank how is vacuum created? Is this truely necessary for lubrication? Or is adequate atmospheric breathing sufficient?

I was going to add an elbow on the rocker cover ( as per early cars) and have this and the tappet cover pipe connected to the catch tank. The tank would then be vented to the atmosphere via a K&N breather. Would also incorporate a drain. Many race cars in Aust have a set up like this. Any thoughts whether this would work well?
Grant Stubbs

Grant,

May be of help

http://www.teglerizer.com/triumphstuff/emissions/emissions_system5.htm

Paul
Paul

About the system that Tegler made... Wow. Seems to be overkill but... Wow.

Insofar as race engines with road-tubes are concerned... They work fine. You just need to remember that the tube needs to be pretty large with free flow both directions or have a second (large diameter) air source on the rocker cover. Properly designed and plumbed catch-cans don't restrict the breathing at all.

Where difficulties arise is converting from engine vaccum systems to open to atmosphere systems and in the engine vaccum systems them selves. If you convert you need to ensure the proper restriction is in place for engine vaccum driven systems or no restriction whatsoever (except a filter) for road tubes.

Looks like Tegler wanted to keep his engine vaccum driven system while limiting the well known issues with oil mist seperation. Pretty clever but... Wow!
I would think (but he's been down several paths here) that just having a large diameter tube (can) with a media for condensing the mist and a return line for condensate would be enough but... Wow.

Personally, I've never had a problem with the Old Mushroom Valve thingy. On the other hand, I've never really tried to stop any and all oil leaks.

Mike!
mike!

Read the thread with interest as I have just purchased a 72 MGBGT and the previous owner trying to trace a misfire closed off the two vacuum pipes into the SU carbs. The vent pipe from the engine is now open running down to ground. While it didn't cure the misfire (that turned out to be a fuel problem) it did reduce the oil used by the engine from a pint over 300 miles to nothing. Now as this has proved the engine is not using oil I wanted to return the breathing back to standard. I've read about valves and vents but from what I can see under the bonnet it seems the vent pipe comes from the engine slits into 2 and then on to each carb. Is that it, piece of tubing and a Y piece? no other valves etc.
Roy Soper

3 bits of tube and the Y piece - real hi tec!
Chris at Octarine Services

Thanks, I had a closer look tonight while adding some missing overflow tubes from the carb float bowls. Noticed that I've got two breather outlets off the engine, one from the lower part of the engine, the sump from what I could see and another off the top of the rocker box. Does one go to each carb?
Roy Soper

Quick update, went through all the paperwork I got with the car and there is a mention on a receipt dated 1992 that the rocker box is not the correct one but as the top breather is closed off it will work OK. So the only breather the car should have is the lower one. I'll get the pipework for it over the next few days and change it back. Then I'll start looking into the misfire over 3,000 RPM.

Old cars, don't you just love them........
Roy Soper

Hi All,
I have slight oil consumption on my 72GT, i have opened up the oil filler cap breather , removed the gauze washed it in petrol e.tc.(no improvement after)
I have opened up the y-pipe from the front tappet cover, that goes to the carbs, and i found steel gauze in There as well!!! (slightly bigger than steel wool, about 1inch in length).
Should it be there , i am puzzled, when i removed it the engine would not start!!
What would the purpose of it be in that location, and why will the engine not start when it is removed????
Please advise if possible.
Thanks
C. ODonoghue

Roy;

In the USA the tube from the rocker cover goes to the charcoal cannister and is filtered supply air to the crankcase. The nipple on the top of the rocker cover should appear to be mostly closed off with a small hole drilled in the end. This is the metering orifice that controls how much air is drawn through the engine and into the carburetors. These engines have a non-vented cap.

Older systems ('64 to '69 USA, probably later UK and elsewhere) used a vented filler cap on the rocker cover coupled with the "mushroom thingy" PCV valve to meter the air to the manifold.

Still older systems (up to '64) had a non-vented cap and a road tube with a hose to the air cleaners for clean air to the crankcase.

Non-USA systems are fuzzy to me but likely work on the same principles.

C;

There really shouldn't be any gauze in the tubes to your carbs. Some "media" was in the oil separator on the tappet cover and either it broke down and has migrated up or some PO tried to reduce the amount of oil being sucked into the carbs by stuffing it in there. I vote for the latter as the oil separation properties of the little tappet cover thingy was marginal, at best.

The car won't run with it not there because it is (in essence) a huge vaccum leak!

The parts must all be matched to the appropriate application. Decide which you want and get the appropriate bits together to accomplish it. If you have the "y" to the carbs you should not have a vented cap but a vented rocker cover with the orifice as noted above.

Mike!
mike!

I am about to replace some worn-out HS4s on my 1967 GT with rebuilt HIF4 carbs. I also have an alloy valve cover. How would I properly vent this valve cover in order to use the Y-connection system and breathers built in to the HIF4s? Would I be better off capping them and retaining my mushroom PCV system?

Thanks,
~Jerry
Jerry Causey

I too have just had this problem of oil leaking. I have a 79 MGB and just replaced the Zenith carb with the Weber DGV set up and have vented per Bob Ford's instructions. After the firt start up it started leaking oil. It was running EXTREMELY rough (more tuning still to follow) which made me think maybe that somehow caused the leak but after reading the above changed my mind.
After a suggestion to run the hose from the oil trap to the carb. air filter it seems to have decreased. Is this a possible fix?


Elizabeth




Liz Martin

Jerry;

If your valve is still working properly then I'd retain it. They have been known to get fouled with oil from the rather lazy oil separator not separating any oil.

Liz;

The oil trap is not very effective and has caused may people headaches. Running the vent tube to the air filter may well work but it isn't really the best way to go. Go down to the local Kragen and get a PCV valve that looks like a small black or silver cylinder. I don't recall what it came on stock but I found one that works well. Plumb that into the line from the seperator as high as you can and still keep the valve vertical. Run the outlet from the valve to your intake manifold. What this does is re-create the '64-'69 system with alternative parts and the air source is still through the charcoal cannister. If you have by-passed or removed the cannister then a line to the air filter will provide clean air to the crankcase. (Plugged cannister will also cause troubles).

Mike!



mike!

I have removed the air canister and have a non venting valve cover with a small 1/16th hole drilled into the non-venting oil cap. Everything seems to run great, what am I hurting.
Bob R.

Mike - I still have the cannister in and the line from the valve cover runs to it. I will ensure it is not plugged.
Quetion: How did you "plumb" the pcv valve into the separator?
Sorry for the ignorance but I'm learning lots!!!

Elizabeth


Liz Martin

The PCV valve was dropped in 1968 for all models being replaced by carb breathing, regardless of whether it was the UK spec with vented oil filler cap or the US spec with carbon cannister and non-vented cap. IMHO it is a retrograde step to go back to it. There are so many of these cars running around *without* this problem that there is obviously some cause, again IMHO it is better to fix the real cause than use some work-around which may not be as effective, and may allow the real problem to bite you in some other way later on.

But like mike! says if fitting carbs with breathers to an engine that has a working valve, I'd keep the valve and cap the breathers. Untill the valve failed, then I remove and block the valve port and use the carb breathers and a Y-piece. There should be a hose from the valve to the front engine cover or front tappet chest cover, which contains the separator/flame trap.

The cap (and the cannister) acts as a filter, a clear hole drilled in the cap will allow dust particles to be drawn in by the vacuum. Much better to fit a breathing cap (and I thought I was tight...)
Paul Hunt

Paul...Your right I am tight, but I did at first order a breather cap from an earlier model MGB. Problem is it would not fit as the whole cap is of a different size, hence the hole in the original which was to be a temporary fix until something else came to mind. My mind is still waiting....
Bob R.

Paul,
Did I understand you correctly? On my '67 GT, I can use my early style breather cap and the HIF4 carb breathers (fed via Y-piece from the front tappet cover) without worrying about running either overly lean due to no restriction in the system or sucking oil into the carbs?

Thanks,
~Jerry
Jerry Causey

FYI:

www.mgbmga.com



TECHNICAL
INFORMATION

Doug Jackson has both a techtip for doubling breath diameter on the valve cover end pipe and an innovative breather rig. I used both on the 1971 cross flow after rebuild. Solved all the breathing snafus immediamente! Vic

vem myers

Doug's web-site won't open.
Darn!
whatzupwiddat?
Dwight McCullough

Dwight
Doug's site will not work if you are using Netscape. Works fine with Explorer. I use Netscape and find that Doug's site and Brown & Gammons site will only work with Explorer.
Jim Lema

Hye Exploder works!
However, Doug is closed form the 7th throught he 15th of September.
So I wait.
Safety Fast
Dwight
Dwight McCullough

Jerry - you should be able to assuming a vented cap and everything working as it should, the position and size of the carb ports control the amount of weakening like the PCV valve used to, and it shouldn't suck up any oil at all. However there are a number of engine problems that can cause oil consumption through the breather.
Paul Hunt

This thread was discussed between 03/09/2004 and 16/09/2004

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