PCV set-ups... can we discuss?

[Boink]

OK, I've read a lot of old threads on this topic, such as: https://www.britishcarforum.com/bcforum/ubbthreads.php/topics/756957/PCV_installation_results . What I guess I don't fully get are all the various set-ups for crankcase ventilation... some with non-ventilated rocker-covers, some with open ventilated oil separators, some with PVC hook-ups to oil seperators, etc. I realize that some are for different engines (notably later 1275s like mine vs. stock 948s).

I've got a 1275 (with dual HS2s) with a front oil seperator that just opens to the air, and a rocker cover that vents by a hose to a carb air-filter. It seems to work. However, I'm wondering if I should consider piping the oil seperator to a PCV to the standard intake manifold access fitting/plug (in the middle between the carbs). Some seem to think this is essential to minimize pressures on seals. Beats me.

I'm not trying to solve an oil leak or anything (well, not right now). Just playing the tweak game. All the various set-ups are confusing my little mind. Once in a while I have blown a little oil into a carb under severe load.

Related questions: if one is using a ventilated rocker cover, does a PCV even make sense? I mean, is any vacuum draw even workable/useful in that situation? I used to have a 1293 Mini with a non-ventilated (sealed) rocker and it worked great. Also, IF this is worth installing, could I use any old barbed PCV from an auto store (vs. the old style metal one with the diaphragm)? In short, what do you use and does it work well?

I value all your opinions. Thanks.

 

[Jim_Gruber]

Mark,

On Bugsy, my 1098 I thought I bought the Exxon Valdez when I fired him up the first time. The Timing cover vent was plugged with a 3" x 3/8" extension and the Valve cover had a broken off spark plug crammed into the hose. Oil just absolutely pumped quickly out of the rear seal. I went the tradfitional flying saucer route with an LBC PCV Valve and everything stopped leaking. The more your rings are worn the more that is pushing past the rings into the crankcase.

 

[Boink]

Mark,

On Bugsy, my 1098 I thought I bought the Exxon Valdez when I fired him up the first time. The Timing cover vent was plugged with a 3" x 3/8" extension and the Valve cover had a broken off spark plug crammed into the hose. Oil just absolutely pumped quickly out of the rear seal. I went the tradfitional flying saucer route with an LBC PCV Valve and everything stopped leaking. The more your rings are worn the more that is pushing past the rings into the crankcase.

Sounds like a novel "set-up." I'd guess that yours had, essentially, NO ventilation (so the leaking makes sense). It's a wonder yours didn't explode like a Texas gusher - a major case of petroleum-based constipation. :laugh: My engine is fresh re-build, and I wanted to do things right... but saw so many variations, it's confusing. Thanks Jim.

 

[Rut]

Mark,
I went thru this when I first got my car and discovered like everyone else that the 1275 needs a little negative crankcase pressure. My engine was leaking excessively from the rear and I'm certain that the scroll seal is not set up correctly so I tried to correct it with vacuum. I finally applied enough vacuum to stop the leak without causing the engine to run poorly, but I think it's just a bandaid. While the engine is out of the car for blasting and paint I'll have it checked out and corrected and/or have my spare 1275 rebuilt correctly.

I tried several things when I first got the car with varied success. The oil separator was connected to the underside of the air filter on the Weber and didn't supply enough vacuum so I put a hose fitting in the boss of the manifold. This gave me enough vacuum, but I needed to plumb a PCV to regulate the flow. After I did that all was ok, but I always wanted to put the original SUs back on the car, so...

My ultimate goal once the car is back from the body shop is to send my spare engine out for a performance rebuild and do it 'right'. I've got my 5 speed ready to go and all the bits laying around to do it, but I'm very low on patience and sometimes leap before I look. Good luck on tweaking your car!
Rut

 

[Boink]

Mark,
I went thru this when I first got my car and discovered like everyone else that the 1275 needs a little negative crankcase pressure. My engine was leaking excessively from the rear and I'm certain that the scroll seal is not set up correctly so I tried to correct it with vacuum. I finally applied enough vacuum to stop the leak without causing the engine to run poorly, but I think it's just a bandaid. While the engine is out of the car for blasting and paint I'll have it checked out and corrected and/or have my spare 1275 rebuilt correctly.

I tried several things when I first got the car with varied success. The oil separator was connected to the underside of the air filter on the Weber and didn't supply enough vacuum so I put a hose fitting in the boss of the manifold. This gave me enough vacuum, but I needed to plumb a PCV to regulate the flow. After I did that all was ok, but I always wanted to put the original SUs back on the car, so...

My ultimate goal once the car is back from the body shop is to send my spare engine out for a performance rebuild and do it 'right'. I've got my 5 speed ready to go and all the bits laying around to do it, but I'm very low on patience and sometimes leap before I look. Good luck on tweaking your car!
Rut

Thanks Rut. Envious of your 5 speed. Did you have a ventilated rocker-cover too?
I guess I should try a little negative pressure from the manifold to the oil separator. Wondering if I can use an after-market PCV valve... and if I might consider a small control valve to potentially adjust that vacuum (or is all this overkill). I suppose I can experiment.

Still, I'd love to learn what others have done.

P.S. - here is what I have:

 

[Rut]

Mark,
My original valve cover was one of those aluminum aftermarket ones with a vented cap. I restored a couple of early valve covers and swapped them out until I got the desired effect. One had a restricted vent tube and the other had an unrestricted one. I'm kinda going for the 'original' look and like the green. There are probably dozens of ways to make this work and I played around with a manometer to see how much vacuum I had and when did the oil quit dripping out the back. I can't remember what the value was, but I wanted just enough to do the job.
If you want to play around this is what I did. I put the original unrestricted vent tube valve cover on, put 'saran wrap' over the oil filler hole secured with a few rubber bands, attached a piece of clear vinyl tubing to the vent tube and marked it in centimeters. I then taped this to a piece of cardboard in a U shape and put colored water in it until I reached my 0 line. I clamped the line from the oil separator to the manifold and started the car. I then released the clamp gradually until the leak/oil stopped and watched the manometer and marked it on the card. I stopped with no leak and no smoking and threw the 'manometer' in a box with a lot of other stuff. It would probably be more accurate if I had tee'ed into the oil separator to manifold line, but it worked ok.
Good luck, Rut

 

[Rut]

Mark,
Just saw your pictures and my oil separator hose went to the bottom of the Weber air filter just like your SUs and it was not enough vacuum. If you do develop a leak you could tee the separator into the line you have running from your valve cover to your air filter.
Again, mine is just a bandaid solution until I can get a proper rebuild and fix the real problem and I did try a LOT of bandaids!
Rut

 

[Boink]

Mark,
My original valve cover was one of those aluminum aftermarket ones with a vented cap. I restored a couple of early valve covers and swapped them out until I got the desired effect. One had a restricted vent tube and the other had an unrestricted one. I'm kinda going for the 'original' look and like the green. There are probably dozens of ways to make this work and I played around with a manometer to see how much vacuum I had and when did the oil quit dripping out the back. I can't remember what the value was, but I wanted just enough to do the job.
If you want to play around this is what I did. I put the original unrestricted vent tube valve cover on, put 'saran wrap' over the oil filler hole secured with a few rubber bands, attached a piece of clear vinyl tubing to the vent tube and marked it in centimeters. I then taped this to a piece of cardboard in a U shape and put colored water in it until I reached my 0 line. I clamped the line from the oil separator to the manifold and started the car. I then released the clamp gradually until the leak/oil stopped and watched the manometer and marked it on the card. I stopped with no leak and no smoking and threw the 'manometer' in a box with a lot of other stuff. It would probably be more accurate if I had tee'ed into the oil separator to manifold line, but it worked ok.
Good luck, Rut

An experimenter (with manometer no less)! I guy after my own heart. Excellent! Thanks. I thought about combining (with a T) to the rocker-cover tubing.

 

[Boink]

Here's another nice post at Spritespot (related to stopping oil leaks - though I don't have much of that).

https://www.spritespot.com/phpBB3/viewtopic.php?f=10&t=2527

It also lists some aftermarket PCV valves.

And at a MG site, there is a nice thread (that is new): https://www.mgexperience.net/phorum/read.php?3,1761422,1774101

 

[smaceng]

The 1275 had a couple, at least, methods for crankcase ventilation. Some SUs have a post into the venturi, and some do not. My 1974 1275 did not. I decided to copy the original design and used the Smith crankcase breather. Seems to work for me and has some original look. Some people instead use a generic "PCV" valve or positive crankcase ventilation valve to do the same thing. Here is what I did. The line coming from the front of the Smith valve is connected to the can mounted on the front of the timing chain cover.
Scott in CA

 

[Rut]

Mark,
Thanks, I'm sure I can use that 1274 number in the future!
Rut

 

[Boink]

The 1275 had a couple, at least, methods for crankcase ventilation. Some SUs have a post into the venturi, and some do not. My 1974 1275 did not. I decided to copy the original design and used the Smith crankcase breather. Seems to work for me and has some original look. Some people instead use a generic "PCV" valve or positive crankcase ventilation valve to do the same thing. Here is what I did. The line coming from the front of the Smith valve is connected to the can mounted on the front of the timing chain cover.
Scott in CA

Thanks Scott.

 

[nomad]

When I did my Datsun A15 swap I had to make up a manifold which I did using a single HS6 SU. On the original Datsun setup the valve cover is plumbed into the air cleaner housing and the crankcase is plumbed thru a PCV valve into the intake directly in the middle under the carburetor.
On my home made set up I tryed plumbing a generic PCV into the back end of my manifold. Generic PCV let in too much and it had to be reduced by fitting a orifice of about 1/8. That would allow the engine to run and idle but back plugs were very lean. I moved the intake from PCV to the middle so that all cylinders recieve the same and I could then adjust the carb to give a correct mixture.
Datsun manual described what happens best. Under normal operating conditions blow by is fairly constant but under hard acceleration the pcv valve can't allow all of the blow by to be sucked into the manifold so, at that time the blow by flow reverses and is fed into the air cleaner.
So getting it right is going to be a trial and error game if your not using stock and you will be sucking air into your manifold which, of course, will make your mixture leaner.
The correct method would be a filtered intake at the valve cover and a metered feed to the manifold thru the pcv that, I think, would have to be one of trial and error.
Kurt.

Kurt.

 

[TulsaFred]

My 1275 plumbs the oil separator on the timing cover via a Y-piece to each SU carb (mine have the vacuum fittings).
However, there is a large opening in the intake manifold that I am unsure what to do with. I also have the vent tube on the valve cover with a small metering hole.

Should I plug the large vacuum fitting on the intake manifold?

Fred

 

[nomad]

My 1275 plumbs the oil separator on the timing cover via a Y-piece to each SU carb (mine have the vacuum fittings).
However, there is a large opening in the intake manifold that I am unsure what to do with. I also have the vent tube on the valve cover with a small metering hole.

Should I plug the large vacuum fitting on the intake manifold?

Fred

Yes, according to Bill M, and I'm quite sure he is right, the fittings on the carb's communicate a far more constant vacuum than any fitting on the manifold . I would think that setup would be fine though I like the idea of filtering any air that is drawn into the engine thru the valve cover.

Kurt.

 

[Boink]

My 1275 plumbs the oil separator on the timing cover via a Y-piece to each SU carb (mine have the vacuum fittings).
However, there is a large opening in the intake manifold that I am unsure what to do with. I also have the vent tube on the valve cover with a small metering hole.

Should I plug the large vacuum fitting on the intake manifold?

Fred

Yes, according to Bill M, and I'm quite sure he is right, the fittings on the carb's communicate a far more constant vacuum than any fitting on the manifold . I would think that setup would be fine though I like the idea of filtering any air that is drawn into the engine thru the valve cover.

Kurt.

Thanks Kurt. Didn't know that. Frankly, I have to check to be sure I have those vacuum access points (hate to confess that never paid attention). If I can, I'll try that.

 

[Hap Waldrop]

Just remember, you not going to get alot of crankcase gas releasing thru cylinder head, the smallish push rod holes are it, this is why it is crucial to vent from the block as well, that will be your main source. if your engine has a mech. fuel pump blanking plate, you can order a really nice baffled aluminum block vent, that you can hook a hose to from Mini/Spridget Mania that will be your purest form of crankcase ventalation. I'm not big on PCV, it's pressure and if properly releived/vented, it doesn't need vacuum to relieve itself, but to each their own. I vent my cars to atsmophere, it's just air, and need some method to catch oil, or oil vapor if it does omit, in a good running engine, it should not, and I like to do that with something other than my carburator and engine, and I can't for the life of me see what burning some oil in thru engine makes is more eco friendly, than filtering it or catching it in catch can, this was another one of the goverment mandated deal, like the smog pumps, it made us feel eco friendly, but in reality it was a farce.

 

[Boink]

Thanks Hap. Yes, my 1275 has a blanking plate (with electronic SU pump back by the tank). Didn't know about a vent at that location. Good info.
As for atmospheric venting, that's what I've always done... but I started reading these articles/posts on excessive oil pressure (and seals).
I may just let it ride for now (given the performance seems fine).

 

[Boink]

Can't believe I've never carefully looked... but it would appear that I can't install one of those Y-branches that pulls some vacuum off the carbs. These would be the "emssions" ports?
Well, unless I'm missing something, there doesn't appear to be any such access except that central plug on the intake manifold.