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PCV valve vs ported carburettor

Posted by kbl444

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PCV valve vs ported carburettor #1
kbl444 Terry Malloch Geelong, Australia	Posted: June 16, 2012 06:18AM Registered: 2 years ago Posts: 42

My car had a PCV (mushroom) valve fitted and the later HS6 carburettors with the crankcase ventilation ports which were plugged. Is it OK to remove the PCV valve and redirect the vent hoses to the carburettors or will this upset the mixture.
To me removing the PCV valve is one thing less to go wrong.

Terry

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Re: PCV valve vs ported carburettor #2
kirks-auto robert kirk Davenport, Iowa, Rock Island, Illinois, Clearwater, USA	Posted: June 16, 2012 11:17AM Registered: 4 years ago Posts: 13,176

I'll allow other to respond and defer to the collective advice. The PCV valve is, imho, the one thing that should be retained and maintained. You have to have a reasonable way to equalize pressures and the engines designed for PCV rely on them to accomplish the task. On the other hand at least two were used, and as far as I know neither is available. I also wish someone could advise what if anything they KNOW will work for this important bit of necessary tech.

Regards,
Robert Kirk
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Re: PCV valve vs ported carburettor #3
two MGCs gary herbold orangevale, USA	Posted: June 17, 2012 07:14PM Registered: 4 years ago Posts: 153

Her is my experience on this subject.

On my 68 GT I removed the factory PVC valve and install a small inline PVC valve on the top of the oil separator using a short piece of hose and then routed copper tubing to the port where the remover original valve connected on the intake manifold. You should use a PVC valve made for the same size engine. Mine is what is used on a Plymouth straight 6 and is a 90 degree unit that makes for a clean run to the other side of the engine.It has worked fine for years. I know of 4 other Cs using this same approach after seeing mine. Don't try to use the PVC valve and connect to the 69 type carb. direct crankcase ports. The one person I know that did this had a car that stumbled until he removed the valve.
On my 68 rds with the tri-carb. setup I have the 69 carbs and run a rubber hose from the outlet on the oil separator to the carb. ports. I have run this set-up for 4 years with no problems.
Here is one item that will cause you problems with ether set-up. If you are running a aluminum valve cover with that very small hole in the cap you most likely will not be inducing enough inlet air to have the crank case ventilating system work properly. I know of two people who have drilled a larger hold in the cap or applied a breather pipe like the original valve cover had .

Hope this is of some help.
Gary

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Re: PCV valve vs ported carburettor #4
Big6Mark Mark Miller Springfield, OR, USA 1960 Austin-Healey 3000 1968 MG MGC "Red Tail" 1968 MG MGC 1968 MG MGC GT "The Wreck" 1969 MG MGC GT "The Lump"	Posted: June 18, 2012 12:23AM Registered: 5 years ago Posts: 1,226

If you have the later carbs with the vacuum port on the side the PCV is unnecessary.

The PCV is needed if it is connected to the manifold as per the factory 1968 setup. That is manifold vacuum and any backfire should be closed off from the crankcase by a PCV valve. This is to ensure that the flow is only from the crankcase and the fire in a backfire in the manifold cannot ignite anything in the crankcase.

The vaccum tap on the later carb is at venturi vacuum. This is not as high as manifold vacuum and, with the SU carb, it is a constant vacuum. Thus there is always going to be vacuum on the crankcase and the throttle separates a backfire from the crankcase.

If you have the later carbs I would use them. I like the spring-loaded needles and the constant vacuum tap. And not needing the PCV is a plus.

C Ya,
Mark

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Re: PCV valve vs ported carburettor #5
kbl444 Terry Malloch Geelong, Australia	Posted: June 18, 2012 05:49AM Registered: 2 years ago Posts: 42

Thanks for your comments.
I couldn't understand why the varying vacuum in the carburetor didn't draw varying fume flows and upset the mixture.
Marks comment that the SU carburetor gives constant vacuum shows how this doesn't happen.
Terry

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Re: PCV valve vs ported carburettor #6
Big6Mark Mark Miller Springfield, OR, USA 1960 Austin-Healey 3000 1968 MG MGC "Red Tail" 1968 MG MGC 1968 MG MGC GT "The Wreck" 1969 MG MGC GT "The Lump"	Posted: June 18, 2012 09:54PM Registered: 5 years ago Posts: 1,226

Probably should include that the inlet at the oil cap on the valve cover is a set size to regulate the air flow through the crankcase. This metered amount of air is compensated for in the needle size in the carbs. As Larry mentioned this may be upset if the hole at the cap is plugged or altered (as in changing to an alloy valve cover) so keep it as it is from the factory.

C Ya,
Mark

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Re: PCV valve vs ported carburettor #7
m.s.macrae Murray MacRae New Zealand	Posted: June 19, 2012 02:30AM Registered: 2 years ago Posts: 143

Was Surprised to hear that the PVC is NLA I have purchased replacement diaframs (no larfing about my spelling) ! in recent years I think the BMC valve might not be engine capacity dependent?

Mark any thoughts on this? And don't ask me where i purchased them I am an old man at least in remembering this fact smiling smiley

one I think was from a moss dealer on line the other from an MG place in UK. but dont shoot me smiling smiley

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Re: PCV valve vs ported carburettor #8
Swamperca Swamper CA Nor Cal, USA 1969 MG MGB GT "Rat" 1969 MG MGC GT "A 69 MGC" 1970 MG MGB GT "Widow" 1971 MG MGB "Ruby" 1974 MG MGB "Groovy B"	Posted: June 20, 2012 08:59AM Registered: 8 years ago Posts: 7,660

Gary has the good idea for still using the PVC.

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Re: PCV valve vs ported carburettor #9
trymes Tom Rymes Concord, NH, USA 1959 Riley 1.5 1969 MG MGC GT ~ For Sale ! ~ 1971 Jaguar XJ6 Series 1 1971 MG MGB GT 1972 Chevrolet C20	Posted: June 22, 2012 02:08PM Registered: 4 years ago Posts: 3,259

Here is my opinion on this subject, gained after buying a car (the red one) that came with both a PCV valve AND the '69 carbs.

If you have '68 style carbs, go with the PCV valve (either original style or aftermarket ala Gary H).
If you have '69 style carbs, eliminate the PCV valve.
If you are not using either solution, you really ought to be. There are effectively no downsides and many upsides.

Another tip from Jimmy H. was to set the mixture with the PCV inlets in the carbs plugged and then reconnect the hoses.

Tom

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