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MG MGB Technical - PCV versus revs problem.

I have just got the engine running after a 12 month lay off. When I got it the rubber diaphragm in the PCV was ripped (it is the 'Mushroom' shaped PCV) which meant I couldn't tune the engine mixture wise, so I can't say if the problem existed at that time.

I got a new diaphragm and now the problems appear. When I connected the pipe from the side cover to the valve the engine was running fine considering I hadn't tuned it at that time.

I then took the pipe of to see if there was any significant amount of crank case fumes evident. All appeared OK. When I did this the revs increased considerably, from about 1000 RPM with the pipe on to about 2500 RPM with it off. If I then put my finger over the PCV inlet and block it the engine dies.

I assume blocking the PCV totally would cause problems as it alters the mixture, but are the other indications normal? In case it has any bearing on the problem, I cant turn the jets up far enough to lean the mixture out. It seems to be running rich all the time. This is with or without the pipe being connected.

Thanks
Tony
Tony Oliver

Tony, You don't say which valve cover your are using. The stock unit for '62 to '67 should have an oil filler cap with a small orifice that allows air to enter the intake manifold along with the blowby gases. If you have a different cap it will greatly affect the idle when the PCV system is functioning normally. Ray
RAY

Thanks Ray. At the moment I am running without a cap on the cover. In fact I was running with the cover off for a few minutes as I wanted to make sure oil was getting to the valves. (I'm a bit paranoid after a problem with a Mini some years ago. Oil wasn't getting to the top of the motor.)

Would this affect the operation? I am obviously dragging huge amounts of air into the system with no cap to restrict it.

Tony
Tony Oliver

Tony, At present you are drawing huge amounts of air into the intake manifold making it impossible to get an effective idle. The valve cover and vented cap must be in place to set the idle and mixture properly. To test run the engine without the valve cover, you must temporally block off the PCV line leading to the intake manifold. Ray
RAY

Thanks for your help Ray. It seems kinda logical now I think about it. My knowledge of PCV systems is very limited. I assumed it was the valve that restricted the amount of air injected into the manifold. As soon as I repair the fuel pump I'll give it a go.

Cheers
Tony
Tony Oliver

This thread was discussed between 19/05/2007 and 21/05/2007

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