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MG MGB Technical - Ported /Manifold vacuum

I'm a bit confused about where ported and manifold vacuum take offs are sited. I read about above and below the throttle valve but I'm a bit slow on the uptake!
Just where are these positions?
I have a flame thrower dissy on my B and a LYNX manifold with 1/34 SUs waiting to be fitted permanently. Have tried the manifold briefly with really good results. The manifold only has a large take off for the brake vacuum (which I don't have) and the carbies don't appear to have any tappings at all for vacuum.
As an aside the manifold doesn't have a port for the PCV either, which to me is a bit of a worry not to connect.It does have a blank at the other end to the brake take off and I thought of joining the two and putting the PCV in with a "T" piece.
Looking forward to your collective and informative replys.

Trackless
J Millsom

Hello,

The easiest route is to fit a PCV valve with that large diaphragm at large take off for brake booster. For carburetter vacuum these replies may help you:


http://www.mgexperience.net/archive/Ported_Vacuum_for_HIF_s/1414014

Cheers,

Jean G.
Jean Guy Catford

Either manifold or ported works ok on the B,and some go to the trouble of drilling a carburettor to fit a small barb but its a lot easier to drill and tap the manifold. Dont use a T as the PCV flux. will affect the vac advance. Use the large one as Jean said for the PCV. The car will idle faster with Manifold and need adjusting down. Its also a lot more important to remove the hose when using a timing light. Denis
D M HILL

Thanks for the comments.
I was afraid that if I used the large port for the PCV it would lean out the 2 cyclinders that the manifold arm fed. The port is right above the inlet as I remember.
Am at work in PNG at the moment so working from memory which at my age can be average.
The engine is around 2000 and a bit ccs with VW pistons. It will pull 6500+ with the big carbs on it which can be a little terrifying but it keeps one young!
Denis may I have a phone # or email please if you don't mind.

Thanks

John
J Millsom

The manifold feeds all four cylinders from both carbs to a greater or lesser degree, it is not split into two separate sections. There should always be a filtered and restricted fresh air inlet to the rocker cover with positive crankcase ventilation to prevent the mixture being leaned. This was originally in the oil filler cap for both PCV and carb ventilation systems, changing to a restriction in the port on the back of the rocker cover and a filter in the carbon canister for cars with a canister. The PCV was only used from Feb 64 to Oct 68, before that ventilation was iffy, after that carb suction was used, with the same filtering and restriction as before. Fixed choke carbs usually don't have a ventilation port like the SUs from Oct 68, so you have to retrofit a PCV. If you have the canister and remove it you also have to do something about the inlet, either sealing the port on the back of the rocker cover and using the earlier ventilated cap, or putting a small filter on the rocker cover port, which makes more sense as it is obvious what has been done and no one will fit a non-ventilated cap not realising it should have a ventilated, as the two are very similar eternally.

The brake booster port on the inlet manifold contains a one-way valve to hold the vacuum in the servo if the engine should stop while moving to give you two or three assisted pedal presses. Try it to see, when stationary! You will hear the servo clanking and hissing, reducing over a few presses, and the pedal getting higher and harder. If you Tee a PCV into the servo hose you will lose that vacuum immediately the engine stops, which particularly on the later master with integral servo will make the brakes significantly heavier.
PaulH Solihull

Paul-
He has just the blanking plug in the manifold.
"The manifold only has a large take off for the brake vacuum (which I don't have)"

John-
Most manifolds have bosses drilled or to be drilled for 5/16-24 threads for vac advance or other small vacuum requirements. Sometimes the holes are used for bolts securing throttle linkage, heat shields etc.

Some carbs are drilled for distributor vac but blocked off, and some are not drilled but could be. Some carbs also have the vac for PCV not drilled but present.

Maybe some pics of your carbs and manifold are in order.

FRM
FR Millmore

Thanks once again.
I'm still at work in PNG at the moment so the piccies will be awhile, don't think my wife will go to the shed for me!
To clarify a point or two.My manifold does not have a one way valve in it and I always thought that the valve was in the vacuum unit in case the hose broke.
I understand that most manifolds have a balance tube. My point was that if the PCV is fitted directly over the head inlet area then I thought it would have to lean out the mixture for those cyclinders. I'm no flow expert but it seems logical to me and I thought that was why the original manifold had the PCV inlet in the middle of the balance tube.
There is a great article on B engines which identifies the need to keep the PCV in the system.
I don't have assist on my car and was only using the brake take-off to get a vacuum source.
Thanks again for all the replys,I have found there is always a better way to do things and many ways to do things I've not thought of.

John
J Millsom

"He has just the blanking plug in the manifold.
"The manifold only has a large take off for the brake vacuum (which I don't have)"

I know, I was commenting on the recommendation to use the servo port, which some might take as a recommendation to Tee into it.
PaulH Solihull

This thread was discussed between 13/04/2012 and 16/04/2012

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