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MG MGB Technical - '80 MGB Engine Breathing / PVC

Hi Guys,

Thanks to all of you guys, my 1980 MGB is BACK ON THE ROAD!! I have just finished doing a Weber carb. conversion and a Rivergate 5 speed trans. conversion.

However, I now have another question!! While cleaning up the engine, I installed an aluminum valve cover. It is a sealed cover with NO breathing tube. It uses my original 1980 oil cap. The car has the standard breathing tube coming from the side tappet cover, which is vented into a vaccum pull off on the intake. Is this suficent? Does there need to be anything allowing the valve cover to breath?

Thanks again to EVERYONE for all the help you've already given over the past month or so.

Danny
Danny T.

Danny. Glad you are back on the road, but do not drive the car too much until you do something to the valve cover. The factory valve cover has a 90 deg elbow with a "restrictor", in it. (Basically a closed over end with a small hole to allow air to enter.) This is attached to one of the charcoal cannisters and forms the "input" side of the PCV system. The older cars used a vented cap for this purpose, but the later ones used this the fitting on the valve cover. You are going to need to either use a vented cap, if available, or hook an elbow up to the rear of the alloy valve cover, then link it to the charcoal cannister. This latter would be my suggestion as the charcoal canister also forms part of the fuel tank venting system. Les
Les Bengtson

Check out this page at Doug Jackson's website.
He discusses alternative breathing schemes for the
later B's. The crankcase does need to breathe
in -- this is accomplished via the carbon cannister
line connected to the elbow on the original
valve cover.

http://www.mgbmga.com/tech/mgb3.htm

I _think_ you should be able to drill a hole for
the K&N adaptor in your existing valve cover, and
connect it via a restrictor to the carbon cannister
line. Restrictors can be made using 1/2" aluminum
rod, cut to a 1/2" or so length, with a suitable
hole drilled in the center. The restrictor then
fits in a 1/2" PCV hose from the adaptor to the
carbon cannister line.

For a temporary fix, you can use the older vented
B oil cap as described on the same webpage.
Ronald

If the tank is still connected to the charcoal canister then it doesn't really matter that the canister isn't connected to the rocker cover as the tank will still vent as normal, but the emissions effect of the arrangement will eventually be lost as 'clean' air is no longer being drawn through the charcoal by the engine to scavenge it it of vapours. For crankcase breathing just fit a vented oil filler cap. What may be more important without the connection between rocker cover and canister is the loss of the anti-runon valve functionality. If you retro-fit the rocker cover port it must have a restrictor as mentioned, this is a hole just 1 or 2 mm in dia. The anti-runon valve also needs a small-bore restricted connection to the inlet manifold, and a pipe from the canister to the float chamber overflow if it is to function correctly. Are you sure you need the PCV on the inlet manifold? Isn't there a restricted breather port on the carb? What are you using for vacuum advance?
Paul Hunt

Hi Paul,

No, there is no access for the PCV tube on the carb itself. There IS a hole in the bottom of the air filter where the tube could be connected. However, there seems to be little or no suction this way, where as connecting to the intake provides a solid constant suction. Currently, the vaccum advance for the distributor is NOT connected. It seems to run well as is. If I need to connect the vaccum advance, where should I connect it? I had thought about connecting IT to the intake and using the air filter for the PCV. What's you thoughts?

DT
Danny T.

The breather port on the original SU air filter was used on the first couple of years cars only and was a non-positive or open-circuit breathing system that was a bit hit and miss. It also had a 'draft tube' connected to the front tappet chest or front cover. I think which way the fumes flowed through each depended on whether the car was moving or not and how fast, hence the hit and miss aspect. Vacuum advance should give better cruising economy and lower running temps. Without a port on the carb, which is generally shut off at idle and only comes into play as the throttle is opened, you would have to use an inlet manifold port. This could be teed with an existing take off but preferable one that doesn't have any flow like the brake booster. The PCV has some flow, but when working properly shouldn't be much. Close to the manifold itself would be best. With manifold vacuum you have to set the timing with the vacuum advance pipe disconnected and plugged at the manifold end, this might need you to increase the idle revs slightly to prevent stalling. Reconnect the vacuum port and reset the idle after setting the timing.
Paul Hunt

OK...I have just been thru this. My 78 B has the same setup with the 38/38 carb. I WOULD use a vented cap for PCV 'intake'. Your weber inlet manifold should have a brass union on the upper right side which I connected to vacuum advance for the distributor and I also Tee'd in a vacuum guage to the dashboard, ( instead of the clock which didn't work anyway). I sealed the carb. manifold distributor advance outlet. The Tappet chest cover, (front) vents to the carb where it's intake creates ample suction to positively vent the crankcase which is what you want. The reason I took dist advance from the manifold is that my low compression engine needs as much advance as possible for any decent performance, the carb take off seemed insufficient.
Pete
78 B
P J KELLY

This thread was discussed between 31/12/2003 and 02/01/2004

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