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MG MGB Technical - Crankcase Emissions - Again!

This is my first post - here goes.
I have a '74 GT that is basically stock except that it has had the air pump, gulp valve, etc. removed. It does have, however, the adsorbsion filter with its connections to fuel tank, HIF4 fuel overflow and valve cover (restricted to <1/8" orfice). It seems to be working OK, i.e. the engine doesn't seem seem to be leaking (blowing out) as much oil. But, when I put a vacuum guage on the carb suction pipes that connect to the vented tappet cover it shows "0" reading at idle and 1-2" Hg. at 2500 rpm. Is this right?
My second question is that my intake manifold only has one tapped barb fitting. Shouldn't I have 2? One for the distributor and another for the run-on valve? The ROV wasn't connected to anything, so I "T"d it into the distributor vacuum line. Of course, if the ROV fails in the open position the distributor has reduced advance.

A photo is attached.

Thanks in advance for your thoughts. Paul

P Simeone

Hi Paul,

There should be a small take off spigot underneath the rear carby, for the vacuum advance.


Herb Adler

The breather vent on the HIF is placed in the carburetor so as not to effect the air/fuel mixture at varying air flows. They dont draw a lot but its enough. I dont know about the spigot as we didn't get pollution equip cars here, but ours were as Herb shows

You have HIF carbs which do not have a vacuum connections at the carb. The vacuum connection is on the intake manifold for the distributor advance. It looks like your carb overflow pipe is below the carbs. This is usually above the carbs with a connection to the carbon cannister. If you are running it to atmosphere make sure it is directed away from the exhaust manifold.
Joe G
J. E. Guthrie

Herb and Denis;
Thanks for the info. I checked the bottom of both my carbs. The rear casting has a small raised circle on the bottom, however, it has not been drilled or tapped. Wrong manifold? Originally, no vacuum advance?
P Simeone

There should be several 1/2" head bolts on top of that manifold. Two of them should be tube connectors instead, one for advance and one for AROV. Threads are 5/16-24 UNF. Hard to see in your pic, almost looks like the holes have been not machined or welded up, very odd since all US cars would have had the front two bosses tapped 5/16-24 to hold the gulp valve and the center boss drilled for a 1/2" elbow for the gulp valve/air pump air supply. The horizontal ear with a hole in it on the front of the manifold is also strange. But a tee in one vac pipe will work just fine.

FR Millmore

Thank you all for your input. When I next have the manifold off, I'll tap it for a second hose barb. Yes, my carb float overflows hang beneath the air filters and athen connect to the adsorbsion canister.
Regards, Paul
P Simeone

This thread was discussed between 21/04/2011 and 03/05/2011

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