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MG MGB Technical - Vacum port on SU Carb

This is probably a stupid question but what should this port be attached to? I can not find anything that looks like a vacum hose should run to. I see in other pictures of SU carbs that the hoses from these ports connect and then go somewhere. But I don't know where.

David Whetsel

Its the overflow outlet. There should be a rubber pipe then metal pipe attached to it and then run down to the front engine mount where any overflow of fuel can escape. there should also be one from the other carb.
Trevor Harvey

I thought it was a vacuum line due to when I put my finger on the one on the back carb the engine slows down and dies. It doesn't do that on the front one. What would cause this to happen then?
David Whetsel

Is the tube on the rear carb in the same location as the one you point to on the front carb?
Sandy
SANDY SANDERS

It is primarily called the "fuel bowl vent", secondarily "overflow". When you plug it you are stopping air from getting in, which stops fuel getting out of the bowl into the engine. The principle is commonly used as an anti run-on preventer.
If only one carb acts that way, then the engine is only running on one carb. It's probably just at idle because of setup errors, but could be at all speeds. One reason for a carb to be nonfunctional is a mudwasp built a nest in that vent!
And please throw those aircleaners far away. See the current thread on the TD board re "1.5" air cleaner". Especially since the missing cover bolt may mean that the internal spacer is about to, or has already, get sucked into the engine.

FRM
FR Millmore

And air is bypassing the filters and going directly into the engine through the resulting hole.

Tape the hole over as a short term fix.

Pete.
Peter Thomas

As Fletcher says it is primarily a vent. When you first switch on the engine and the float chamber fills it allows the fuel level and hence the float to rise and shut the float valve when full. With the vent blocked the pump keeps going and pumps fuel up the jets and down the throat.

American cars with anti-run valves use it to apply a small vacuum to which lowers the level of fuel in the jet and instantly stops the engine.

Otherwise it allows fuel to escape when the float valve sticks open or float sinks. And for that reason there should definitely be pipes on them taking any overflow down past the exhaust, if they aren't plumbed to a charcoal canister.

Momentarily blocking an overflow with the engine running shouldn't do anything as the fuel level should be relatively constant, although I suppose if you catch it just as the float valve has opened it could cause the pump to keep going and flood that carb. Sounds unlikely though, although I haven't tried it. Could be that carb doesn't have any fuel in it and the engine is sucking air in through the overflow until you block it! Whatever, both carbs should always be the same.
P Hunt

The primary purpose of the vent is to allow air in so fuel can go out through the jet - that's why the anti run-on system works. If you block it with your finger, you cause it to build a vacuum = no fuel. If the pump comes on then, it will get fuel, but the float will soon shut the valve so the pump stops, and you get vacuum again = no go.
As stated, mudwasp = dead motor!

FRM
FR Millmore

This thread was discussed between 22/12/2009 and 23/12/2009

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