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MG MGB Technical - US MGB '74 & half Emission stuff

I am currently doing some work for a neighbor on their '74 1/2 MGB.

Someone at some time has (sensibly) removed the Gulp valve and all that powersapping stuff from the car (must go faster just having got rid of the junk) but they have left the stub pipe that comes out of the inlet manifold venting to the atmosphere!

Now, having a '63 UK that I imported from the UK, all this stuff is 'alien' to me, but I am thinking to myself that this cannot be good to have just an open unblocked pipe.

Should I cap this off with a closer?

Thanks

Paul
Paul Barrow

Naturally, no vacuum lines on the intake manifold should be left open. Vacuum ports that wind up being 'extra' should have vacuum plugs put on them. If that big stub pipe is wide open I can't imagine the car even being able to idle, so surely that's plugged already...?
Sam

Hi Sam

I would have thought so too, but stranger things happen! I will investigate some more tomorrow and if not will certainly block it off.

Thanks for confirming my thoughts.

Paul
Paul Barrow

>all that powersapping stuff from the car (must go faster just having got rid of the junk)

For the record, no, it won't. I've driven both ways (my B had no emissions gear installed when I bought it; I replaced it all). With everything in good repair, you won't be able to tell the difference from the driver's seats.

The gulp valve cannot affect performance -- it only operates with the throttle closed. It helps prevent the mixture going rich when the throttle snaps closed.
The air pump will consume a little bit of power, but as I said it's nothing you'll be able to feel. Some say it does nothing more than dilute the exhaust to fool the sniffer; the official line is that it adds a little oxygen to the exhaust stream to allow combustion to complete in order to reduce unburned HCs. I think if it was the former the car makers would never have been allowed to get away with such an obvious dodge.
The evaporative controls have absolutely no effect on performance and do a good job of trapping smog-causing unburned HCs.

IMO, there's no legitimate reason to remove emissions controls (never mind that it's also illegal...)

Cheers!
Rob Edwards

Hi Rob

Thanks - that is interesting. I was always given to believe that a) the power (bhp) output of the 1798cc engine was severely impared by the addition of all that stuff, contributing to the demise of the MGB in appeal terms to the US market, and b) that once a car reached vintage age it could be customized to suit the requirements of the owner. In WA USA there are no safety or emmission tests on vehicles, and that is very evident from some of the wrecks that you see running around here! If it were/is illegal to remove this stuff, the WA State Patrol have many more pressing calls to attend to before they begin to get too upset about the odd MG pottering around without its 'clean air' kit - plenty of US metal falling off vehicles instead, but I take your point with interest and thanks for your contribution to the debate.

Regards

Paul
Paul Barrow

FWIW you can often mal-adjust the carbs to compensate for a vacuum leak at idle, but you will end up being way over-rich when running.
Paul Hunt 2

Hi Paul (B),
The power did drop significantly on later Bs, but that is the fault of the horrible combined intake/exhaust manifold used on the single-carb models (and the ZS carb itself), and changes to the ignition advance curve. AFAIK, the cam was the same (or very nearly so) over the entire production run, but on ZS cars, it was advanced several degrees to boost mid-range power to compensate for the loss due to the other changes, but of course that mid-range boost came at the expense of the top end.

Power did decline very slightly over the run of twin-SU equipped cars, but only very slightly (and again, less than the average person will ever notice). Any loss can be easily compensated for by making sure the tires are properly inflated and by shifting a little later! ;-)

And yes, I agree there's a difference between legality and enforcement. ;-) It seems many states have stopped checking (NC among them), but the Federal law is still on the books....

Cheers!
Rob Edwards

Paul B. Yes, any vacuum leaks at the intake manifold should be plugged. The "evaporative loss control system", i.e. the charcoal cannisters and their connections, must be left in place. If not, the carbs and the fuel tank are being vented to the atmosphere under the hood. Not a big thing with the carbs as they vent a very small amount of fumes. A rather large thing with the fuel tank though. A significant amount of fumes can be exhausted into the engine compartment if the connection from the fuel tank is just left disconnected. A hot day, causing the tank to vent as the heat expands the fuel and vaporizes it, and a spark plug lead with a little insulation breakdown could make for a very interesting condition. But, not from the point of the person trying to start the engine as the explosion goes off.

As to the air pump and gulp valve, I do not like them. The air rail and pump get in the way of easy access to the spark plugs and alternator. This is undesirable, but may be acceptable if they do anything of value.

If the air pump actually adds fresh air to the still burning exhaust gases, causing them to continue to burn during their passage through the exhaust system, we should be able to document this by the observed temperatures within the exhaust system. I have not been able to document that a vehicle without a catalytic converter demonstrates a higher exhaust system temperature when the air pump is connected than when it is disconnected. But, I only have access to a hand held, infa-red thermometer to do my experiments with. Laboratory grade test equipment might demonstrate a small temperature increase due to continued burning of the exhausted gases. I suspect, however, any increase would be very small or it would have been demonstrated under my less sophisticated testing.

The gulp valve is an interesting piece of engineering. Its purpose is to open a channel between the air pump (thus diverting some of the air being pumped to the cylinder head) into the intake manifold. When the mixture flows through the intake, small droplets of fuel condense out, especially when the intake manifold is not yet fully warmed. When the throttle is closed, the increased vacuum causes the droplets to be sucked into the cylinders resulting in a richer mixture than normal. Under these circumstances, the gulp valve is opened and provides extra air to lean out the overly rich mixture. But, not in any really controlled fashion. On my two cars, and my daughter's car, we had to plug off the gulp valve, to meet the emissions testing requirements. The uncontrolled input of the air caused a spike, due to an excessively lean mixture, in the unburned hydrocarbons after the throttle was abruptly closed. Plugging the line between the gulp valve and the intake manifold actually improved the emissions testing results quite significantly.

When a catalytic converter is installed into the exhaust system, the addition of additional air input makes some sense. It assists the cat in burning the small residue of unburned hydrocarbons and the results can be documented with the IR thermometer. Slightly higher exhaust temps behind the cat with the air pump connected than when it is disconnected and the hose to the cylinder head blocked off. But, a well tuned engine will pass the original emissions requirements without the air pump in place, so it becomes a personal choice matter to me on whether to remove the air pump on cars equipped with cats (75 model year onwards in California and 76-80 cars in the "49 States" version).

Les
Les Bengtson

Subsequent investigation has shown the pipe to have been superficially blocked with a pad of material. I will make sure, before it goes back, that it is properly sealed up.

Thanks everyone for your help.

Paul
Paul Barrow

This thread was discussed between 17/07/2007 and 18/07/2007

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