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MG MGB Technical - Gas Vapor Collector & Carbon Canister Removal

I was in a discussion the other day at a gathering of British car folk, and I mentioned I ocassionally had gas fume odor when opening the trunk. I further added that I was going to replace the hoses in the trunk that went to the vapor collector.

It was suggested that since my 72B already had the bulk of the emissions control devices removed by previous owner, including the Gulp valve, I may as well by pass the carbon canister and vapor collector too.

Presently the tube coming from the side of engine that would have gone to the Gulp valve, is connected to a hose that is simply venting to the ground.

Also, the bottom hose coming from the base of the carbon canister is venting to the ground.

The center hose on the top of the carbon canister is connect to the tube on the valve cover.

The suggestion is to take the hose that vents the gas tank to vapor collector and simply run it through a hole in the trunk and directly vent it to open air below the car. I assume I'd have to plug the hose going to the collector to close the system.

Yes, I realize all this is adding to global warming, etc., but when getting < 20 mpg, this venting isn't adding much.

My question is, I can understand the gas tank vapor hose being vented directly to ground instead of vapor collector, but am I going too far? Those suggesting the change have their cars this way.

But how does all this affect the line running from the float bowls to the carbon filter, and how does it affect the air supposedly moving from the valve cover to the top of the carbon filter?

For some reason I stare at these suggested alterations and wonder how the valve cover and float bowls will be vented.

I stare at all these lines & hoses and wonder if because the bottom hose in the carbon filter is directly venting to ground already that the vapor movement in float bowl line and the hose from the valve cover are already compromised. My thought is that nothing may be moving in these two lines already.

More free advise since the bypass suggestion, is to get a T connection installed that accepts both the hose that vents from the side cover on engine and the hose that comes from the valve cover to a new PCV valve that is connected to the either the original Gulp valve location (which is presently plugged with a brass plug), or the vacumm port next to it on the intake manifold.

Does any of this make sense? What can of worms am I heading into with all this free advise. Yes, this is all coming from MG owners.

I'm still in the "stare at it" mode.

Someday someone else may get this car and open the hood only to bemoan my efforts as a PO.

Thanks for your patience and any response.

Your thoughts?

BobA

R.W Anderson

The canister and it's immediate plumbing has no effect on performance but does help to keep fuel fumes out of the atmosphere. As such the only benefit in removing it is to gain some space in the engine compartment.

Isn't the gulp valve originally plumbed to a port (originally the vacuum advance port) on the rear carb? If it is that hose that has been disconnected from the valve and left hanging that will be introducing a significant air leak to the inlet manifold.

The hose on the front tappet chest cover, if that is what you mean, would normally go to the breather port on the carbs. With that disconnected there is no scavenging of the crankcase so you can get condensation and corrosion, and unless the carb ports have been sealed that is another source of a vacuum leak.

You seem to be asking if both the rocker cover and tappet chest covers should be teed toghether and connected directly to the inlet manifold, which definitely should NOT be done. It will place a large vacuum on the crankcase which is very likely to cause high oil consumption as well as suck dirt past seals, and that will be the only crankcase ventilation.

The bottom hose of the canister being open to atmosphere is correct, that is the fresh air inlet to purge the canister and breathing for the crankcase.

If you disconnect the canister you need to do something about the tank venting. Any open port in the engine compartment, trunk or underbody is likely to cause fumes, the tank port should really be sealed and a vented filler cap fitted. That will vent fumes of course, but they won't get trapped by the body.

The carb float chamber ports/overflows will need piping down past the engine, particularly the exhaust. There needs to be a free flow of air to these or the float cannot rise to shut off fuel.

The hose between canister and rocker cover supplies air from the canister to the crankcase for breathing, not the other way round. You should fit a small filter on the port at the back of the rocker cover, which will act as the fresh-air inlet to the crankcase, and ensure the carbs are sucking on the front tappet chest cover to ventilate the crankcase.

Paul Hunt 2010

Thanks very much for the response Paul.

Now on to clarifications.

I have no loose vacumm hose from former Gulp valve, it must be plugged off, but I'll look at the hoses again tonight or tomorrow.

I will NOT install the suggested T.

As I understand your explanation, air is entering the carbon canister from the hose on bottom of canister, which then provides air to the valve cover tube, of which all should exit by way of the breather port of carbs.

I'll look again to confirm but I think there is a tube coming from carbs that goes to top of carbon canister. Your explantion suggests that this line from the canister to carbs should be drawing air from the canister.

Your explanation also suggests that the tube & hose coming out of the front tappet chest cover should not be open to the air as an exit point for air, but should actually have a vacumm on it drawing air out and back up to carbs?

At least for now I know NOT to T any of this. THANKS.

More after next inspection.
R.W Anderson

Hi Bob,

The carb float chambers were connected to the top of the canister. As the float chambers are filled by the pump at switch on air is pushed through that hose into the canister, which trap fumes. Likewise the tank vent also connected to the top, fumes from fuel expansion in the tank also being trapped by the canister. Then when the engine is running suction from the larger breather ports on the carbs, sucking on the front tappet chest cover, via the rocker cover and the port on its rear also to the top of the canister. That purges the fumes trapped by the canister as well as venting the crankcase on the way. Fresh air also goes via the canister (from its bottom port) into the tank vent to replace fuel as it is used.

Hope that clarifies,
Paul.
Paul Hunt 2010

Often, the gas fumes may be coming from the fuel filler neck
hose connector.

It's difficult to diagnose. When filling up, fuel will quickly
evaporate and leave things dry. Try wrapping the area
with a collar of paper - then at your next fill up open the
trunk (boot) lid and see if any stains show up while filling.

Daniel Wong

So that we can close this discussion out, I'll provide this update.

I returned as much as possible to original intent last night. Unfortunately for me, I had listened to too much free advise years ago, and had placed a brass plug in the intake manifold where the Gulp valve went and had hoses open to the air in several locations. My original question to this posting was on also disconnecting and by-passing the vapor canister in the trunk.

Well it took 2 hours to get the plug out, and another to clean the brass filings out of the intake manifold, but with patience and grease on Q-tips I got the passage ways clear. And well lubed!

Gulp valve is in. Vapor canister vents to charcoal canister. Float bowls vent to charcoal canister. Fresh air is coming in at base of charcoal canister with hose extending down past engine. Valve cover is connected to charcoal canister and tappet side cover is connected to inlet top on Gulp valve. Thus drawing all the vapors into the valve cover and exiting from side cover hose into intake manifold.

Bottom tube on Gulp valve is plugged, as there is no air pump on car.

Car runs fine, and smells leaner and cleaner.

Now to see if I have gas vapors in the trunk anymore.

Now to get back to the axle seal, and the speedo.
R.W Anderson

This thread was discussed between 21/04/2010 and 04/05/2010

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