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MG Midget and Sprite Technical - Charcoal Canister

Getting somewhere with the rebuild of my 1971 1275 midget. It had an air-pump which I ditched but I'd like to keep the evap loss system. I've read lots of posts on this BBS about changing the charcoal but my unit seems to be sealed apart from the very bottom filter - no access to the charcoal. Are there any options/tricks here?

Also I might be missing some pipework... The air vent at the base of the canister had a piece of what looks like heater hose about a foot long attached to it - where does that go? Does it connect to anything?

The pipe from the gas tank to the engine bay ends up poking up by the chassis leg pointing towards the front of the car. This seems obtuse - have I got it misrouted somehow? And how does this then connect to the canister? There is a very short flexible hose coming out of the canister that should somehow connect but why are the hose and the pipe so far apart? And what goes in between?

Simon Fryer

On my 73, the charcoal canister is connected to the overflow hose from the carbs. So if the carbs flood (which they will, i have found, if a float valve gets stuck), the excess gas goes directly to the charcoal cannister to be absorbed, and if you overfill what the charcoal can absorb... it runs out through that bottom hose onto the ground! (maybe not it's actual function... but how it works). The charcoal cannister as I understand it is just to prevent, as much as possible, gas fumes from getting out into the air.

the pipe from teh fuel tank that comes up at the front of the car is the feed for the carburetors. the charcoal cannister is downstream of the carburetors.

on the 73 (not sure if this applies to the 71 or not) there is a seperate evap recovery tank in the boot (on the right wing). As I understand it, this just catches any vapor from the gas tank, and allows it a place to condense and run back into the tank.

I hope that helps?
Chris Edwards

I think that the catch tank in the boot is intended to collect liquid fuel drawn into the vapor line due to overfilling. That is, if you top off the fuel into the filler neck, the level is then above the pickup point for the vapor line, and raw fuel will be pulled into the vapor recovery system... but it will only go as far as the catch tank.

And Chris has got it - the charcoal canister (or two for later 1500s) is intended to collect vapor only, not liquid fuel.

Simon - were you able to unscrew the bottom of the charcoal canister? At least that's how the later ones can be opened.

Gryf Ketcherside

here is a close up glamor shot of my new lines that shows the evap line from the tank.

It is the one that comes up under the battery tray, and goes behind the ignition coil bracket (note: this is a LHD car, so the clutch line runs by, underneath it, where it bends over horizontally).

Sorry that this angle isn't great to answer your question, but this vapor line doubles back to the right (behind the ignition coil bracket in this photo), and then bends upward (between the starter solenoid and the charcoal canister bracket) to be connected to the charcoal canister. That end of this hose has a black rubber plug in my photo.

The OTHER line from the tank comes from the fuel pump, and is maybe the one you mentioned "poking up at the chassis leg", that Chris pointed out is the fuel pipe. You can see it in my photo right at the bottom, crossing over to the LH side of the engine room.

The large diameter rubber hose you mentioned, coming out of the bottom of your charcoal canister, is the air intake, and it just points toward the ground (it routes just rearward of the ignition coil bracket shown in my photo). As Chris mentioned, if your carbs flood, soaking your charcoal, the excess will run out this hose. If that happens, you ought to replace the charcoal as it will be saturated and won't work as well as fresh stuff would (after you sort out why your carbs flooded).

To replace the charcoal, you unscrew that bottom cover, exposing the filter (as you stated), then the next part beyond the filter is a perforated 1/4 turn piece that can be removed, exposing the charcoal. An old canister might be sticky, making it tough to shift this piece.

At the top of your canister, there are three nipples. The two outer ones are "intake", one attaches to the fuel tank vapor line we were talking about, and the other attaches to the carburetor float bowl vent tubes (there ought to be a steel line that runs from them to a clamp near your heater blower motor). The middle nipple connects to your valve cover (see this link, page 320):

The vapor separator in the boot is as Gryf said, to prevent liquid from flowing up the vapor line.

hope this helps,

Norm Kerr

thanks for your comments guys.

As I understand it (and I don't really) the evap system is intended to be a closed loop. The unit in the boot captures vapour from fuel expansion in the tank and allows it to condense and return. The charcoal canister traps vapour from the carbs, and crank case (via the rocker cover connection) processes it through the charcoal (and returns the product to the gas tank?). Is the vent at the bottom an air intake rather then a drain..? It seems like a surprisingly large bore hole for such a small purpose...

There are two pipes running from the fuel tank forwards on the car. One does indeed carry fuel to the carbs and is on the driver's side. The other runs up the passenger side and I'm pretty sure, is part of the evap system. But what is it actually doing and how is it connected..?

Gryf: yes I have undone the bottom of the canister and removed it and the cloth filter but that seems to be all that can be done. I can't find a way to get to the charcoal.
Simon Fryer

you and I posted at about the same time!

the second one that you are pretty sure is part of the evap system is the one I was talking about.

The reason why that air intake is so large diameter is because, well, actually, I am not sure why they made it so big. But the metering device for how much air is drawn into your carburetors is the 9/64" hole in the end of the tube on the rocker cover that the middle nipple is connected to.

The vapors from the fuel tank and carburetor are drawn from the canister, into the engine through that tube, and that happens because of the pipe connected from your carburetors to the front cover. The vacuum from the intake draws the vapors in, and also helps a little bit against engine oil leaks.

Norm Kerr

>>> (and returns the product to the gas tank?) <<<

Not exactly - any vapors collected by the charcoal, be they from the tank or the float chamber, are drawn into the intake air stream and burned. Norm hinted at this, and he's spot on.

Gryf Ketcherside

Well, foo. I added a bit to the above post, and the BBS wouldn't accept the update. Here 'tis -

While the engine's running, there will be a steady flow of air through the charcoal canister toward the engine, due to the vacuum line that runs to the carbs. This air flow is what pulls fuel vapors from the charcoal into the mixture. When the engine's off and the airflow stops, vapors generated by fuel in the tank and carb find their way to the charcoal and are adsorbed.

Used to be, fuel vapors in the float chamber or tank simply vented to the atmosphere, and the vapor recovery system is a simple and effective prevention for that. Usually the system just does its thing unobtrusively, but let the float chamber overflow, and problems start... the charcoal canister fills with fuel, blocking the airflow to the carb or intake manifold, and raw fuel can then be pumped right up through the jet(s) and swallowed by the engine. Ewww. Been there.

Gryf Ketcherside

Thanks everyone - think I can put this together now (famous last words). Norm - the photo of your car is humbling. What can I say..?

Simon Fryer

My Midget is the first one I have ever seen close up and I purchased it almost completely disassembled or like I often say, with some assembly required.

Because of threads like this I feel there is a chance I will someday have it back together at least somewhat correctly.

Thanks to you folks.

follow up:

1) that pretty midget only took me 20 years (here is a "before" picture). I rebuilt it mechanically and drove it until the rust got too far along. Then I had a really good body guy, and then a really good painter work their magic (I can take no credit for the wonderfulness of their work). Being a mechanic, and as Dirty Harry told his boss, "A man's got to know his own limits.", I would never have tried to do what they did!

My hat is off to you folks doing body work and painting yourselves.

2) the vapor recovery system is not only simple, it is amazingly effective. 40% of the hydrocarbons exhausted by a car are the vapors it gasses out while just sitting around, this charcoal canister eliminates 99% of that.


Norm Kerr

This thread was discussed between 27/07/2010 and 28/07/2010

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