You are here:

MG Car Repair/vented gas smell


QUESTION: I have purchase an 1978 mbg last dec 2013, I put it away for the winter time in a garage last year with no problems and drove it this summer 2014. This year I just garaged it and notice some gas smells. I went to check the charcoal canisters and notice that they where removed. There is just a long tube running from vent line where the canisters where to the front and down under(outside) the car behind the radiator. I checked all the gas lines and vent lines in the boot and everything is ok.
The car has a weber carburetor and has been running fine.  Is there anything I could do for the winter storage in the garage. Until I can decide to get the canisters install back on or anything else.  
Would it help if I connected up a charcoal canister or some filter on the vented line where the old
canister where?  Last year I filled the gas tank only half full with fuel additive for water removal. The car just seats there for the winter storage and I don't drive it until summer .

Any suggest would be great


The MG's that had charcoal canisters were there to filter the vapors from the gas tank and from the float chamber of the carburetor. They also routed the vapors from the engine crankcase to these charcoal canisters. (two on some of the MGs) All it took to destroy the canister was for the carburetor to flood and raw fuel would go down the vapor line and that destroys the canister. Also owners would over fill the tank on a cool morning and then park the car in the sun and the heat and expansion of the warmed fuel would flood the vapor line and destroy the canister. Earlier MG's had no vapor lines. The fuel tank just had a vented gas cap. The two US carburetors ran lines from the float chambers down the side of the block venting to the ground under the car. If you are smelling fuel and don't have any leaks it is most likely coming from that oepn fuel tank vapor line. It is not a good idea to run that vapor line to the small air filter supplied with the Weber conversion because as the filter gets dirty there will be a vacuum formed inside the filter and you do not want that vacuum to be transmitted to the fuel tank. Even a minor one pound per square inch vacuum is multipied by the number of square inches of the tank. One PSI of vacuum in the tank will crush the tank but that may not happen because the fuel pump can ony pump from 1.5 to 3 PSI so the engine will quit running with a vacuum in the tank.

BLM struggled to meet emission standards here in the US and all of the mid 70 cars were very poor running so thus aftermarket companies started making intake manifolds with Weber and other carburetors to try to improve the perforamnce. This also upset all the vapor lines.


---------- FOLLOW-UP ----------

QUESTION: Howard, thanks for the quick reply. I was thinking that I would purchase the original canister for the car. And connect the vapor line in the bonnet where the missing canister should be to the new canister and plug or close the other connections on the canister. Would that collect any gas vapors and not create a vacuum because the other connections are closed off. Now I'm only using this as the car is seating there for the winter time,and not ever running, I would remove it if I wanted to start the car.
What are some good ways to check the vapor line leaks? Are the lines stainless steel?

thanks again for the fast reply


As best that I can remember the canister on that year MG was open on the bottom with a screen over the bottom. If so, you can block off the other lines. I don't remember if the Weber has a vent line from the float chamber. If it does you should connect it to the canister also. Some of the MG canisters had a closed bottom and a large tube port at the bottom. This type had a anti-run on valve which would apply manifold vacuum to the canister to cut the fuel supply to the jets via vacuum applied to the float chamber fuel.

Do this, look at the weber to see if there ia any port for the float chamber venting. Then look at the new canister to see if the bottom has a screen in it and/or there is a large hose port at the side of the botom. Also look at the ports on the top of the canister (should be three) and see if they are labeled.

With this info I can probably give you a good connection diagram.


---------- FOLLOW-UP ----------


QUESTION: Howard,   going to order the canister this week, should I order some hose lines also. If so what should I order? Is it vacuum line hose I can get at a local parts store? I looked at the weber carburetor and I didn't see any ports on it. I can sent you pictures of everything if that would be easier.

A little more history on what happen. I put the car in my mother-in-laws garage and a couple of weeks later. Her son said he though he smelled gas in the garage.When I when to over to check it out i though I may have smelled it also. But I noticed the key in starter was turned on to the first position and i could hear the fuel pump running. So i though that may have been the problem that the pump was running.I don't remember leaving it that way. But anyway I removed the car from the garage and ran it for about an hour and left it outside for a couple days.
The smell in the garage when away, so i put the car back in. Now i still smell something but I don't think it's gas. But the car always seem to smell like that once in a while even in my garage.

again thanks a lot for your help


When you get the canister get hose to match the pipe fittings on the canister. These do not need to be pressure hoses like gas lines.

If it has the original SU fuel pump, it runs until it builds pressure and then slows down to just an ocassional click. So if you heard the pump running, it must have been flooding and thus the gas smell. Probably on the floor of the garage and evaqporating.

A standard test of the fuel system (if you have an SU fuel pump) is to turn on the ignition switch to "ON" (not Start) and listen for the fuel pump. It should click rapidly for several seconds and then slow down and finally just click very slowly or just randomly.

An SU fuel pump that clicks rapidly and keeps clicking that way before you start the engine indicates something is wrong. Either fuel is laking somewhere or the float in the carburetor is not shutting off the fuel. Or you have a leaking one way valve in the fuel pump.

The fact that you smelled fuel means most likely the float was not shutting off the fuel and it was flooding.

Most Weber carburetors I looked at only had one port on the air filter for a crankcase breather pipe to go on so you will also need a plastic "Y" connection and connect the side case breather pipe and the valve cover pipe to the air filter.

Your car is old enough and the fact that someone has put a Weber conversion on it means they may have put a different distributor on it too.

Look at the vacuum advance unit and see if it is a vacuum advance or a vacuum retard unit on the distributor. If the vacuum port on the unit is pointing away from the distributor, it is a vacuum advance unit and must have "Ported" vacuum. (comes from the mounting flange of a carburetor. If the port on the vacuum unit is pointing back towards the distributor then it is a vacuum retard unit and that must receive direct intake manifold vacuum.


MG Car Repair

All Answers

Answers by Expert:

Ask Experts


Howard M. Fitzcharles III


MG from 1956 (USA versions only) up and Engine theory.


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

Associate member SAE EAA member

Import Car magazine

ASE Master Auto with L-1 certification up to 2000

©2016 All rights reserved.