MG Car Repair: MGB Carbaration, charcoal canisters, fuel vapors


Question
1980 MB B

When I remove the gas cap to fill up, there is a loud "whoosh" of air. This has happend several times.

Answer
Hi Doug,
On the later MGBs they vented the gas tank up to and through two charcoal canisters so that fuel vapors would not get out to air. As you drive the fuel sloshed around and that makes it vaporize more than normal and if the road is warm that adds heat to the tank. There should not be that much pressure build up in the tank. Either you have a pinched vent line going forward, the lines on the canister are connected up wrong or the charcoal canister is stopped up.
1978 on had two canisters, a primary and a secondary. If either were stopped up with wet fuel or because neither were changed (most likely) it could cause high pressure in the tank. This could also cause a vacuum in the tank when fuel was used up in cold weather. A fuel separator is located in the trunk which helps condense fuel vapors back to fuel but that is not usually a problem item and requires no maintenance. The float chamber of the Stromberg carb is vented to the canister also so if the car has ever had a float stick it will flood the canisters which stops them up.
I hope this helps, if you need a diagram of the hose connections contact me at fitzcharlesh@bellsouth.net and I will e-mail you one.
Howard