Don’t Let Your MGB Burn
by Dennis Trowbridge
The MGB has a reputation as a car that catches on lire. This reputation may be true: however much of the reputation is made of rumour and horror stories. But MGBs can burn and the fires may almost always be prevented by regular maintenance.
There are a number of reasons MGB fires start. They can be seen and prevented if you pay attention to your car. Leaking gas, bad carburetter adjustments, altered wiring and design ?aws all may cause fires. Except for the design ?aw problems. everything else can be prevented by just opening the bonnet and looking. The design problems can easily be fixed.
Leaking gas is the main cause of MGB fires. The worst of the problems pertain to the 1975 to 1980 MGBs equipped with the Zenith-Stromberg carburetters. Gas may leak from these carbs in several places and when it does, gas drips onto the hot catalytic converter or exhaust manifold. One area that gas can leak is from the automatic choke assembly. The three copper colored screws that hold on the auto choke assembly will work loose over time. This will allow gas to drip out between the choke and the carb body and the gas will drip onto the hot converter and a fire will result. If you can wiggle the auto choke assembly with your hand, these three screws should be tightened with a screwdriver. The auto choke assembly should be checked on a regular basis.
Another potential gas leak comes from the bottom of the choke needle housing. There is a small brass plug on the bottom of the choke metering needle housing that can leak of fall out completely causing a dangerous gas leak. The choke must be removed to replace the plug. Only replacing this plug with a new one will fix this problem.
Most Zenith-Stromberg carbs have a brass covered plastic plug in the bottom of the float bowl. Over time the “O"-ring that seals this plug shrinks and the gas leaks out the bottom of the float bowl again onto the hot catalytic converter. The "O"-ring is part of all rebuild kits. Care must be taken when removing the plug to replace the “O“-ring.
All carburetters can over?ow and spill gas out the ?oat bowl vent tubes. This is usually caused by the needle valves in the float bowls sticking. However if the correct hoses are in place the gas will be routed away from the most immediate and dangerous fire ignition sources. These hoses should be in place as a safety and emissions control matter.
Dirty, neglected Zenith-Stromberg carburetters will start to run increasingly rich. Soon this over enrichened mixture will cause the catalytic converter to overheat and glow cherry red. This condition can melt the plastic parts in the bottom of the carburetter causing gas to pour onto the catalytic converter instantly causing a ?re. MGBs with the emissions controls intact are especially prone to this failure. Incorrect ignition timing can also cause the catalytic converter to be flooded with gas causing it to glow. It is imperative that the carburetter and the ignition system be maintained in top operating condition to avoid this dangerous problem.
Over time all MGB gas tanks will leak. The design of the tank ensures that it will rust-through on the top of the tank. Water and dirt are collected in an embossed depression on the top of the tank and rust develops in this area. Small pinholes also develop on the top of the tank that leak gas when the tank is full. While this leak seems frightening, I have never heard of it causing a fire. The only way to stop the leak is to replace the gas tank. Moss produces he strongest replacement tank for the MGB.
The newest of our MGBs is over fifteen years old now and time has taken its toll on many of the car’s parts. Something that is often overlooked is the fact that rubber hoses don’t last forever. All hoses dry out and get brittle and these old hoses break. A broken fuel line will spray gasoline all over the engine bay and a fire is almost sure to result. Please don’t let the originality craze go too far. If your MGB has its original hoses, replace them now!
Electrical problems are the second major cause of fires in the MGB. However, most of these problems can be traced to someone messing with the wiring. There is almost never any reason to add extra wires to the MGB’s electrical system or any reason to cut into the wiring loom. Poorly done extra wiring and connections are a breeding ground for electrical fires. Shorts are possible, but they are not as common as the stories would lead you to believe.
The exceptions to this rule are cars equipped with the seat belt interlock system of the mid-seventies, cars with non-fused overdrive circuits and some alternator failures.
The seat belt interlock system can short out when the wires that run under the floor mats are worn through and ground out against the car body. These wires should be inspected on a regular basis or better yet, disconnected.
The overdrive circuits were never fused at the factory. Fitting an inline fuse to this wire is an easy task.
The last major electrical problem is caused when the diode pack in the alternator begins to fail. The warning sign of this condition is an ignition light that stays on after the key is turned off. If the light stays on, the battery must be disconnected or a dead short at full battery amperage will result in a major electrical fire. A bad alternator diode pack can also cause a chain reaction of shorts that will eventually result in a short that will cause the starter to crank continuously and cause a fire. Proper attention to the battery, battery connections and the alternator will prevent these fires.
The seat belt interlock circuit and the lack of a fuse on the overdrive circuit are examples of what I call “design faults.” The last and worse of these “design faults” is a so-called safety device that has caused more MGB fires than any other cause.
1977 to 1980 MGB’s have a mechanical fuel impact cut off valve under the bonnet, just to the left of the master cylinders that is designed to shut off the gas flow to the carburetter in case of an accident. The problem with this valve is that when it fails, and they all eventually fail, it sprays a fine stream of gas directly onto the catalytic converter. A very large fire results from this failure.
This valve is redundant because of the electrical impact cut-off switch that shuts off power to the fuel pump in an accident. The easiest solution is to simple remove the valve and route past it with new fuel hose. Regular inspection of the unit is a must if it is left in place: however, I strongly recommend that it be removed.
Pay attention to your MG and stick with a regular maintenance program and your car won’t burn, but still carry a fire extinguisher for those MG drivers who aren’t as careful as you!
Retyped text copy provided by Douglas Gaither in California - (May 2016).