QuestionI have a 1978 MGB GT, I broke down recently during a journey on the motorway. The symptoms were a severe misfire with accompanying backfiring, followed by a gradual loss of power and eventually a total stop. Now back at home the car will start and idle, albeit rather roughly, but as soon as it is revved, even slightly, will cut out completely. I've taken the air filters off and checked that the carbs are moving correctly and they seem to be. My next thought was a fuel pump or fuel blockage problem, which I am yet to investigate due to the sub zero temperatures outside. Any suggestions would be gratefully received.
Gregg
AnswerSorry--holiday time!
It's almost certainly nothing to do with the timing (the usual cause of back-firing).
As you half-suspect, I think you have fuel starvation at one or the other carb. In fact I would bet that if you'd opened up the top of the carbs immediately after it stopped, one would have been empty or nearly. Suspect sticky fuel residue in the float valve and seat. Let's do them both...so, for each,
1) Take the top off the float chamber (don't lose the screws!)
If the fuel tubing allows it, dangle the top over a can (otherwise, a wad of paper towels), and have a helper turn on the ignition. Jiggle the float up and down. When down, fuel should pour out; when pushed up, it must completely stop.
2) While the lid is off, clean out the float chamber as much as possible. Use a lint-free rag (e.g. old T-shirt) around a pencil or pointy dowel. Put it back together, making sure that the gasket around the lid is still in place around the rim.
3) Now you've done them both, do this anyway. Take the air cleaners off. Get the car running. For each carb in turn get your helper to get the revs up to 2500-3000, and on the shout "now", (i) helper mashes the pedal to the floor and (ii) you put your hand (or other fairly solid object) over the carb intake. The car will die. That's fine. Restart and do the other one.
4) Pour a can of fuel injector cleaner into the gas tank. I'm suspecting sticky goop in the fuel system, and the jet is the most susceptible.
Let's hope you've cured it. The reason I think it's carbs is the sudden onset. Timing (which can easily cause backfires and stuff as well) doesn't come on suddenly.
Jo
PS My mother (in England) told me about the weather!
PPS It occurred to me to mention that this of course assumes that fuel is getting to the carbs. A quick check: disconnect the fuel line at the front carb and dangle the end in a jam jar. Turn on the ignition. Fuel should pour out. If it just slowly trickles, you have a problem in the supply, either pump or filter. If you let the jam jar settle you can also check for water in the fuel, which might have frozen in the cold. JJ
