QuestionMy MGB 1975 (left hand drive American Model) does not drive smoothly in 3rd and 4h gear independant of how long it has been in the gear, instead it jerks, almost lunges with a boost of power followed by a lack of power. When the car is cold the problem is not very evident but when it heats up the problem becomes most evident in 3rd gear and slightly less so in 4th gear, it does not happen in 1st or 2nd. Our local mechanic in Italy is at a loss as to what causes it, he has tried to fix it a number of times and when driving out of the garage all is fine but the problem returns within a day or so. We also cleaned the plugs which also seemed to help for a short interval. Please help !
AnswerHi Andrea. Surges of power are usually a symptom of a weak mixture,
1)It could be that the carburetter jet is worn oversize. To fix it, the needle has been adjusted far too high so the piston is touching the face of the jet. This closes off the fuel completely, the engine dies, then suction lifts the piston and the engine surges, then the cycle repeats. The solution is to replace the worn-out jet.
2) Do you have a USA car with all the American emission control equipment and a single Stromberg carburetter or have you converted to the UK specification with twin HIF4 SUs?
If you still have the USA emission control equipment, it may be a faulty air pump gulp valve or a faulty air manifold check valve.
The USA equipment reduces the power from the engine by about 20 percent, so rather than try to diagnose and fix the fault, why not fit the UK specification carburetters in order to solve the problem AND give yourself more power?
