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MG MGB Technical - Vapor locking

My 1979 roadster has develo0ped a tendency to vapor lock. The fuel pump seems to be delivering OK. It does not appear that any lines are close to the engine. Any thoughts.
William Snyder

I was fooled for a while that my fuel pump was ok and looked else where for the problem. Eventually i disconnected the output from the fuel filter and put the pipe into a plastic bottle. The flow was ok for a few seconds then it dropped back to a trickle. Turned out to be a dirty fuel filter although it was only a few months old. I had changed the fuel pump and i can only guess that the increase in flow moved some debris in the pipe work and partialy blocked the filter.
John

Are you sure it is vapour lock?
Could be an intermittant blockage in the fuel line perhaps the tank side of the pump if it is picking up crud from the tank
If the lines are clear of heat sources I would suspect a blockage
Hope that helps

Colin
C J Bryan

William. As the others have said, vapor lock is a rather rare animal with MGs. Classic vapor lock symptoms are that the engine will not restart after having been run for a period of time, then turned off. The fuel in the carb bowl(s) becomes heated to vapor as does the fuel in the mechanical fuel pump attached to the engine. Thus, the fuel pump cannot pump fresh gasoline up to the carb, pushing out the vapors in the float bowl.

The SU carb is an electric fuel pump mounted in the rear of the car and is designed to push fuel ahead of it (rather than pull fuel up to it as a mechanical pump does). As a result, any vapors in the fuel line or the carb(s) are pushed out when the fuel pump pressurizes the system. To actually have a vapor lock, you would have to vaporize the fuel all the way back and through the fuel pump. I have read of one person who claimed to have experienced true vapor lock with an SU pump. I, however, have not seen it in something more than 35 years, nor have I ever met anyone who has. I would suggest that your problem lies elsewhere, but, without more information than you have provided, cannot help you define what the problem may be. Les
Les Bengtson

I had a similar occurance with my 1969 MGC. It turned out to be a defective (new) SU points-style fuel pump.

FWIW

rick
rick ingram

Just about the only way you can get vapour lock on the MGB is on HS carbs in the pipe from float chamber to jet, if the heat-shield is defective, and only then in very hot ambients. Vapour anywhere else will be pushed by the pump through the float chamber and jet as soon as the float valve opens, to replaced by cooler fuel from the tank. First confirm with a timing light clipped to the coil lead and each plug lead that you are getting a spark to each cylinder. After that fuel starvation can be caused by a number of causes, including a blocked tank vent via the canister on emissions equipped cars, faulty pump, faulty pump 12v and ground supply, blocked strainer in the tank, etc.
Paul Hunt

This thread was discussed between 29/12/2004 and 30/12/2004

MG MGB Technical index

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