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MG MGB Technical - vapor lock

I have searched the archives and can't find much on this.
I have a '79 B with a Austin Marina manifold and a HIF44
carb but lately after driving around in the heat and then parking the car for 20~30 minutes it won't start if I dump some water on the carb it will then start.Sounds like vapor lock to me.
I have put in a new facet fuel pump,made a heat shield,repositioned the fuel lines so they are not near to the manifold.I don't know what else to do .Any ideas out there?
Thanks Pat

patrick bailey

This shouldn't really be possibly, especially with an HIF where the bottom of jet is stiing right in the float chamber. It's said that HSs can suffer from it in the exposed jet pipe, but even then it should only be if the heat shield is damaged as thousands of these (well, factory engined cars) have run in desert states for many years with no problems.

With modifications all bets are off, even though yours isn't that much of a departure from standard. There are also other factors that can lead to excessive heat that *might* cause carb problems that wouldn't happen in 'normal' temperatures.

As the end of the system in both HS and HIF is two ports open to atmosphere (the float chamber vent and the top of the jet) I can't see how any vapour pressure can develop to prevent fuel flowing in to the float chamber of the HIF, and even the jet in the HS, if the float valve is open and the pump is applying pressure. It could be that the float valve is stuck closed from the heat and sticky residue, and cold water contracts the metal enough to free it.

Have you dumped this cold water on it while the ignition is on? Have you noticed what the pump does i.e. does it start chattering suddenly? Although that will only happen with SU pumps, not some after-market types.
Paul Hunt 2

Could the increased fuel pressure from the Facet pump be playing a role? If I remember correctly PSI should only be around 1.5 ?
James

Have the carb & manifold got an insulating (black plastic) spacer between them? It could be conducted heat from the cylinder head causing the problem. When the car is moving there is probably sufficient cooling from the moving air. Without an insulator, when the car is stationary, heat is trasferred to the card body.
Allan Reeling

This thread was discussed between 08/08/2007 and 09/08/2007

MG MGB Technical index

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