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MG MGB Technical - Fuel leaking from HIF Carbs

'72 MGB - Fuel pouring out of vent line from front carb when the fuel pump is pumping. Motor starts then cuts out and fuel continues to run out vent line into the left front wheel well until I turn off ignition. I assume only vapor is supposed to flow through this line, but obviously something is wrong. It only leaks from the front carb vent line. Help!!
J Rosen

From your description, it sounds like the needle valve operated by the float is not closing properly or is bad. Could also be the float has a leak and sinks. This keeps the needle valve from closing.

Remove the carburettor and check to see if the float has any fluid in it. Remove it carefully so as not to bend any of the 'tabs' that adjust the level.
werner haussmann

You describe it as a vent line; I'd say it is an overflow pipe and functioning as it should. No big problem; Werner probably hit the nail on the head.

You can only reach the float by unscrewing the four screws of the bottom lid, so you have to take of the carburettor.
Willem van der Veer

Its primary function *is* a vent, if it weren't there the fuel level couldn't rise (due to air pressure) to lift the float to cut off the fuel at the appropriate level. Being an overflow is its secondary function, when the float or valve doesn't do its job.

An HIF will really need to be removed to deal with this, but if by chance you have HSs it doesn't. With an HIF, *because* removal may not be required, it is worth disconnecting the electrical supply to the fuel pump and running the engine to empty for float chambers. Then reconnect the fuel pump. When the ignition is turned on the resultant rush of fuel through the now wide-open float valves may be enough to dislodge any dirt that may have become trapped. If that stops it overflowing then all well and good. But if not, and they are HIFs, then it will need to be removed. For HSs simply remove the lid. Check the float *carefully* to make sure there is no fuel inside. Took me a couple of goes to spot this on one of my HIFs. Check also for any debris inside the float chamber, and if there are any particles then the problem is dirty fel, possibly from a rusted tank. Otherwise replace the float valve. Use Viton-tipped needles, not Grose jets.
Paul Hunt

Paul;

Thanks for the helpful hint. I tried the "easy" way first, got a lot of gunk out of the tail pipe but unfortunately it did not fix the problem. I guess i need to replace the needle/valve.
J Rosen

You don't say anything about your fuel pump. One of the reasons for the symptom that you describe is having a fuel pump with a pressue that is too high. Are you using an SU fuel pump or an after-market unit? Many after-market units require the use of a fuel pressure regulator because their pressure is too high.
glq Greg

This topic is well discussed on this forum, I've suffered from this over the years and eventually switched from HIFs to HSUs, but I still got it, except it was easier to fix. But now it hardly happens, because I drive it a lot more. My theory is that the lead replacement in the petrol leaves a residue after the fuel in the float chamber evaporates,and this causes the needle valve to stick
c cummins

I had repeated problems with my HIFs as well, whereas none with the HSs. Eventually I discovered the problem was a float with fuel in it, which was difficult to spot. In theory the valves in the HIFs shouldn't be any more prone to gumming than in the HSs as they are the same type. The float valves themselves aren't in a bath of fuel but above the level, so are only 'wet' when fuel is passing through them and for a relatively short while afterwards. Americans talk about fuel deterioration and gumming, but their fuel is to pretty some weird formulations by all accounts. I can have the same tankful of petrol in the roadster at least for the whole of the winter and don't get any problems on the occasional runs during that period.
Paul Hunt

This thread was discussed between 12/09/2008 and 25/09/2008

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