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MG MGB Technical - Fuel Overflow

MGBBT, '73. While trying to troubleshoot a starting problem that acted almost like a vapor lock or flooding, I ran into an even more baffling problem. I was able to start by placing the accelerator pedal to the floor while cranking. It runs rough, and fuel seems to be leaking through the drain that comes out of the overrun control valve.
Seem to have plenty of spark (but don't want to push that part with fuel leaking).
Any ideas/suggestions for troubleshooting problem most appreciated. Isn't the line for the overrun valve AFTER the carbs? Not sure how it could be overpressure from the fuel pump--I even have a pressure regulator after the pump.
Pat
Pat

Cleaned both carbs (looking for stuck needle). Still leaking somewhere--appears to be in the vicinity of the overrun control valve, and near the starter.
Pat
Pat

Pat: In the normal course of events there should not be any fuel in the overflow line. Its real function is to provide ventilation to the float chamber(s) so as to maintain atmospheric pressure there, and in the event of a stuck needle valve, to allow fuel to overflow outside of the carb body instead of up through the jet and into the engine.
On the early cars the overflow lines simply dumped over the side of the carb, but on the pollution controlled cars they are connected to the anti-run on valave. When the valve is closed ( by the oil pressure switch) it creates a enough of a vapor lock in the overflow line to prevent fuel from entering the float chamber when the stock fuel pump is being used.
Since you mentioned a fuel regulator I suspect that you are using an aftermarket pump. In no case will an SU pump need another regulator (it has one built in). If the float needles are OK as you say, then I would say that your problem is either due to excess fuel pressure (more than 2 psi) and/or float levels set incorrectly.
Andrew Blackley

Another cause could be that one or both of your floats have small holes and have filled with gas. FWIW Clifton
Clifton Gordon

Andrew & Clifton:
Thanks. To add to the mystery, have HIF4 carbs. The pump is aftermarket, as you mentioned, but is a 2 psi type, and the regulator is also set at 2 psi-- a previous pump did not have the 2 psi limit. So it would take both to fail at the same time, to lead to this problem. Not impossible, but...
I was beginning to suspect a broken supply line from the pump to the carbs (has a near parallel path-near the apparent location of the leak--to the after-carb line to the emission stuff). Haven't found any breaks there, and the only joint visible seems to be sound.
Still baffled and searching. Thanks again for the info. Guess I should put a guage on the fuel supply line just prior to the carbs, but I'm worried about engaging anything while this leak is present.
Pat
Pat

HIFs are no different to HSs and it doesn't need two problems. Even if both float valves are leaking it is probably only one problem i.e. dirty fuel. Over-full float chambers will cause a rich mixture which is probably why it only starts with a wide open throttle - which is the normal fix for flooded SUs. Disconnect the fuel pump and run the engine till it stops through lack of fuel, then reconnect the fuel pump. The resulting rush of fuel through the wide-open float valves usually clears any debris from them. But if the seat valve is worn or damaged it will start leaking again straight away so they will have to be replaced. However very dirty fuel could be causing them to leak straight away, and so will new valves. If you have a fuel filter replace it. Disconnect the overfdlow pipes from the carbs while diagnosing, much easier than having to wait to see if it drips out of the anti-runon valve.
Paul Hunt

Paul,
Thanks. I suspect you are right about the dirty fuel. Carbs were really gunked up, and it hasn't been that long since they were cleaned.
Last evening, I couldn't even get the car to start, using the previous procedure.
Will try your suggestions for the pump and overflow pipes, as well as the filter.
Thanks again,
Pat
Pat

Paul,
If you are still checking this thread--followed your advice on disconnecting the fuel pump. Car started up immediately, and ran smoothly for a couple of minutes, then stopped when the residual fuel ran out. Reconnected the pump and tried to start again--no luck, and it overflowed again, but after considerable cranking.
If it is dirty gas, would the car run so smoothly with pump disengaged? If it is the seat valve, would it have happened so suddenly (worked fine one day, wouldn't start the next morning), with no early symptoms?
I guess the next step is to follow the rest of your procedure, and maybe even put a pressure guage on the supply line.

Thanks for the previous, and any additional info,
Pat
Pat

Still watching, Pat, but replied to the email before reading the BBS.
Paul Hunt

Paul,
Thanks. Am at work right now, but will check out my email tonight. Hope to continue with your diagnosis recommendations later this week.

Pat
Pat

This thread was discussed between 08/09/2002 and 12/09/2002

MG MGB Technical index

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