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MG MGB Technical - Carb chokes and overflows

I have a '77B with dual HS4s. It was laid up for a month. It started fine, yesterday, but soon started overflowing from the front carb, and the car choked and died. After the overflow stopped, the car started fine again, then repeated the overflow, choking and dieing. I measured about 1/3 cup fuel from the overflow. I removed the carbs and inspected the floats, but everything looked fine. After reinstalling, the problem continued for a while. It finally started working. This happened once about 4 months ago (a single overflow event).

What do you suppose was the problem, since I'm sure I'll be seeing it again?
Fred Doyen

It is the float valves playing up. Possibly overpressure from the pump. There is a lot about this in the archives
S Best


Had similar problem after long layup. Replaced both check valves with grosse jet ball type from Moss, 1/2 the price of a new standard check valve. Replace the aftermarket plastic floats with some reclaimed older nylon SU floats with the stainless steel clips and reset the adjustment; works fine. The new fuel mixture in the metro NY area uses 10% ethanol, in lieu of the MBTE, and could have affected the float material.

Good Luck: Rich Boris 67B
Rich Boris

Fred I saw this same problem on another B that had a stock SU fuel pump. I would check fuel perssure at the fuel filter. It only needs less then 2 psi to operate, anything more than that will cause fuel to overflow. Good Luck!

80 B Roadster
68 C GT

I know this one's old, and you thought you were through with it, but I've still got the problem. It's obviously not the top priority in my life.

I've asked three major parts stores in my area for a fuel pressure gauge, and none have any. I ordered needle and seats from Moss, today, but I still would like to measure my fuel pressure.

Where should I look for a gauge?
Fred Doyen

If you've got an original style fuel pump the pressure should be fine. Generally if it works ok most of the time I'd think the pressure is ok and something else is wrong.

Do you have a fuel filter in the line? Flooding can be caused by small pieces of dirt preventing the needle valve in the floats from seating. The needle valve could be worn and not seating properly all the time too. If they haven't been changed before they're pretty cheap.

Never tried it, but I believe you can use a vacuum gauge to measure fuel pressure.
Mike Polan

If the fuel pump is a SU pump, the pressure is going to be ok. The maximum that they put out is 3.7psi, which is not going to over pressure the needle valve if they are not damaged. There is also no way that a SU fuel pump will put out excessive pressure - insufficient perhaps, but never excessive. MikePolan is correct about using a vacuum gauge to measure the pressure from the pumps. All of the vacuum gauges that I have ever seen will measure pressure up to about 10psi. Good luck - Dave
David DuBois

Thanks, Mike and David. Since I've already ordered the needle and seats, I'll wait to buy the vacuum gauge. Others in this thread indicated they have seen overpressure from the pump cause the overflow, though. I hope you're right.
Fred Doyen

I have had failure to seal with standard valves and an SU pump. The factory manual shows a manometer arrangment for testing flow and head. However for flooding static head would seem to be what matters.
S Best

It was the needle and seats.

Also, I need to correct some disinformation I provided, in that I have dual HIFs, not HS4s.

Thanks for your help.
Fred Doyen

This thread was discussed between 23/04/2005 and 10/05/2005

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