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MG MGB Technical - Carb flooding puzzle

The rear HIF carb on my 1980 LE roadster is giving me a real headache. The carbs were a reconditioned pair from MGBHive six hundred miles ago (two years!). It began intermittently flooding, so as a precaution I bought a new needle valve. When I fitted it I found a small piece of plastic jammed in the seat. I fitted the new valve anyway.
All was well until now as It has started flooding again. Yesterday I opened the float chamber for inspection. the float was removed and checked for leaks. It appeared normal and no bubbles of air came out when immersed in water.No fuel could be seen inside. I cleaned the needle valve and seat and made sure the float moved easily on it's pivot. I set the float level per the book and checked that the flow was cut off when blowing through the fuel inlet and raising the float.
I re-fitted to the car and the fuel pump just kept on ticking with fuel coming from the rear overflow pipe.
What have I missed??


Absolutely no help at all, but my 72 MGBGT does the same, all appears fine until you fit the carb to the car. Except mine is the front carb ! I'll watch this space !

Sounds like new valves and seats are in order.Not much in there except those to keep fuel in the bowl. If you inspect them under a magnifier you might find ridges worn into the taper part from constant bumping against the seat.

I think a lot of owners have had this problem with the HIF. I see you only did 600 miles in two years, modern fuels do not suit carbs, or so said an enjoying MG article. Gumming up residues could be a problem. But the HIF is a PIG to work on. I dumped mine for a pair of second hand HS4s that cost me 30. The car performs the same, and I get better mpg probaly because I don't lose so much through leakage. If they do overflow, and it's only done it once it's a minute or two to fix.
c cummins

You don't want to take any notice of what Enjoying MG says if it comes from the pen of Roche Bentley. I've questioned a couple of his statement lately, only to be told point blank that they hadn't been in Enjoying MG. When I quoted the issue and page numbers I got the response "Oh, that's Roche" as if that explained everything! Someone else has said that they usually catch some of his wilder pronouncements, but some still get through. He does own the whole shooting-match, so they do have to be a bit diplomatic Two of his recent ones have been fuel 'going off' and storing batteries on a concrete floor ruins the battery. I've regularly have the same fuel in my roadster tank for several months over winter, only starting in every 4 to 6 weeks when the weather is inclement and never had a problem. And the battery and concrete issue refers to storing *old* batteries on concrete, where if they leak it ruins the *concrete*!

The HIFs *are* a pain compared to the HSs, and if anyone were tossing-up between the two I'd say HS every time. All HIFs give you is a slightly better environment and less mixture variation between seasons, which is only slight anyway. But they should be no more prone to leaking than anything else. I had several bouts of overflowing on my V8 and it was only after the 2nd valve change when it leaked again (usually worked OK for a bit before it started leaking again) I spotted the float had some fuel in it (not easy to see) and no amount of heating or squeezing would get it out to find the leak. Since fitting a new float some time ago it has been fine. Replacement valves (like anything else) can be faulty, it can be dirt in the fuel, possibly float positioning, and many after-market pumps put out too much pressure for SUs. If it only happens when the engine is running then vibration could be the issue. Why only one carb? Depends on the valve. If you fitted new valves to both they both might leak.
Paul Hunt 2

Actually it's a Moprod solid state fuel pump on there that I fitted when I rebuilt the car a few years back. I do have a spare SU pump that I may try. Perhaps as Paul says the pressure could be too high and is overcoming a faulty float??
I may replace the float and valve to see if they are at fault, Then if there is no improvement I am very tempted to go down the HS4 route.
ph barton

If the Moprod is the same basic appearance as the SU, except plastic bodied, then it probably is the correct pressure. My V8 came with one and was fine for quite a while. Then it started short-stroking from time to time and causing fuel starvation. As these 'pointless' pumps don't respond to a quick thump like the point ones do I was stuffed until it decided to start working again. After the third time I fitted a reconditioned pointy type from the roadster and have had no more trouble. I have kept the pump to use as a backup should it be needed, but would never fit one (of any type) again for full-time use. I'd rather clean the points once every 20 years!
Paul Hunt 2

This thread was discussed between 21/02/2008 and 24/02/2008

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