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MG MGB Technical - Overflowing carburettor pots

I have an on-going problem with overflowing carburettor pots. Both carbs. have been overhauled (2008). When the engine is running all is fine as the fuel is going straight into the engine,...but with the engine stopped and the ignition full on,...the fuel pours through the two overflow pipes. I try knocking each pot to hopefully make the small float close the valve, but this does not work. I am changing the braided hose that connects the two pots as this may be worn and breaking up internally,..sending small pieces of rubber into the pots and jamming the floats. Find that hard to believe but worth a try. Any ideas most welcome. The floats seem to rock well on the small pins.
JP Mitchell

JP, what type of fuel pump are you running. SU pumps have a max 3.8PSI output but some of the newer aftermarket pumps can exceed 5PSI. SU carbs are a bit fussy at pressures over 4.5 to 5PSI.

Tony
Tony Oliver

If the pump is standard, it might be the valve/seat. I simply took the lid off and blew through the fuelpipe while pushing the valve closed. If that works does the float float, and is the gap-(what type of carb? HS4 or HIF) correct. Haynes or www.burlen.co.uk/tech gives specs and how to do. Drill bits are a useful "tool" to set the gap. IIRC HIFs are less tolerant of wrong gap.
Michael Beswick

As both carbs are affected it's either too high a pressure from a non-standard pump, and should be the first thing to eliminate, or dirt/debris which can be coming from anywhere in the fuel system from tank to carbs. You don't give the year, if you have HS carbs then it is easy to remove the float chamber lids and see if there is any dirt in the bottom. Much more difficult with HIF. You could try disconnecting the fuel pump wire where the white from the rear harness joins the main harness by the fusebox, running the engine until the carbs empty, then reconnecting the wire. The resultant rush of petrol through the now wide-open float valves (they are normally only open a little to nothing) can sometime clear debris from the float valves, although that is usually only worth doing if one carb only floods occasionally. A fuel filter as close to the carbs as possible will also help if dirt is coming from the tank side of that. But none of the above will help if you have the wrong pump.
PaulH Solihull

If they are HIF4s there are lots of threads about the problem. No matter what I did I could not stop overflowing with them. I fitted HS4s on the reasoning that cleaning out the bowls when it overflowed would be a minute or two compared with one hour for HIF4s. But strangely enough they do not overflow, despite being years older.
c cummins

HIF's have adjustable floats. In my experience, setting the correct float level is often missed when carbs are re-conditioned. I have also seen floats put in upside down in the wrong chanbers! They flood!
Allan Reeling

I've also recently heard of someone putting the floats in the wrong carbs - they are handed on HIFs but not HSs, which caused intermittent overflowing on one of them.

I had repeated intermittent overflowing on one of my HIFs, until on the third investigation after two replacement float valves I discovered there was some petrol inside the float ... But only by shaking it as they were dark brown and totally opaque.
PaulH Solihull

I had this problem with HIFs, fitting a pressure regulator solved it, I tried new float valves, Grose jets etc none of them worked for long, in my case the main cause is an aftermarket electronic pump.
If you are fitting new float valves try and get the ones with a viton tip rather than the old solid brass ones, they are much better.
Ron
R. Algie

Gentleman,...many thanks for your comments. Most helpful. Several new ideas to work on to solve this problem.
JP Mitchell

I have not posted in this thread before as this is all covered in the archive. However I will say that I do not recommend some fuel pressure regulators. I posted here about my one developing a leak past its diaphragm which meant neat petrol was pumped over the exhaust manifold while we were driving along. I was horrified when I stopped to check the fuel smell, and very lucky the car did not burst into flames when I opened the bonnet. There is a pic of the guilty unit in that thread,
Viton tipped needles do the job fine on their own.
Stan Best

An inexpensive fuel-pressure gauge, in line, will tell you what your pressure is....On HIF 4's, 1.5 to 2 lbs, is more than adequate....Any more, and you will have problems....
Look closely at the photo (sorry it's a little blurry), and on the left side of the engine, you will see the filter, followed by the small liquid-filled gauge...
Edward


E.B. Wesson

Here's a better view (I think)


E.B. Wesson

This thread was discussed between 03/08/2011 and 13/08/2011

MG MGB Technical index

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