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MG MGB Technical - Carb overflow-AGAIN!

I wonder if you can get hollow ground through heart stakes for problems like this? I have just fixed my overflowing HS4s again by re-installing the Viton tipped float needles. We were on the MG club stand at the Uxbridge show on Sunday, always an amazing range of vehicles(my personal favourite was this 1950s Thames 100E van looking like it had just rolled out of Dagenham). On the way home the B went onto 2 cylinders briefly and the fuel pump started to run continuosly. I came home avoiding the Motorway and when we got here found an intermittent overflow on the front carb. Turning the fuel regulator down had no effect so today I refitted the Viton tipped needles. I also tried the pressure regulator at min and max settings and it was working (I ran a pipe to where I could both seal it with my thumb and turn the ignition on and off.) Just changing the front valve stopped the leak although the pump still ticked once in a while, changing both stopped the pump completly. I checked the mixture and it needed weakening one flat on both carbs. While doing this I discovered that at it's lowest setting (1/2 PSI) the regulator will eventually let the carbs run dry! Thats me out with metal tipped valves, Viton only from now on. No help needed but this is such a regular on here that I thought I would update you all and it seems to me the plastic seal needles are the way to go, I'm not sure the regulator is needed but it's staying there to help them out a bit.

Stan Best

Stan - What year is your MGB? Are you using a SU fuel pump? If so, is it the early L type pump or the later AUF 300/AZX 1300 style pump? The early L type pumps have an output pressure of 3.8psi while the AUF 300/AZX 1300 series pump are 2.7psi (some of the AZX 1300 series pumps used in the Jags were also 3.7psi - if you can send me the model number on the pump, I can find out what you have). Either pump should work fine with your carburetors and shouldn't need the pressure regulator. Cheers - Dave
David DuBois

Carb incontinence has been a recurring problem on my car for last 10 years. I think modern fuels etch the needle valve until eventually they stop working. As you may know I have owned my car since 1969 and until unleaded fuel was introduced (= foisted on us) it had only ever had one set of needle valves in it's life. The fuel pump on now is a Burlen rebuild with solid state points, however it first developed the problem with AFIK the correct SU pump for its year fitted , on that occasion even new valves failed to stop the leak.
Stan Best

There has *got* to be some other problem. There are thousands of us with SU pumps and the original steel valves as well as viton-tipped without pressure regulators without overflow problems.
Paul Hunt

Have you always gotten your fuel from the same place? Maybe its poor quality fuel. There could be silt in the garages tanks.
Ross Kelly

Dave, sorry for not responding sooner, the car was built in Dec 66 and registered in Jan 67 so depending on how the question is put it is a 1966 or 1967 car. Paul I agree it should not happen, on this occasion when I got the metal tip valve out I pushed it shut so the spring on the pin was about 1/2 it's travel and tried to blow through it, good seal! The first time it had this problem I was close to tearing my hair out before the Viton tips fixed it, they worked without the pressure regulator and might well do so now but I am not going looking for trouble. At one point I could take one valve (new)apart and put it back when the other carb would overflow. This was after swapping out the fuel pump! And Ross the float chambers were quite tidy, I have seen a lot worse without this problem. The tank is stainless steel so no rust particles.
Stan Best

I haven't been following your thread so this
may be a rehash, but...

Have you checked the float pivots and pins?

If they are worn and oval, then there's your bug.
Daniel Wong

I'd been having overflow problems on one HS4 myself and tried the usual fixes -- fuel pump, pressure regulator, float chamber needle and seat -- but none of these worked. That's when I started looking at the float. The float looked in good condition, so I called Joe Curto, the SU expert, and asked him if a bad float could look good. He replied that the float itself is good unless it contains fuel. He went on to say that the metal float arm often gets worn so that the float wiggles too much and can't exert a steady pressure on the needle valve. So I got a new float.

The new float has a different construction and no metal arm, which bothered me at first. But after confirming that the new design is meant to replace the old one, I installed the float and the problem is solved. -G.
Glenn G

I have gone back to the original metal arm floats after trying the current offering, the one piece moulding. The pins are fine and the metal arms are a pressing that does not bear tightly on the pins. I also set them with the drill as per the workshop manual!
Stan Best

Stan - 66 or 67, doesn't matter, the original pump for those years (and all subsequent years) was the AUF 300/AZX 1300 series pumps with an output pressure of 2.7psi. There is no way a SU fuel pump output pressure can increase with age, so you have either 2.7psi of something lower.

Here is another thought, how old are the mounting busings that isolate the float bowls from the carburetor bodies, and are they the correct ones? The busings are used to isolate the float bowl from vibration that can cause sufficient agitation of the fuel in them that they will overflow. An incorrect bushing will position the float bowl at the wrong angle, which can cause overflow or low fuel level depending on which was the bowl is tilted. I thought that since we are grasping at straws at this point, I would toss the above out for consideration. Cheers - Dave
David DuBois

I forgot to mention that I replaced the float bowl mounting bushing prior to replacing the float on my errant HS4. I also replaced the fuel feed hoses and carefully cut them to lengths that would not disturb the positioning of the float bowls, which should be nearly level. The hoses and their braided covers are difficult to cut, even with a good pair of razor-blade utility pliers. I managed to leave a small piece of stainless wire inside one of the hoses, which found its way into the needle and seat and made its presence known as soon as I started the car. -G.
Glenn G

Thats an interseting point, yes they are the correct bushings as I have never changed them. Are they in good condition, well they seem to be however they may well have hardened with time so I will try changing them. I put new jets on when I changed to the = to AAA needles (these are fixed jet so it's a very similar taper with a different name) I'm sure the jets came complete with the pipe to the bottom of the float chamber and washers etc so this does just leave the rubber bushes.
Stan Best

Vibration is a good point. If it starts happening again with the engine running switch off then straight back on again so the ignition is on with the engine stopped (or just pull a coil wire). If it stops leaking then it *is* vibration. HSs have rubber mounted float chambers, it should be giving some flex, is it rock solid or alternatively very loose?

FWIW HIFs have integral float chamber and so are solid mounted, but they don't leak either.
Paul Hunt

Perhaps unrelated, but I have seen several examples of "Genuine SU" (i.e. new stock from Burlen) float needles that have a little bit of flash on the plastic "fins" that center them in their seats. You can't see it, but you can feel the sharp edge. This flash can cause the needle to hang open or shut. I had this problem with a rebuild kit I bought for my TD, and also saw it on a set of current production SU carbs purchased from Burlen. In both cases, just a little gentle scraping with a knife cured the problem.

Just an FYI...
Rob Edwards

Something I should have said ealier, I had decided that this was insoluable, but that word does not seem to exist on this board, thanks a lot for the brilliant suggestions when I was out of ideas. I will look at the needles and may take a picture with macro on. From memeory when this began several years ago I could get the carbs to overflow alterantley just by taking the valve apart putting it back then switching the ign on, but it was a long time ago!
Stan Best

Glenn G's first remarks fits the best ( from my experience)regards SU overflows. Visual float check-test can fool you. Remove them both from the carb, put in clear glass of water and do a buoyancy test... they will both float but the one lower likely will be the culprit. Shake and feel the float to check for fuel inside float.
Suggest, rule out the float issues first before dealing with seats and jets and fuel pump and pressure issues. Good luck.
J P Wyc

Paul Hunt and Glenn G's proposition sounds relevant: on mine 64, I found the rubber bushing between carb body and float chamber really weakened and in poor condition. I had to replace them to give the float chamber a good fixing.

I'm reviving this thread from the archives because it is a pretty good summary of the fixes for overflowing HS4 carburetors, and I have one fix to add. Now that I'm driving my '67 GT about 200 miles each week, I'm finding more things that still need attention, including the fuel overflow problem.

As mentioned above, I tried nearly everything and thought it was fixed, but was still having problems on warm days. Whenever the engine idled more than a few minutes, fuel would spew from the rear carburetor overflow pipe on acceleration. I was able to reproduce the problem so dependably that I could witness it in my garage. When I leaned over the hot engine and revved it, I could see fuel and air spraying out of the rear overflow pipe under some pressure.

I began to think that pressure was building in the fuel line from the ambient heat. When the engine idles, the fuel moves slowly and has more time to get hot. Vapor in the fuel line then expands, resulting in more pressure. I imagined the fuel flowing past the inlet needle into the float bowl with such speed that it could depress the float, allowing more fuel to enter the float bowl and creating a miniature tempest that exited the overflow pipe.

My solution was to relieve some of the pressure by drilling holes in the sides of the inlet seat. I got the idea from looking at some old HS6 inlet valves. I avoided putting holes in the sides facing the overflow hole to keep any airborne fuel from flying out. I've had two warm days to test, and so far it seems to be working. -G.

Glenn G

One more photo. -G.

Glenn G

I've been reading this thread with great interest as I am having a very much related problem with the rear carb on my HS4 equipped '65 B. In my case, it seems that the valve is sticking in the closed position. I've been experiencing this problem, only sporadically, over a long period of time, and believed it to be water in the fuel as it would go away and not reappear for several months or even years at a time.

I discovered what I believe to be the sticking problem after it happened again just a couple of days ago and decided to clean out the float bowls and generally check things out. I even drained the gas tank still thinking water. While checking out the float/valve assembly, I noticed the valve was sticking in the closed position but would eventually fall to the open position if I prodded it with a screw driver or similar tool. In addition, there was not a fuel filter in the system but there is now. It looks like a valve replacement is the answer but was wondering if anyone might have another solution.

Very long statement with a most likely short answer. - Dennis
DLR Dennis

Yes, I would first try replacing the needle and seat assembly. -G.
Glenn G

One thing that nobody has mentioned is that a blocked overflow pipe can cause fuel to be pumped straight through the jet/needle and into the motor as the air can't get out of the float bowl. I have come across both squashed & blocked pipes. First came across this on an MGA which had had a plain fibre washer fitted under the overflow banjo. I have had more trouble with Viton tipped needles than with the original brass type. I have done 300,000 miles in my GT & only replaced the needles & seats once. Some seats have a larger hole for greater flow which means that the PSI per sq" is greater so if a fuel pump has too higher a pressure it can open the needle. I have seen pumps fitted with the wrong spring from a different model pump.
Garth Bagnall

If the fuel lines are getting hot and the fuel vapourising, the increased pressure can overcome the float chamber valves. Modern fuels are more volatile, check that the heat shielding is still in place and that the tranverse fuel line is behind the shielding at the base of the heater box.
Allan Reeling

This thread was discussed between 21/07/2008 and 16/04/2011

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