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MG MGB Technical - Carburetor Float Valve Sticking

I have been fighting a needle and seat float valve sticking problem on my SU HS4 carbs for more than a year. I have (I think) tried everything. I have used 3 different types of valves, the grose jet type, the type with the brass cylinder, nylon piston and metal tip (after market brand that was included in a rebuild kit) and the brass cylinder, aluminum piston and Viton rubber tip (Lucas brand) that are currently installed. All of these valves have stuck but would break free with a few moderate taps on the float bowl covers. The engine would run fine for the next couple of weeks then the problem repeats. In addition to trying both types of piston valves, I have modified both piston types by increasing the clearance between the piston and the cylinder by lightly sanding the fins to reduce their diameter. I have also tried two different types of aftermarket fuel pumps along with a pressure regulator. I adjusted the fuel pressure to about 3 PSI. Much more than that causes the carbs to overflow out the vent pipes. I have also replace all fuel line and added extra filters to ensure that the fuel is clean when it arrives at the carbs.

The problem may be that the pistons are to light. A brass rather than an aluminum piston may be the solution if they are available. Does anyone know of a source? If brass pistons are not available then the only other solution that I can think of is to add some weight to the pistons. I can do this by using JB Weld Epoxy to glue some small diameter copper or steel wire to the pistons in the area adjacent to the fins. I think that I can fit this wire and still maintain the original diameter of the pistons. This added weight, albeit it small, may be enough to help gravity drop the piston as the float level drops. What do you think? Any and all suggestions and/or comments will be appreciated.

Frank Grimaldi
Frank Grimaldi

Frank - Are you saying that the needle valves won't open when the float level drops or are the needle valves sticking open? If they are sticking open, the problem may be excessive float drop, allowing the needles to become jammed against the top of the float. I have not heard of this happening in the HS carburetors, but it is quite common in the older H carburetors. As for fuel overflow due to too much pressure, you would be ahead of the game if you just went with the original SU fuel pump, which will not over pressure the carburetors. Cheers - Dave
David DuBois

Dave, thanks for your reply. The problem is that the pistons are sticking in the closed position, cutting off fuel flow. It happens primarily to the front carburetor. Do you think that adding weight to the pistons will help? Do you know if brass pistons are available?

I am not having a fuel overflow problem. The pressure regulator solved that problem.
Frank Grimaldi

Frank - Was your fuel tank ever sealed with a sealer? If so, the sealer may be dissolving in today's fuel and acting like a good glue to keep the needle valve closed. This happened in our TD and I had to get the tank dip stripped to get rid of all the sealer. No a heavier needle in the valve won't help - you can't pile on enough weight to make a difference, plus the needle is being acted on by the 3 PSI from the fuel pump. There is something sticky on your needle valves that is causing them to stick. Try using a Q tip and some strong solvent (lacquer thinner, acetone or MEK) to clean the valve seat and the needle and see if that stops the sticking. If it does and if the sticking reoccurs after a period of time, there is something in your tank, lines, and/or pump that is causing the valves to stick closed. Cheers - Dave
David DuBois

I'd have thought the pressure in the line should be much more important than gravity. Seems you've ruled out the valve as the real problem; is the pressure regulator set too low or the fuel pump the issue?

Yes, I did use a gas tank sealer. I used the one supplied by Moss Motors. Has anyone else had problems with this brand of sealer?
Frank Grimaldi

Odd that it is just the one carb, with multiple valves which include both needle and seat. That would tend to rule out contamination from tank sealant in my mid. I'd be looking at excess material on the float catching on something, binding on the hinge pin, or a bent hinge pin. Swap float and pin between carbs and see what happens, easy-peasy on HSs.
Paul Hunt

Hi , I replaced the faulty SU fuel pump on my 1980B with a HUCO recomended by The Sports Car Centre in JHB. The carbs are HIFs. When testing the fuel poured out the carb overflows like Victoria Falls. I removed both carbs and cleaned both Gross ball valve/needle and seats which are about 2 years old. There was no gunge in the float bowl. All back but same problem. The local importers of Huco have replaced the pump which produced over 6 PSI when measured at the carb fuel line. The fuel line was connected to the gauge so there was no flow but the line was bled to remove any air induced errors. The replacement pump I am told has been tested and is to spec at 0.25bar/+- 3.5PSI. The pressure I have been advised should be around 4PSI max., thus it should be within limits.
I have also read that the Gross valves at some stage had problems but not what these problems were. It is a schlep getting the carbs off and on so I would rather do the whole job, IF I had to , new pump + new valves but which valve / needle and seat is the best? And what is the max pressure that the HIF float can close off? Any other suggestions? Thanks
R. E Bester

The SU fuel pumps put out either 2.7 psi or 3.8 psi, depending on the model. Either of these pressures will work fine with the SU carburetors. Rule of thumb is 2.5 to 4.0 psi is what the SU carburetors are happy with. From everything I have read, introduction of a pressure regulator is a very iffy thing unless you get a high quality (read expensive) unit.

The Gross jets, since the company was sold had a tendency to stick closed. I would not recommend them to anyone roe use in the SU carburetors. Cheers - Dave
David DuBois

Hey Frank, not a carb guy but......I replaced the floats, valves and the spindles that the float rides on. My thought was that it too was worn. At any rate, no more intermittant issuses. Regards, Tom. Oh yeah, the "O" ring too was replaced.

Tom, what "O" ring?
Art Pearse

Frank - I was just rereading all of the posts and noticed that you keep referring to the pistons being too light and then about the piston sticking in the closed position. What piston are you talking about? The only piston I know of in the carburetors is the one in the vacuum chamber that controls the venturi opening and the metering needle in the jets - it doesn't have anything to do with the needle valves in the float bowls. Please clarify what pistons you are talking about. Cheers - Dave
David DuBois

The O-ring seals the base of the float chamber to the body of the HIF carb, and should be replaced, at least the first time you open it up.

Conventional needle valves with Viton tips are recommended over Grose these days.
Paul Hunt

Usually with the balance, or lack of it, shown by venerable B series engine sticking carb float valves are impossible. My Viton tipped ones work fine, as did the brass SU ones before, at least in letting fuel in. Before reading the thread I was going to vote for the glue theory.
Stan Best


The parts that I referred to as the piston and cylinder should have been called the needle and seat. Sorry for the confusion.

For what it is worth, I decided to replace the entire choke lid assembly (includes new float, needle and seat) with a new one from Moss. It came already assembled. Ironically, would you believe it, the needle was stuck in the closed position. It easily broke loose but would re-stick when normal pressure was applied. I had to clean the Viton rubber with a solvent to get to release properly.

It is too early to tell if this solves the problem. So far it seems okay, but time will tell.
Frank Grimaldi

I have AUD 135 SU's (fixed metering needles) on my 65. I thought my float needle valves (older Crose jets) were sticking closed for a couple of years. It usually happened on the front carb shortly after a choke start. The engine would basically try to run on only one carb acting like fuel starvatio. A rap with a small hammer on the float chamber and it would run.

Well, when I decided to rebuilt my carbs I found the metering needle was sticking in the jet in a raised position. New jets and proper more problem.

My diagnosis. With the jet dropped for choke startig the piston would fall properly. Then as the jet was raised the needle would bind on the jet raising the piston..not enough depression especially with a partly warm engine. The rap with the hammer dropped the piston to the proper position. Once the car was warm a slightly stuck piston was masked by rough running due to tired carbs.

See if this is the problem. On an outside chance it might be. You'll probably need to run a drop test on the pistons in the chambers. I had a fair amount of varnish on mine that had to be removed to get them in tolerance (they were too slow). Carb cleaner and a cloth do the job. I also had to switch pistons to get them both the same.

Good luck. Hope this helps.
Robert McCoy

With adjustable metering needles always check centering with the jet raised from its normal running position (count the turns), and make any adjustments there to get the definite click as the piston drops. If it doesn't bind then it isn't going to bind when it is in the running position.
Paul Hunt

Hi Folks
Over the years I have found that in my cases, the sticky HS4 fuel valves are usually caused by bad floats, such as leaking float, worn hinges on OEM floats and the terrible plastic replacements that dissolve due to alcohol in the fuel. When I install a decent used oem float, my problems go away. The last time I talked to Joe Curto he told me was procuring the new foam floats with the original metal hinge. I will contact him again to find out if he has any stock available.

Good Luck:
Rich Boris
67 B roadster
Rich Boris

This thread was discussed between 13/07/2009 and 31/07/2009

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