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MG MGB Technical - Overflowing SU carb. pots

For some time now I have been troubled with overflowing SU pots. I try the quick-cure method (a light series of taps with a small hammer) but usually end up removing the top assembly, blowing through, checking the small needle valve closes correctly, then back together again. It is probably fine then,..until the next time. I did hear of a certain needle that can be used that is very good, replacing the small brass-type valve. Can anyone help me with the name of this item?
JP Mitchell

If your carbs are overflowing and then are OK for a while after cleaning out it seems that dirt must be coming through from the tank. An in line filter just before the carbs would cure this problem
Iain MacKintosh

Hello JP,

Replacement for original float needle is Groose jet. Here is link usefull:


Jean G.
P.S. Lain is right with filter addition
Jean Guy Catford

Gentleman,..many thanks for your comments. Much appreciated. Groose jet was not the name I was looking for.An in-line filter may well solve the problem though. I also have an MGC roadster, and at a recent event with many of those cars, noticed quite a number had a small filter just before the carbs.
JP Mitchell

JP - While Gross Jets and a filter may solve the problem in the short term, it is still treating the symptom. If you are getting debris in your carburetors, there is only one place it can be coming from - the fuel tank. Consider a new tank, or at the very least having your existing tank completely cleaned and sealed. By the way, Gross Jets are not the same animal that were so great many years ago. the company has been sold and the product is no longer as good as it used to be. Cheers - Dave
David DuBois

Further to my problem of overflowing pots, I have been on the SU website. The name of the needle I was looking for is called a Viton tip. The needle in my carbs. is a simple brass one with no Viton tip fitted. I shall buy two of these (very cheap) and see if this cures the problem and advise.
JP Mitchell


In many of the carbs I service, the metal tip is beaten to a cylinder with a small point. I think you will like the sealing ability and life of the viton tip. However if you are still overflowing... well, Dave DuBois is correct.

Dave Braun

To David (Wash. State) and Dave (Minn.) many thanks for your advice - much appreciated. I am going to try these Viton tip needles. Really hope it cures the problem. I am quite optimistic. Thanks again for your help.
JP Mitchell

I recently had the same problem with my MGC. Replaced the Gross Jets with the Viton tipped needles and the overflow problem went away. I have a new gas tank, fuel lines and filter so I attribute the problem to poor quality Gross Jets.

Interestly too, Viton is not a well rated elastomer in ethanol, go figure...

Best regards,
James Budrow

Hello John
Good MGC website


Grose jets (jets?) were said to be better than the original brass-tipped valves, but those only gave problems when they were worn, like anything else. Then the owner of Grose changed and the manufacture of both changed also, now it's Grose jets that have the poor reputation and the later Viton-tipped valves that have improved - or at least are better.
PaulH Solihull

Whereas the Grose Jet valves have a brass body and use two stainless steel balls, as the valve "wears" it is simply pounded into a match shape to the harder ball it mates with. I run with Grose Jets I installed well over a dozen and more years ago, some of the original manufacture, with no problems still. But take note of what others have said, the ones made and sold since at least the late 90s are of indifferent quality and user satisfaction is NOT what it once was.

Occasionally, castings will develop cracks where a threaded object is screwed into them. SU carbs are not exempt from this occurence, and if some prior workman has wrenched down hard on the float valve body, a crack could easily have occurred, and one that leads directly into the fuel inlet. As the valve is then tightened, the crack may grow and lead to flooding.

Also, observe the status of your float valve's sealing washer. They too can have problems that can lead to unexpected leakage as well. The attached photo shows such a float valve washer and one can see that this one was not making full and complete contact on its mating surface. Could be a bad washer - as cheaply as kits are put together these days, it is not an impossibility.

Somewhere WestofLaramie

SU carbs do not normally have sealing washers on the float valves.

But if that washer came from that location, there is a machining fault at the seat.

FR Millmore

Whether they serve as shims or as gaskets, they still should be able to perform a job as a seal between the float valve cage and the float bowl lid casting. Today we have at least 3 float designs to contend with, two of which allow adjustment via the metal arm that connects them to the pivot, and one (and a relatively common one still) that requires shims for adjustment.

The washer shown was from Weber/Edelbrock carb, and I probably should have said so, but the point in showing it was that there are other ways to cause a leak at the float valve that go beyond the "habitual suspects". Everyone seems to blame the needle and its seat right off, and on occasion, they are NOT the culprit.

By this point in the lives of these cars, much less the Original parts that have come to us, there is very likely a history we don't know about the parts themselves, their flaws, or how they have been handled by multiple owners and mechanics. That leaves a bit of lee way for the unexpected and atypical problem to emerge. It is simply a different way of looking at a problem.

Somewhere WestofLaramie

This thread was discussed between 03/06/2012 and 23/06/2012

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