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Sticking carb float.

Phil Siddall

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This happened when I had my Spit some 40+ years ago. It was a known fault then that then needle valve seat was poorly made. The solution given to me was to use Jaguar needles which had a fibre tip so it gave a better seal. Sorted the problem but I'm sure there are better solutions now.


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SU do seem to have this for variety of reasons

small ridges in the float needle platform............but you have a new float

top cap distortion mainly down to years of over tightening 

poor needle seat    but you have a new set   Q are they viton tipped ??? needles 

fuel pump excess pressure (  max 2.5psi)

slivers of hose that get cut of when you insert the tubes into the  hoses jam  the back of the float valve ............ very common 

a real lumpy idle that shakes the float into submission





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Dirt, and as Pete says slivers of rubber tube, can prevent the needle valve seating properly and cause flooding. The front carb can suffer worst because it's the first in the pipe run from the fuel pump. Dirty sediment in the bottoms of the float chambers is a tell-tale sign.

Check and clean the gauze mesh filter in the fuel pump. Also consider fitting an additional fuel filter just before the fuel pump. With clean fuel and viton tipped needles all should be well!


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Our 66 Spit has an issue at 100 to 104kph only that range, we get a sniff of petrol and resulting moisture on the rear float chamber top cover only never been able to completely irradicate it.

The original needles and seats were badly grooved so I changed the needles and seats to US Grosse there NBG nowerdays, then new SU types which came with the full carb rebuild kit (SU Midel here in Aus), but I don't think I've tried the later Viton tipped units or stay up unsinkable floats, must get some, daughter advised me she got a petrol smell at the 100kph again just recently. 

Remember I did have issues with the float chambers moving from vertical as the nylon locating bush had aged and cracked, and the float locating bolt became loose. I have never had rubber slithers and frequently clean the float chamber out. I have a separate fuel filter before the pump.

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