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Mixed SUs on a Vitesse

Ian Bailey

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So, last run out, my Vitesse Mk2 Convertible started spewing fuel all over the road. Looks like the float to the front carb was stuck. Time to overhaul the carbs and replace the floats with some unsinkable ones. I have twin SUs fitted, I think from a 2000. Looking at the little tabs on the float chambers, the front one says AUD604F. A quick Google confirms this is an HS4 from a 1974-on Triumph 2000. However, the rear SU has the number AUD147. A similar Google tells me this is an HS6 from an Austin 1800 from the mid-60s. But the two carbs look identical. Is it likely that the rear one is an HS6, or more likely that it is the wrong tag or just the float chamber is from an HS6? Any thoughts welcome!

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Hello Ian.


The quickest way to check if you have a 6 instead of a 4 is to take them off the manifold and measure the throat diameter. The 6 (1 3/4") is bigger than the 4 (1 1/2") I would only use the tab number if all your physical checks with the carb are positive.


Burlen carbs are producing ethanol proof floats.


The float may not be at fault but the gap maybe - I am not sure what the gap should be, perhaps 6mm you will need to check a WSM or another Forum user should be able to assist with that. Personally I think your float got jammed hence the discharge of fuel.


A full carb strip down is no bad thing.


Good luck.



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Hs4 have a 1.5" choke bore throat the HS6 has 1.75" Hs4 has 2 fixings most HS6 have 4


needle valves dont last for ever and look closely at the top float contact if there is the slightest wear ridge

its likely to jam the valve and puke fuel

Always remove the needle valve and check no floating slivers of rubber hose are not ending up in the back of the needle assy.



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You don't need to. 


As Pete said, a quick look will suffice.


HS4 have 2 manifold mounting holes on the diagonal. HS6 have 4, but often only 2 are used.


Chances are somebody has swapped tags around.


Worth checking they both have the same needle, Undo the dashpots off the carbs, lift the piston and spring offand take the needle out. Number stamped on it at the top. Not sure what spec you should have, but it depends on the actual engine number. And also air filters (some 2000's used a very thin filter box which is very very restrictive, and results in a very lean mixture over 2-3000rpm Ask me how I know!)



After all that, Pete is again on the money with teh needle valves 90% the most likely culprits.

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There is a post somewhere that I came across re vitesse/GT6 with SU carbs requiring different float bowls front & rear due to the angle these engines sit at. It may just be your floats that are different.

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