TR7 carbs on a Sprint - will they work / what changes needed?

[gmsclassics]

I’ve been having issues with SU carbs that I sourced from a very early 75 built Sprint, possibly as that car had done c250,000km. With no tags to identify, I imagine these are AUD661 or 680. I have remembered / found a pair of FZX1242 carbs off a TR7 engine (that was in a Dolomite) and wonder how simple it would be to use these on the Sprint? These carbs appear to be very similar to the FZX1257 carbs fitted to the later Sprints. The only difference I can pick in the setup of these two is down to needles and springs – BDM with red spring for TR7 and BDQ with yellow spring for the Sprint. In fact, Burlen’s website says FZX1257 were also fitted to some TR7 with the exact same Sprint spec.

The height of the suction chamber, dashpot & piston assembly is too tall to fit under the Sprint bonnet (that is the assumption I have always made) so the Sprint one would need to be used. Question then is does anyone know if the assembly from the earlier Sprint carb (AUD661/680) will fit and work correctly if I fit it to the FZX1242 from the TR7? Also does the following logic work – if the FZX1242 on a TR7 can be replaced by the FZX1257 with exactly the same spec as fitted to the Sprint, will the reverse work, the FZX1242 in TR7 spec work (without needing to alter needle and spring spec) work on the Sprint?

Incidentally the TR7 rubber carb mounts are identical to the Sprint ones and these must be pre 1995. They are in as new condition with no cracks or deterioration so that is a good find.

We’ve just ended day one of our four week Covid 19 lockdown in NZ so I need to find things to do!

Geoff

[cliftyhanger]

Yes, you can swap stuff about with no problem at all, just keep it matched. I would be inclined to keep the needles and springs from the sprint though.

[xvivalve]

You’ll need to swap the linkages too as for some reason the spacing between carb centres on the two cars varies by about half an inch.

[cleverusername]

I though the Sprint carbs were just standard Hs6 carbs and the only thing that varied between cars was the linkages and needles. Are all SU carbs of a particular design, ie HS2, HS4, HS6 ect identical or were they sometimes modified to fit in certain cars?

[cliftyhanger]

I though the Sprint carbs were just standard Hs6 carbs and the only thing that varied between cars was the linkages and needles. Are all SU carbs of a particular design, ie HS2, HS4, HS6 ect identical or were they sometimes modified to fit in certain cars?

I think there are some odd ones out there. Andrew Turner turned his nose up at some donkeys years ago. But the specific info has evaporated from my memory.

[xvivalve]

There are all sorts of subtle differences, for example, there are various designs of float chamber, with the mounting boss at different heights to suit the geometries of various cars they were fitted to...