Hi chaps, I have an A+ 1275 bored to 1310 with a 521? coopers s cam, a lightened flywheel
and a standard mk 2 S head. So as you can see its nothing fancy. currently I'm running with
a set of twin 1 1/4SU's. I do have a set of twin 1 1/2 SU's and a lynx manifold that will take
either a 1 1/2 or 1 3/4 SU. My questions are, what would be the most reliable and economical
set up? Is there much difference between them? Also what needle would be the best in the
recommended set up? This is in a daily driver which does approx 250 kms a week with most
of it on the open road to work and I do like to scoot along . sorry for all the questions but
I'm a bit clueless when it comes to carbies.

cheers

Anything I could say about the 1.75s I couldn't say from first hand, but I I finished a set of twin 1.5s and put them onto my 1275 about twelve months ago. I have to say I enjoy them immensely over the twin 1.25s. My engine set up is nearly the same, with a std flywheel and a different camshaft.

The idle is nearly identical, and the pick up is so much better through the mid to high range. I cannot say it has damaged the low end performance in any way.

Reliability will be the same, as they're the same carbs, just slightly bigger, and fuel consumption is undoubtedly a little worse off, but all worth it.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

IMO performance with the twin 1.5s will be better than a single 1.75 SU on an old Lynx manifold.
With the right needles economy should be just as good as the twin 1.25s.

re needles for the 1.5s, either CP4, #6 or #7 should be on the money. use with red springs.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Thank you Mick and Dr for your input. You've helped me make up my mind.
Twin 1 1/2's it is.

i have a1310 using twin 1.5 running with 6s and have recently swapped out for a single hs6 on a standard minispares manifold and a bbv? needle(oa6 would be better). Performance is similar .... both work fine...

looks go to the twins but keep a look out for the routing/bracketry that will fit, as you may need to buy extra ontop of the synch bar....there is no one standard way to attach choke and accelerator cable and lining it up properly , and similarly 2 airfilters/cages can be extra cost . the twins are reappearing on another build so i still like them..
edit to make sense

I've had twin 1.5s on my cooper S for years, I think they are running CP4 needles.

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1969 Cooper "S"
1967 Mini Deluxe
1973 Clubman Van (the fleet spare)
1978 ex 1275 LS ("Wizard" Eaton Supercharged)
2015 HSV GenF GTS(occasional drive & tow car)
2019 MINI F55 Cooper S

ditto

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.