Twin HS2 SUs do work great on a 1098. But they want to be reconditioned or in good nick, otherwise they are a PITA to keep in tune.
I feel they have better throttle response than a single does, and they look great too.

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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.

I would fit a 40 or 42 Weber or Dellorto before the twin su's.

Twin SUs in restored condition do stay in tune. Worn ones suck air around the throttle spindles making tuning and balancing difficult.
The downside to SUs these days is new ones are expensive, and proper restoration/reconditioning is not cheap either.

I like Webers & Dellortos too (I run a 45 Dellorto) but yes they are noisy. They also drink a bit more than SUs do, even when tuned correctly.

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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.

The Cooper S cars I have owned were both running the original 1 1/4 SU's and Air boxes. They were quiet and fairly economical but the setup is not the best to get HP. Had to play with the mixture every 12 months when I did the points and plugs.

Your engine specs look good. What airfilter setup are you looking at, K&N filters with stub stacks?

Id say stay with a single carb.
I know lots of people that are changing to singles from twins because they reckon it feels better and stays in tune alot better!
The RE13 cam in a 998 would be rather insane... I have a mate with one in his 1100 and its lumpy as all hell but goes like stink!

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2000 Holden Rodeo TD - The Daily
1998 JDM Mitsubishi Lancer Evolution V -The Weekender
1977 Leyland Mini 998cc, 1293cc in the works - The project
1977 Leyland Mini Van - Sold
1966 Morris Mini Deluxe - Sold
2004 MINI Cooper S Chilli - Sold

A couple of guys in the club say that a single 1 3/4 inch SU is a really good setup on an 1100. I have found that the ramflow filters work well.

Not many have taken the time to learn to rebuild SUs. Massively overrated job, and massively overpriced for the simplicity of fitting the bushes for the first time. After a few rebuilds, you find that a careful hand will drill the carb body for the bushes really easily. You drill through the sizes from 1/4 to 5/16ths in small steps. The bushes will sit in nicely, and a touch of loctite will ensure they don't move. Push the bushes through until they're proud of the inside bore and file back with a jewellers file to suit the curvature of the bore. if the alignment of the bushes are imperfect, use a 1/4 drill in ones hand to carefully hone through both bushes at once a little by hand. I have a jig to hold them secure as I drill them vertically as required if I feel the need.

Once the twins are new and in tune, I have not felt the need to touch them for years. The set for the Mk2 have kept in tune for five years now. Every now and then I get the urge to have a play, but only find that I am bumping them out of tune, and bring them right back to the original setting in the end.

I love twins for the look of them alone, so I'll go the extra mile sometimes to rebuild a set.

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SooperDooperMiniCooperExpertEngineering

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

Sounds like you just signed up to rebuild everyone's twins Mick.

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You're so square
Baby I don't care

You sure can. The rebuild kits don't have very many parts in them at all, which is a clue.

Frankly, the ONLY part which is a problem besides the bushes, is remembering which way the little springs in the linkages go in. Take two images each side of the choke linkage and the throttle linkage, and you will be fine.

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SooperDooperMiniCooperExpertEngineering

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

I have a very similar setup to you and run twin HS2s and wouldn't swap them for anything.

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"In two years time your car will be like a lady's clothes, out of date, my car will still be in fashion when I am dead" - Sir Alec speaking to Pininfarina

For a 998 I would go the twin HS2s. IMO twin HS4s are a bit too big for a roady 998.

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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.