My brother has a spare S&S B series and I want to run it on a 98 FLHT, any ideas. Any set up problems or flat spots, appreciate any help. Cheers

Norton the S&S B series was superceded years ago by the E series. I thik the E series would be a much better carb for your EVO motor. Not saying it couldn't be done but I personally have no experience with the B series. Maybe another here can help.

You didn't give much info-- is it a worked over motor with headwork and/or cams, exhaust, breather etc or a stock motor? I have been running a E carb on my worked over 1996 FLSTN for more than a few years and it's been OK. Tell us more and you are likely to get better assistance.

B series are a great carb.
They do stick out more, which can be annoying, depending of how you are set-up foot control wise, etc.
Most do not have the pump option fitted, which made little difference in my experience.
They make almost exactly the same power (within 1 hp) compared to an E, same bike dyno-ed twice, with just a carb swap.
As most are older units now, throttle plate shaft wear (including the carb body) can be a big issue.
You can mod the air bleeds like an E, which can help.
They are better at top end than an E, though that would likely not be noticeable on a less modified set-up.
E series new come in an easy to fit evo mount kit, so that helps make them appealing.
Flat spots are related to bike set-up, and carb "style". As in, in this case, a fixed jet/butterfly only type.
If the carb is in good shape, then you can make it work ,and work well.
If it is not, then it will stutter, refuse to return to idle, and clonk and knock a lot.
They do not last forever, and need serious refurbishment from time to time.
Just like anything.