Twin carbs only flow a bit more than singles, so if a Single 40 is too small, Dual 40s will be too small.

Sounds like 44s or 45s are the way to go for a modified high RPM L20B.

Unless modified, twin 38mm S.U.'s are too small for a worked over L20B. Even a single 45 Weber or 44 Mikuni would be pushing it I think. With twin 40mm mikuni's on the dyno my engine couldn't get enough fuel at the higher RPMs. For a worked L20B I would go for twin 44 Mikuni's or twin 45 Weber's. Just my two cents.....

A single 45 on a plenum manifold will flow more than twin 38mm SUs.

The twin SUs should be more response and smooth, if both setups are tuned correctly.

air filter use k/n part number 569096 with min mod to bolt holes on base plate

run the su-s i run them on my L-18 work great

I'm leaning towards the single 45mm webber at the moment, just for the simpicity more than anything else.

Ok - this is a big if - but if it's just a single weber - the main 'gain' from webers (well aside from the fact they are a pretty decent design in general) is where you have one carb throat per cylinder/intake port. If you don't have that (which you won't with a single weber) then the gains over twin SUs is much smaller. THe weber (if you optimise both setups) will still outpower the twin SUs, but not by very much, and i'd bet money nobody would be able to tell without lifting the bonnet, just by driving.

the big if, however - is as Matto says - the weber is easier to setup (since it's a matter of altering jet, idle jet, emulsion etc (though I'd bet you'd get the emulsion right in 1-2 goes tops), With SUs, you need to buy 2 needles per 'experiment' till you get it right. If it were decades ago, more than a few workshops would have enough needles on hand that they could dial it in and just bill you for the time and final needle choice, but now, good luck, and the needles are expensive all things considered.

That said, try giving SUmidel (I think there is a website too) a call. They are (or were) a firm in NSW if I recall, who specialised in the SU carbs and might just have enough experience to get you right in the ballpark as far as needle selection, possibly enough to be acceptable (if not perfect) for a lot of apps.

i would take the webber anyday. they preety much are the same thing (twin carbys) but the webber is so much easyer to get parts to tune it.

im going su's on my set up, only cause ots stright lpg and already have the set up on the a15 su manifold (will bolt onto L16 one too)

Anyone here got an l-series in a 1200 running a single s/d? Any pics?