I've got a mini stash of these early style flat slide carbies...a couple of Lectrons, an EI, a couple of Blue Magnums and one I've never seen before, a chrome plated 36mm Posa. I went through a bit of a craze of buying these gizmos on eBay when they were as cheap as chips ($10-$20) but stopped the madness when they started going up in price for some obscure reason.
I had Lectrons for a while on my YZ 400 and SC500 dirt track bikes and had a Blue Magnum for very short on the VB 250 Monty I owned in the USA(it was on it when I bought it) and always liked their simplicity. A mate gave me a 36mm EI that's set up on methanol a month or so ago so I may try it on a dirt track bike, maybe my upcoming Antig/Maico 250. Like Graeme indicated, they certainly do produce the HP but only in those sports that don't require a lot of throttle action like dirt track or drag racing.