Originally Posted by Ben
I'd not want to get involved in a draw through or blow through set up with a turbo and carbs. They just don't run well.
While I've never turbocharged a motorcycle, I do have some exposure to carbureted turbo setups by way of the air-cooled VW community.
The earliest such setups were of a draw-through design, typically using a large two-barrel (or small, progressive 4 barrel) carburetor of Holley / Rochester design. These tended to work rather poorly when not in boost, and rather particularly poorly at idle, as gasoline which has only just achieved an atomized state does not, it would seem, enjoy being drawn around the complex and rather tight curvatures of a compressor wheel and housing. And, of course, forget about running a bypass valve. Additionally, turbochargers rather dislike having vacuum pulled across them (such as by a closed throttle plate) to the extent that when exposed to such conditions, the oil seals within them tend to adopt a rather casual attitude towards their job.
A rather ingenious blow-through design was then undertaken, wherein the entire carburetor was enclosed in a sealed box, and the box itself simply pressurized by the compressor. Of course, it was necessary to add a carefully calibrated boost-referenced fuel pressure regulator, to prevent all of the fuel from being blown backwards out of the carburetor when in boost. This was, to say the very least, cumbersome. Not to mention that a concept which we take for granted (fuel enrichment under boost) is simply a foreign concept to a carburetor of this design.
Later efforts refined this concept employing discrete-barrel Webers, eliminating the box and carefully sealing the carburetor against the outside world, manipulating the pressure applied to the emulsion tubes in complex ways to achieve boost-referenced enrichment, and produced a system that did in fact work, but at the cost of being indescribably finicky and bothersome to tune.
At about this time, the concept of electronic fuel injection was discovered and, despite the exceedingly high cost attached to aftermarket systems of the day, was universally accepted as the Right Thing™, whereupon all attempts at carbureted turbocharging were unceremoniously binned.
I can think of few bikes better suited than the Road King, with its vast panoply of unoccupied space and its robust electrical system, for an EFI conversion.