the Mikuni VM18 was suggested because of its price and availability.
of course if you can make a 18mm I.D. manifold that would be better.
please dont anyone comeback with the old wives tale of bigger carburetors causing worse running at low rpm. god, will that fairy never die?
nope. not saying that at all. oversized carbs DO cause bogging when opened up wide when operating at low RPM but. no old wives there, just plain, well documented truth.
im saying that the port only has a cross section of 16mm, 17 at the most. theres not enough material there to try and enlargen it, even by a whisker, and ive made properly matched manifolds before. at which point, the studs and the nuts really start protesting about clearances... then try bunging a weld in there... have to stick that tube to a flange somehow...
make the port WIDER, and you cant get the flange to match, without some tomfoolery with the studs. oh yes, its possible, but awkward. look closely and youll see where i had to bash it to clear one nut...
make the port higher, or deeper...there simply isnt enough metal left to create a good seal... i like my gaskets to have at least 2mm of contact all around...
you can just see that that manifold is curved smoothly, not "cut and welded" through some angle, and the angle it attaches to the flange is the same angle the port itself has... i get awesome flow... but cant improve it with a dremel, at all. other than to take the seam out of the tube, maybe...
now compare my manifold flange to the best "port matched" flange you can find on the market... ha!
nothing about worse running at low rpm. everything to do with minimising flow restriction.
its like sticking a brick behind a high reynold number wing... the wing makes a lot of lift (suction on the jet in the case of a carb with a properly shaped throat, such as the mikuni, which is , really an airfoil shaped as a cylinder) that just gets broken into destructive turbulence afterwards.
if you can TAPER the manifold smoothly from the 18mm (or even a 20 if you can get the taper right...) to the intake port, then its all good.
an abrupt step... sharp edges dont have great airflow. its why we port match.
and whats that bit about sound waves being reflected by sudden changes in diameter? hmmmmm?
ill stick to my stock standard NT
possibly taper bored.