looks like that will breathe better but IIRC it's a 15mm carb trying to push air through a 13mm tract. If what you're trying to do is get more power, try getting a bigger carb with bigger intake manifold. Someone here did dual carbs, no idea how it worked out for him, but I imagine there wasnt much gain. Similar to blowing air through a straw, if you squeeze part of the straw, it'll get harder to blow through. This is similar to a 15mm carb going into a 13mm intake (it's actually 12.7mm, or 1/2"). The trouble is, though, that unless you have a GT5-a or GT2-a from grubee (or super rat/super mouse), the maximum width you can open up your intake port is 7/8" (22mm). Being 32mm spacing, 6mm bolts, leaves 26mm between bolts. 22mm wide leaves 2mm on each side of the intake. BUT remember those bolts to screw it down are 10mm. so -5mm from both sides gives you 22mm O.D. maximum. The stock intake is 3/4" O.D. (about 19mm). So even though the stock intake manifold is too small for the 66cc motor, it's also as big as it can possibly be.
The solution? Well you have two options. You can get reeds (Arrow reeds give you tons of space to custom make an adapter for the carb), or you can get a new jug (
www.motorizedbicycle.ca has GT5-a jugs). If you have a type D motor, this will set your ports 1mm higher, giving you a bit more high end power. Both intake and exhaust are matched at 1" wide as well, and the intake bolt spacing is 40mm, meaning you can get a bit bigger than a 1" O.D. pipe, which will fit up to a 21mm carb (which combined with reeds should easily be enough to feed the motor all the way up to an insane 11,000 RPM, or without reeds probably 11,500 or 12,000 RPM).
You're on the right track though, these motors get huge boosts from being able to breathe better, thats why these motors love the expansion pipes so much (it gives it an "extra breath" each stroke thanks to the reverse wave). However I wouldnt spend too much time finding an air filter to breathe better, as stated above, your weak point is in your intake manifold, not your carb.
