Wrong ^^ its the pipe that makes the power . 56mph with my polini maxi 15 mm bing derestricted Estroil no porting.
I hit 60 mph with rev run pipe 15 mm bing
21 mm is a waste on a 70 cc anyway even if tuned unless it's ported
The inside of the intake port is = to about 18.4mm
So a 19mm carb is fine
I run 19-21 phbgs
And mikuni 24mm carbs too
so I'm not stupid
I ran a 19mm dellorto on a dmp 70 cc and it was fine
It was a little faster and had more mid to hi rpm power then a 14-15 mm bing
but not double and a 14 mm bing with rich needle just killed it off the line .
Each to his own I'm tired of emails of how to jet/tune on a 70 cc kit with a phbg and local kids wanting me to fix tune their treats "party pack combo".
It's just easier and cheeper to get a bing and 3 jets (14mm 70,72,74 ), ( 15mm bing #78, 80, 82 jets)
Then a 21 mm phbg and 10-15 main jets, 3-4 pilot jets, 2-3 slides (though# works good for me) 2-3 needles ( though # 8 seemed to work good for me)