Re: +7 mph by fixxing poor quality on 66cc
The OP did many things that need to be done. My suggestion would be to get rid of those pretty acorn nuts and replace with shouldered hex nuts. Also he should not rely on a metal to metal seal at the carburetor/intake manifold. A good gasoline resistant sealer should be used at that point. I like SealAll as opposed to most of the silicone based sealers which are not gasoline compatible.
The welds on the intake and exhaust flanges have always been a problem and should be ground away to allow proper seating of the fasteners.
Sounds like he'll have a reliable installation if he pays the same attention to the other parts of his install.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"