I didn't have a 67 but tried a 66 and it improved further. It slightly bogs at the very extreme top of the rpms where, frankly, the motor is screaming. I misstated my top speed earlier. It's closer to 30mph.
I did try it without the air filter and did not notice any difference.
How do I determine if the exhaust is blocked? The bike only had 700 miles on it when I received it so would it be normal to have a fouled up exhaust pipe?