well, by drilling a hole in the stock air cleaner cover, you won't really increase flow, the stock air filter is pretty much a peice of sponge, and really isn't very good.
if your engine "fizzles out" on the top end now, that tells me that it may be too lean because you have a better flowing exhaust. when you get to wide open throttle and the engine just falls on it's face, looses power and bogs down, 9 times out of 10, that is a sign that it is too lean. if you have the older style stock n.t. carb, these carbs are designed for the smaller 50 c.c. engines, and every one i've ever seen has been jetted too lean for the 66 c.c. engine. (in my situation, i had to richen my n.t. carb quite a bit by going from a .70 jet, to a .74 jet). my 66 c.c. bike gets 32 mph, and my 50 c.c. bike gets 28 mph. both of these are 20" bikes with 20" wheels and 41 tooth rear sprockets. if i drop to a 36 tooth sprocket i would probably get 35 mph out of both of them.
tuning these carbs is a pain to get them right, but when you do, you can actually get decent power out of these cheap little engines. the n.t. carb is a porr design in my opinion, but you can tune them without a lot of work. I tried a cns carb and i was not happy with the way it performed on my 66 c.c. engine.
i am assuming that you removed the main jet, and put the washer under the jet to lower it?
if this is what you did, it will have no effect at all, because when you are at w.o.t. the needle is 100% out of and away from the jet.
I'm not really following where or why you put a washer in the float bowl.