Do you have (or did you have) a CEC solenoid on the carb? These were designed to keep the throttle plate from shutting fully when you take your foot off the gas; this reduced emissions. Some people took them off since they hindered engine braking.
I'm curious as to why you want to shift into neutral and coast at 45 mph. Our motor vehicle act prohibits the practice of coasting, but I have never hears of anyone getting a ticket for it. Probably some relic from the past...
