Mike the Pipe

Afternoon all from a nice sunny London, Hoping that i can pick somebodys brain regarding the above, Ive got a 1987 Grey import, That is finally running on all 4 cyls, But just took it out for a run, and i do think that there is a restrictor of some sort liked into the speed side of things, Revs lovely through the gears, up to about 45/50 mph, then its as if it retards itself, Any help finding this pesky device would be greatly appreciated, Thanks Mike