I drive a Ford Escort 1.3 cl (1986) and lately it has developed a nasty trait...when you 'accelerate' (if you can call it that) it hardly reacts to the throttle but at a certain high rpm it suddenly starts pulling twice as hard...

Now it has new sparkies, airfilter, oil and filter and a new exhaust EXCEPT for the backbox wich does seem to be rather full of crap...

Could that cause a restriction that causes it to hardly wanna move low down in the rpm range and better at the top???

Sounds like a daft question maybe but that's the only explanation I can come up with...