No start!

[John Cornforth]

Failure to rev under load and severe lack of power normally points to weak ignition for some reason e.g. failing coil or condenser BUT that is normally accompanied by jerky running and misfiring, none of which have been reported.

If you have a vacuum wiper era car you could borrow and connect vacuum gauge to the take-off on the inlet manifiold, and the reading pattern will tell you if the exhaust is blocked.

Alternatively, take off the downpipe. It will be mighty noisy but should have minimal back pressure......