I am running a pre 95 1.8 rebuilt zetec on twin 40 webbers and Alpha ignition.
Approx 3000 miles ago iw was set up on RR with all correct jets,chokes etc.
It starts,ticks over and runs Ok but feels if it "gone off the boil". Can it or does it "go off" and if it does what goes off.
I know someone will have an answer !

Fresh Petrol in the Tank or Old ?

Just wondered if cars been stored over the winter with old fuel in the tank and it's lost some of it's Octane rating or become contaminated ?

Describe "off the boil"
Does it feel 'flat' is it 'missing' etc

Bent or burnt valves ? do a compresion test or better a cylinder leak down test.

check the carbs are balanced

if is seems a bit quiet check the exhaust isnt blocked

From Dave Ashursts comments about his xflow powered seven i'd say .........

"is the exhaust any louder?"

He notices that as the packing gets blown out, the back pressure changes in the exhaust which affects the engine which in turn affects the carbs. Repack the exhaust and the performance comes back.