Running too lean

[Brett Nield]

We have a Caterham with well tuned 2 liter Zetec(280 BHP at the flywheel) . It has just decided to run really lean, banging ,farting a the occasional flame. Checked it and found badly cracked exhaust and assumed this was the problem. That's now fixed but still the same .
What does the panel think, any opinions gratefully received

[Brett Nield]

Thanks Panel
The engine was rebuilt by Rick Wood in Stockport, my pal is on the way there now to stick on his diagnostics. Will let the panel know the outcome for future reference.
Many thanks
Brett

[Brett Nield]

The fuel potentiometer had gone into default mode. No one seems to know why this happened.