coil swap - made it worse

[FlipFlopGriff]

Swapped my coil, as I had a spare Bosch, to see if it would improve my running. It made it run much worse straight away so I swapped it back immediately. It is the correct Bosch number so why could this be. Tempted to buy another but would it just be the same. Its for the BV not the 500 if that makes any difference.
Bought it off Tim at ACT as a spare only at the time.
FFG