Actually, serious answer - if you must have it set to 2.5 turns, get a new needle that suits the engine modifications.
The reason you need to drop the jet to 4 turns is because the modified engine needs more fuel. Dropping the jet allows more fuel. A different needle can also allow more fuel. What needle you need depends on where it is rich / lean when you set it to 2.5 turns down. Does it run lean all across the rev range ? Just at idle ? If you're changing the needle, you need to answer these questions to determine if you need a needle with more taper, a needle that is "thinner" everywhere, or e needle that is just thinner in various locations up and down the needle.
Best bet is to put it on a dyno with a dyno operator that understands SU's and go for it.
Or accept that it runs perfectly OK at 4 turns down, and setting it to 4 turns down is a perfectly acceptable solution.
EDIT: Yeah, do what Cozza typed while I was typing ^^^^^^
Edited by 1600dave, Yesterday, 07:00 AM.